13 ms·

The magic of DC-DC voltage conversion (2023)

392 points by _Microft 2y ago

vagab0nd 2y ago
Can we convert from DC to AC, manipulate voltage, then convert back to DC?
crote 2y ago
> Between the resulting thermal management issues and reduced battery life, linear regulation is seldom worth the pain. I'd argue the exact opposite. The article is targeting "enthusiasts", and a very large portion of enthusiast projects are going to be powered by a 5V USB charger and consume in the order of a few 100mA of power. LDOs are dirt cheap, widely available, have pretty decent output characteristics, and incredibly easy to use. If you have basically unlimited 5V and want 100mA of 3.3V, why not use one? On the other hand, buck converters require you to actually do some actual engineering. You can't just haphazardly throw in a single IC and expect it to work flawlessly on your first try. You either have to use an (expensive!) fully-integrated module, or do a decent bit of math and part sourcing yourself. Neither option is exactly attractive to a hobbyist building a fairly simple one-off PCB.
- quacksilver 2y ago
  Linear regulation is also very good when you have situations where you want to avoid generating unwanted noise or stray RF. Cheap buck converters are very noisy and annoying if you are building an audio or radio related project, or have such things nearby.
  - f1shy 2y ago
    In my experience, not only noise in the RF sense, but also audible. I put together a little audio amplifier, and the sound of the DC/DC makes it unusable in quiet situations. The 12kHz (coming physically from the converter, amplifier off) really hurts the ears!
    - arghwhat 2y ago
      That’s magnetostriction, components under switching load (caps, inductors) need to be secured in place with an appropriate glue/putty. Using a higher switching frequency can also help, plenty to choose from.
      - schoen 2y ago
        Can that also help with the emanations security issue where an adversary might be able to extract usable data from the audio produced by the electronic components?
        mschuster91 2y ago
        TEMPEST and other side-channel hardening is hard to do if you lack access to anechoic/RF isolated chambers, sensitive scopes/microphones and knowledge.
        kragen 2y ago
        yes, but the audio usually doesn't travel as far as the rf; you'd almost have to be in a situation where the adversary can't put equipment near you but has managed to subvert a microphone
        foldr 2y ago
        The usable data would just be "DC-DC converter is on/off". In theory, if the converter uses a variable frequency or duty cycle, you might be able to extract some information about that too. But that's not very interesting.
        arghwhat 2y ago
        A DC-DC converter always uses a variable duty cycle to maintain the target output voltage (or for CC, current). Without it, the voltage would vary wildly depending on load. For something like an audio amplifier, obtaining precise power supply load would in turn give you a curve over amplifier load, which effectively gives you the speaker amplitude. Input caps and filtering will likely remove the high frequency components entirely, but you might be able to construct at least part of the played waveform.
  - Animats 2y ago
    > Cheap buck converters are very noisy and annoying... Yes. This is why the good ones have more parts. It's a totally fixable problem, and the parts cost to fix it isn't high, but it takes extra engineering effort.
    - shiroiushi 2y ago
      All true, but for a hobbyist it probably isn't worth it if their goal is to just build some little audio project.
    - atoav 2y ago
      You are right. Yet, if you asked me how to get less noise on your audio circuit the LDO is the easier answer that will cost you less time to implement and likely give you the superior result. Especially for beginners without a ton of measuring equipment and experience having potentially bursty high frequency components in series can be an interesting way to not get the thing they were planning done, but instead have to deal with an entire new set of problems whose existence they didn't even know about. Technically you are correct, but "just slap a LDO on it" is probably the better advice.
      - Animats 2y ago
        Agreed. I had to learn a lot to build one. I had an application so unusual (driving antique teletype machines from a laptop) that I had to do a unique design. For most low-volume applications, it's not worth the trouble.
- roaringraster 2y ago
  And 3.3/5 is approximately 66% efficiency, which isn't too horrible. So even if you get your buck converter working, getting those 95%+ efficiency numbers you see in datasheets out of the circuit is not trivial.
- _fizz_buzz_ 2y ago
  If you have 5V and have to step down to 3.3V using an LDO is a very reasonable choice (at 100mA you have about 170mW losses). However if you have e.g. 24V and need to step down to 3.3V, an LDO can get annoyingly hot (at 100mA you now have over 2W losses). But I agree, this is really a "it depends" situation.
  - klysm 2y ago
    Most LDOs straight up won’t tolerate those conditions without very careful thermal management
- deleted 2y ago
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- michaelt 2y ago
  > On the other hand, buck converters require you to actually do some actual engineering. You can't just haphazardly throw in a single IC and expect it to work flawlessly on your first try. It used to be a hassle a few years ago - but these days you can haphazardly throw in a R-78K3.3-0.5 - which has the pinout of a classic three-pin 3.3v linear regulator, but it's actually an 80% efficient DC-DC converter with 500mA output and an input range that goes up to 36v. That's enough current even if you've got something like an ESP32 that needs 250mA - and for any type of hobby project, the $2.40 is fine.
progbits 2y ago
I can highly recommend the MIT 6.622 Power Electronics course recently released on OCW: https://youtube.com/playlist?list=PLUl4u3cNGP62UTc77mJoubhDELSC8lfR0 https://youtube.com/playlist?list=PLUl4u3cNGP62UTc77mJoubhDE... https://ocw.mit.edu/courses/6-622-power-electronics-spring-2023/ https://ocw.mit.edu/courses/6-622-power-electronics-spring-2... Prof. David Perreault is excellent. While the course gets into pretty advanced topics that simply won't matter unless you are designing multi-kW systems, it covers all the fundamentals and builds understanding from ground up so you will know what makes sense to use and when.
- kleiba 2y ago
  Thank you! I was just going to ask about recommended resources for getting into electronics. I've never been able to find anything that I personally found useful - often times, introductury courses are too basic and slow to keep me focused, or they lack exercises or are too theoretical, etc. There are many hobbyists who have learned all that stuff and can design and implement their own circuits (say, audiophiles or model train enthusiasts), so obviously they have all been able to get there. But I have never managed to learn anything about electronics, although I would really like to.
  - progbits 2y ago
    Yeah it can be hard. As a self-taught hobbyist I've found a mix of university courses (not whole curriculum, just pick and choose and don't feel bad fast-forwarding over some of the math theory), books (art of electronics, practical electronics for inventors), and high quality youtube channels (eevblog, phil's lab, robert feranec, microtype engineering) to be a good way to learn. Also eevblog forums are great. I don't post much but just reading through the discussions you get a lot. My greatest annoyance is the flood of very low quality Arduino tutorials everywhere that polute the search results. Not to be ungrateful, Arduino got me into the hobby, but if you just learned about resistors last week the world doesn't need your blogpost on how to connect it to a breadboard.
    - Max-q 2y ago
hcfman 2y ago
I've been working with audio recently and found so many of the devices that convert 3.7V to 5V for example inject noise into the rail that make's it in the microphone input source. The battery support from pisource does this terribly. But so do many battery sources. It's not just microphones that get affected, but also other sensitive sensors like accelerometers. I hope that other people making DC-DC convertors put some effort into making sure the supply is so clean so as to prevent this in future.
- euroderf 2y ago
  Isn't this like 95% fixable with a capacitor ? Aren't there cables with small embedded caps ?
  - dragontamer 2y ago
    Oh hell no. I think a lot of people are overly cautious of DC-DC conversion in this topic, but you've gone full-tilt in the opposite direction and are severely underestimating the problems that occur. 1. Its not "power-conversion" that's hard per se, its EMC that's very hard and not taught very well at a bachelor's level. 2. DC-DC Voltage Converters usually handle the entirety of your board's power, meaning they are the highest power component. 3. High power and high-frequency is a difficult EMC problem. This means that a bad design will absolutely send your electrons / energy out and radiate out like an antenna. And if things on the same board pick it up, it will be called crosstalk. And if things off-board pick it up, its called electromagnetic interference which almost certainly leads to a compliance problem. --------------- 1. Hobbyists don't care about compliance. So bam. We are already dealing with the biggest problem by simply not caring about it. (Maybe you can care and go into deeper studies, but... if you're a beginner just don't care. Learn this very difficult stuff later). 2. Prevent crosstalk by following good board design rules: have a 4-layer board. Use Power+Signal / GND / GND / Power+Signal stackup. Use two vias (one for signal-1 to signal-4 traversals), and a 2nd via for GND2 to GND3 traversal of the return current). Thinking of both the forward current and a tightly bound reverse current is basically all you need to do to avoid difficult crosstalk problems on board. Done. Point#2 requires deeper studies than is typical in bachelor's level electrical engineering. But it truly isn't very difficult once you learn the theory. Tight ground-planes reduce crosstalk (and EMI problems), and furthermore thinking of the return-current explicitly prevents problems. Now you could have some truly difficult "ringing" from trace inductance and other such nasty problems... but that tends to occur beyond 100MHz. I'm thinking most beginners are going to be under 20MHz for most of their designs and thus never deal with those advanced "PDN" / Power Delivery Network problems. Though if you do go into PDNs, its obviously a tough subject with huge amounts of study and reading involved. But most of the problems truly are at very high frequencies and/or at EMI compliance. Beginner Hobbyists avoid the most difficult issues entirely by nature of beginner (aka: low-speed) and hobbyist (and therefore don't have to follow regulators). ---------- I'm not a professional. But my understanding is that top-level EEs who work on PDNs will simulate the circuit-board itself to figure out trace inductances / capacitances in the board itself. (Closer planes of ground/power will create more capacitance. Long traces tend to increase trace inductance, etc. etc.). And tight simulations are the only way to truly understand the PCB and how it interacts at high frequencies with high-power. But such methodologies are gross overkill for a 1MHz boost converter with a pre-made PCB Layout, and a list of capacitors + inductors already picked out for you. (ex: https://www.microchip.com/en-us/product/mcp1640 https://www.microchip.com/en-us/product/mcp1640) Seriously: Page 17 (https://ww1.microchip.com/downloads/aemDocuments/documents/APID/ProductDocuments/DataSheets/MCP1640-Family-Data-Sheet-DS20002234E.pdf https://ww1.microchip.com/downloads/aemDocuments/documents/A...) already gives you the PCB-layout you need for this, with recommended components. Don't overthink it, just copy the design from the document.
exar0815 2y ago
I do work in automotive EMC testing and it's nearly always the voltage conversion at fault when you fail tests or influence other devices. Buck-Boost converters are a noisy and finicky thing, and not easy to debug if you use a monolithic IC from the cheapest vendor. Quite annoying discussions.
Animats 2y ago
DC-DC converters are hard, but fun. The basic concept is that when you put current through an inductor for a while, then disconnect it, you get a big voltage spike. That's a classic auto ignition system. You can put that spike through a diode and use it to charge a capacitor to get DC out. The neat thing about switching power supplies is that there's very little resistance in the power path. That's why the efficiencies are so good. The not-neat thing is that they are a dead short across the input for part of the cycle, which is why failures can cause fires and why you may need an inrush current limiter and/or a fuse. There are boost converters, buck converters, and ones with transformers. With a transformer you can isolate the input from the output, which is mandatory for safety if you're driving the thing from the AC power line. Here's one of mine. USB 5VDC in, 120 VDC out, to operate antique teletype machines that need 60mA 120VDC.[1] The basic circuit is simple, but there are multiple surface mount ferrite beads and small capacitors to keep the spikes from coming out via the input USB, output, or as RF. LTspice simulation was needed to pick the values for those, so as to minimize noise in both voltage and current. [1] https://github.com/John-Nagle/ttyloopdriver/blob/master/board/images/schematic.png https://github.com/John-Nagle/ttyloopdriver/blob/master/boar...
- deleted 2y ago
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- rkagerer 2y ago
  Can't you also charge up capacitors then slam them together in series? Is there a name for that kind of supply?
  - caf 2y ago
    Yes, it's called a charge pump. There's one specific sub-type called a Cockroft-Walton voltage amplifier.
    - pfdietz 2y ago
      They won the Nobel prize using this invention (which wasn't theirs). https://circuitcellar.com/resources/quickbits/cockcroft-walton-voltage-multiplier-2/ https://circuitcellar.com/resources/quickbits/cockcroft-walt... Cockcroft went on to great acclaim for "Cockcroft's Folly". https://www.bbc.com/news/uk-england-cumbria-29803990 https://www.bbc.com/news/uk-england-cumbria-29803990
3dGrabber 2y ago
There exists an interesting connection between Boost Converters and Hydraulic Rams [1]. A Hydraulic Ram is device that can pump water from a stream to a higher location by harnessing the kinetic energy of the stream, no other power source required. The equations for the two devices are essentially the same, only the units change. 1 https://en.wikipedia.org/wiki/Hydraulic_ram https://en.wikipedia.org/wiki/Hydraulic_ram
- agumonkey 2y ago
  I love analogies between fields like this.
  - deleted 2y ago
    [deleted]
  - nraynaud 2y ago
    There is a whole area of multi-domain simulation, where the simulator seamlessly jumps from one form of energy to another as long as the units match. I have always loved that.
    - agumonkey 2y ago
      oh nice
    - 3dGrabber 2y ago
      Modelica comes to mind. https://media.springernature.com/lw685/springer-static/image/prt%3A978-3-030-44184-5%2F12/MediaObjects/978-3-030-44184-5_12_Part_Fig1-140_HTML.png https://media.springernature.com/lw685/springer-static/image... https://en.wikipedia.org/wiki/Modelica https://en.wikipedia.org/wiki/Modelica
  - SoftTalker 2y ago
    Water flows in pipes, valves, etc. concepts transfer to a lot of basic electrical circuits and concepts. E.g. voltage is analogous to pressure. Current is analogous to the volume of water flowing. Bigger pipe (wire) can carry more current. Valves are like switches or resistors. It works to de-mystify concepts for kids who have no concept of what electricity is but can think about water flowing in a pipe.
mikewarot 2y ago
I've learned that the magic search word for 150ish volt boost converters is "Nixie". My friend needed that voltage for a Geiger counter B+ battery replacement.
moffkalast 2y ago
96% efficiency sounds great on paper for synchronous converters, but as SBC current draw just keeps increasing and BLDC motors can run at higher voltages it starts to create a major heating problem when you have to supply both from the same source. Something like 12V down to 5V at 5A creates a managable amount of heat, but going higher, 20V, 30V on the battery side and things start to melt all around from heat losses from that large a drop. In some cases I've had to resort to using cascaded rails, stepping first down to 24, then 24 to 12 and then 12 to 5 just to keep the heating spread between different buck converters even if it multiplies losses. Would love to hear what the expert solution is to this that isn't just a massive heatsink.
- mschuster91 2y ago
  > Would love to hear what the expert solution is to this that isn't just a massive heatsink. A smaller heatsink with active cooling and parallel MOSFETs. At a certain power level, it's just physically impossible to rely on convection cooling alone - just look at audio amps or your average CPU/GPU... banks of MOSFETs, caps and inductors it is. While the BOM part count may be higher, you need lower-capability parts. The danger is, you need to carefully grade and match the MOSFETs, otherwise you risk them failing sequentially in a very short time if you're operating too close to their rated current - one burns out, the load distributes to the others, and then they fail because they cannot handle the additional load (or one fails into dead short instead of open, which instantly kills all of the others).
- posterboy 2y ago
  you forgot to mention it should fit under a thumbnail, probably
  - moffkalast 2y ago
    It would be a nice plus :P Honestly the size isn't such a big deal, as long as it doesn't weigh as much as two African elephants like the average mains PSU of this amperage.
- michaelt 2y ago
  You can get 48V DC -> 5V DC 6.5A converters that are 92% efficient [1] You're dissipating 25W from your SBC already. You can dissipate the 2W from your DC-DC converter the same way. [1] https://www.meanwellusa.com/webapp/product/search.aspx?prod=NID65#1 https://www.meanwellusa.com/webapp/product/search.aspx?prod=...
ziofill 2y ago
I'm a theoretical physicist and I swear electrical stuff is so hard to understand! I have a lot of respect for electrical engineers ^^' (and electricians)
mglz 2y ago
For beginners it is super annoying that many tutorials say "there is a magical switch or oscillator here which is integral to the function of the boost converter, but we will not tell you how to actually realize it". Additionally, that needs to work at the voltage level you are starting out from and in many cases should be galvanically isolated from the converter. This is a lot to keep in mind and it is actually not trivial. The answer here is usually to find an IC that works at your desired input voltage or to have a linear regulator provide a small amount of power for the PWM generator. Also be wary of just running with an AI generated answer. Claude 3.5 Sonnet suggest you connect an Arduino straight to 230V and after some back and forth generates circuits which contain strange elements like "antiparallel diodes" which makes no sense.
- posterboy 2y ago
  sounds like a spherical cow on a frictionless plane.
  - mglz 2y ago
    It is a very hairy cow, which likes to bite and is stuck in the mud. Also it has a wierd high-frequency response. There is a description of tractors to get it out, but we'll skip how the controls work for now.
- awjlogan 2y ago
  The TI Power Designer[0] is a great resource. Obviously it will only show you TI parts, but it's very helpful to get a base design. You can filter by complexity (roughly BoM count), size, cost etc based on the parameters (input voltage range, output voltage range, power etc). The designs usually have a reference layout as well. 0: https://webench.ti.com/power-designer/ https://webench.ti.com/power-designer/
  - mglz 2y ago
    Very convenient, thank you!
saltserv 2y ago
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londons_explore 2y ago
I have often wondered if ideas from a buck/boost converter could be applied to a mechanical gearbox. Voltage and current in electrical circuits (where voltage x current = power) is completely analogous to torque and speed in mechanical shafts (where torque * speed = power). Every electrical component has a physical counterpart. Spring = capacitor. Inductor = mass with momentum. Resistor = friction brake. The goal would be a variable ratio gearbox using a fully mechanical system, using a spring and a hammer type mechanism to convert one torque/speed to another torque/speed. This is already done in impact wrenches, but I would hope that rather than having an impact rate of say 5 Hz, you have an impact rate of 50 kHz or more, allowing a smooth conversion from one speed to another. Obviously, the difficulty is in the details - designing parts to withstand 50k hammers per second for years of operating without failing from fatigue. Various other mechanical things already operate at high mechanical frequencies. SAW filters vibrate things mechanically at Ghz and don't suffer fatigue failures.
- hwillis 2y ago
  You're overcomplicating it; you only need a single clutch and in/out springs[1] to do this. If you're spinning at 4000 rpm and your springs cover 6 degrees of rotation, then your clutch needs to be able to actuate at 4000 Hz. When the clutch is engaged, the engine-side springs compress to supply the torque and match the speed difference. When it's disengaged, the springs expand back out as it returns to engine speed. The obvious problem is that clutches do not smoothly click on and off like a transistor. However there are more specialized devices that use stick-slip dynamics like piezo actuators. Since there is a much more rapid transition between "on"/"off", they can be very efficient and allow relatively weak devices to exert very large forces. They're just only able to take very small steps. [1] Labeled 4 here: https://haynes.com/en-gb/sites/default/files/styles/blog_landing_header_687/public/info_anatomy_clutch.HEAD_.png?itok=ktUUuTqa https://haynes.com/en-gb/sites/default/files/styles/blog_lan...
  - londons_explore 2y ago
    > When it's disengaged, the springs expand back out as it returns to engine speed. What is it? I think you need an intermediate flywheel, with springs and clutches on each side. The intermediate flywheel's mass is tiny, so might be formed by just the masses of the springs and clutch mechanism.
kbouck 2y ago
I want to power my 12V devices with USB PD. Looks like 12V is optional in the spec and is supported only by some devices (eg. UGREEN), and not by others (eg. Anker) Given a USB PD power supply which supports 15V but not 12V, and a usb-c/barrel-jack cable configured to negotiate for 15V, what would be the simplest (yet safe) circuit i could add via barrel jack to regulate the to voltage down to safe/consistent 12V? is a simple linear voltage regulator (LM7812) sufficient? would i need capacitors to smooth it out?
- klysm 2y ago
  A 3V drop over an LDO is usually reasonable with low enough currents. Some LDOs require capacitors to be stable, and it’s usually a good idea to have some capacitance on your power rails anyway.
- 15155 2y ago
  The one important thing missing from your query here is: How much current do you need? If you need, say, >100A, the possible architecture looks very different than ~1A or less.
- _Microft 2y ago
  It might be cheaper to get a power supply that supports later PD standards? E.g. IKEA is selling some cheap here for either 8€ (Sjöss, 1 USB-C port (max. 30W, up to 3A)) or 15€ (Sjöss, 2 USB-C port (combined power output of 45W, up to 3A, also on a single port)). Both support PD 3.0 and PPS (that's the fanciest PD standard that implements requesting arbitrary voltages from the power supply) They also stock nice and cheap USB-C cables. These power supplies work fine with USB-PD trigger boards set to 12V.
- a96 2y ago
  Linear loss obeys Kirchoff's and Ohm's and power laws pretty simply. If you need to lose a certain voltage at a given (max) current, the power lost is P = U * I. For example, 3 V * 0.1 A = 0.3 W. If your LM7812 can run at that overhead (yes), take the voltages and currents (yes) and dissipate the 0.3 W as heat (almost certainly yes) then it'll work. Caps will depend on how fast the response needs to be. If your load jumps too much too fast and there's no reservoir, you'll get a voltage drop. If it feeds too much back, you'll get a voltage climb. Caps may help and are pretty cheap and IIRC 78xx chips are very stable, but the datasheet will have limits and recommendations. All the information will be in the data sheet and making some test circuits is easy for these, especially if you have some measuring equipment. Anything below 1A will probably be easy, anything above might be risky, as a rule of thumb. Very small or very large voltage differences will also be tricky or impossible.
cushychicken 2y ago
A decent article, but there’s a ton of misunderstanding in the comment section. For one thing: LDOs can be more efficient than buck converters, especially at very low current consumptions. If you’re drawing sub 1 mA, like a battery powered system, an LDO is going to be a more efficient step down converter, because it doesn’t have switching losses. Bucks are only better choices for stepping down voltage at higher currents because the switching losses become negligible. Second: a ton of people here are vastly exaggerating the difficulty of designing a step down buck converter. Integrated designs from TI or analog devices will tell you all the compensating components, output capacitor values, inductor values, etc. for common step down output voltages. Most will include reference layouts with a four layer six layer or even two layer stack up for optimal performance. It’s really not that hard to get a one spin win out of most common buck designs. Don’t be afraid. Just follow the manual. You’ll be fine.
- klysm 2y ago
  The challenges I’ve had with buck or boost conversion is on mixed signal boards where I have very sensitive analog circuits. The ripple and switching noise on the output can make you lose bits on ADCs, or show up on a DAC quite easily. It’s easy to get a buck converter doing the right thing without horrible EMC/EMI if you’re careful and follow the manual, but it’s a lot harder to optimize for something like low noise without utilizing LDOs
  - cushychicken 2y ago
    True, but also not the whole story. For one: most LDOs don’t have a high enough PSRR bandwidth to completely eliminate buck switching noise. Most LDO PSRRs roll off sharply in the low hundreds or high tens of kHz. That’s generally below the switching frequency (and noise frequency) of most buck regulators. If you’re dealing with audio that’s generally fine. RF, however, is a separate problem. Second: there are more cost effective and wide bandwidth solutions for noise reduction. Capacitance multipliers are one that spring to mind. Ferrite beads are another great means for tamping down high frequency noise. Third: layout and current return paths are often just as much of a problem as the buck itself. Couple a high current return path into an audio chain with a shared return, and you’re gonna have a problem no matter which way you slice it.
minkles 2y ago
Lots of things in here which kill me a little: 1. You don't get a voltage spike when you disconnect an inductor. The field collapses and induces a current. If you measure it across a high impedance then it looks like a voltage spike. If you measure it across a low impedance then it's not necessarily much of a spike. Ergo depends on load impedance. 2. SMPS designs are not necessarily noisier than linear power supplies. It's always a design trade off. In fact you see SMPS in all modern RF test gear which is generally far more sensitive and has far more bandwidth than anything back when linear supplies were common. Also there is a lot of noise coming off the diodes in a basic bridge rectifier as well! Noise is a whole-system design consideration that has to be made. 3. Don't use any LLMs for designing circuits. Please go read a book on it designed by experts, not stuff scraped from thousands of idiots. I've seen some horrible stuff out there. 4. I'm sure I'll come up with more over time.
- hwillis 2y ago
  > If you measure it across a high impedance then it looks like a voltage spike. If you measure it across a low impedance then it's not necessarily much of a spike. "Disconnect" implies an open circuit and high impedance.
  - klysm 2y ago
    No, it’s the input that’s disconnected
    - hwillis 2y ago
      Irrelevant. If it's not an open circuit, then the inductor is connected to things in parallel, and the impedance increase creates a voltage spike. If the load impedance is significantly lower than the thing being disconnected, then you're just disconnecting something that doesn't matter to the circuit and it's silly to be that pedantic about an irrelevant situation. You're bending the statement from "disconnecting an inductor" to "disconnecting something from an inductor (while something else is still connected)"
      - 2y ago
marcodiego 2y ago
A commonly used alternative in the microcontroller world is to simply stack a few diodes. Very simple alternative which I have seen being used a few times.
shellback3 2y ago
Interesting, but I had expected to see a comparison of generating DC voltages using tubes, which were used in my university Electronics course, with solid state. In those days to generate a DC voltage from another DC voltage required generating an AC voltage from the DC and then rectifying it.
stonethrowaway 2y ago
Friendly warning to people who aren’t electronics savvy: this blog post is written in a “now draw the owl” sort of way. I’m not sure who the audience is. Anyone who can read this stuff at the level presented inherently knows most of this and then some. Everyone else will need a book and that book will cover this material as it’s fairly fundamental and will derive equations used in here as well so you can make sense of it.
- anthomtb 2y ago
  Maybe the target audience is those with lapsed circuit theory knowledge from undergrad and no hobby or professional power electronics experience afterwards? Describing myself, of course (most of icamtuf's stuff is up my alley, fwiw). One would think the title including "DC-DC voltage conversion" is enough of a squirrel-catcher to stop folks who either 1) Know nothing about what it means or 2) know exactly how to do it, from reading the article.
dgacmu 2y ago
I've moved a lot of my home computing to home-brewed 12V UPSes using these. LFP charger --> Battery --> 12V or 5V DC-DC buck or boost/buck regulator --> device. Most UPSes are designed for high wattage, short runtime, but things like my firewall or small proxmox box for SDN+DNS benefit from low-wattage, long-runtime, and getting the inverter out of the picture substantially improves runtime. Said proxmox box uses under 10W and gets about 20h of runtime from a $50 battery.
- kijiki 2y ago
  What charger and battery did you use?
  - dgacmu 2y ago
    For the moment some random cheap ones from Amazon - I haven't run it long enough to see if it will last more than a few months of full-time use. The battery was a freebie, so I don't actually recommend it; the charger is an "ULTRAPOWER 4a". We'll see. What it's augmenting is a pretty cheap AIMS power brand modified sine-wave inverter/charger (meh, should have gone pure sine, it causes some of the power supplies to make funny noises) hooked up to a Litime 2kWh LFP battery (very happy with the big battery). TODO on the experiment is swapping out the inverter part with direct 12V conversion for some of the things on the inverter, but I wanted to test the regulator approach with a non-critical component for a month or two first before hooking it to something that can deliver 100A, obviously with a fuse. :-)
- thehappypm 2y ago
  Does it outperform a $50 UPS?
  - dgacmu 2y ago
    Kinda depends what you're asking. There are IP camera 12V UPSes you can get that are ok -- I have a few TalentCell 98Wh units -- but they don't actually deliver regulated power, they just tap directly off of the li-ion cells. Those are about $65. With an inverter UPS, $50 gets you a unit with a single 12V / 8ah or so lead-acid cell; 90 or so watt-hours _could_ be reasonable except that the inverter draws upwards of 10-15W while running, so these units actually won't run even a router for more than a couple of hours. Also, lead-acid dies after about 3 years, and then you're in for another $15 or and a bit of a pain replacing it. I'm really sick of doing surgery on old UPSes at this point and trying to move entirely to LFP for the 10 year lifespan alone. If you want really long runtime, you basically have to DIY, but it's fortunately quite straightforward. Larger LFP batteries are about $250 per kWh, sometimes less. DIY'ing it, you can pick your capacity/runtime independent of your wattage, vs pre-packaged UPSes that frequently couple the two. (I have a big pile of APC UPSes of various sorts, some with external expansion battery packs, and it's quite a hassle to get truly extended runtime from any of them -- I don't want to have a generator, I want 20 hours of extended runtime. I realize I'm a little weird, but I like to hope I'm just ahead of the curve a little on switching. :-)
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the__alchemist 2y ago
I suppose the downside is the complexity in circuit design. If the datasheet has a good reference schematic and instructions, or if you are good with electronics fundamentals, it's not a big deal. If not... rolling the dice if it will work. These circuits have been one of the biggest sources of me having to respin boards. Requires a lot of attention to detail to get the passives correct in terms of values, net connections etc, compared to an LDO. Then you may go down rabbit holes like "The one at the switching frequency I want went out of stock. Can I use the one that switches slower? Can I substitute this capacitor in instead of the nonstandard value the ref diagram recommended?" They should IMO be the default for most designs, but double-check everything!
coding123 2y ago
Many people use DC-DC to down-regulate a 48 volt battery system to 12-volt for their RV where they remove the original battery pack and replace it with Lithium. Not everyone does that, some use a 12-volt lithium system. Less 4/0 cables needed when adding inverters and other fun stuff to the RV.

392 points by _Microft 2y ago

Can we convert from DC to AC, manipulate voltage, then convert back to DC?

> Between the resulting thermal management issues and reduced battery life, linear regulation is seldom worth the pain. I'd argue the exact opposite. The article is targeting "enthusiasts", and a very large portion of enthusiast projects are going to be powered by a 5V USB charger and consume in the order of a few 100mA of power. LDOs are dirt cheap, widely available, have pretty decent output characteristics, and incredibly easy to use. If you have basically unlimited 5V and want 100mA of 3.3V, why not use one? On the other hand, buck converters require you to actually do some actual engineering. You can't just haphazardly throw in a single IC and expect it to work flawlessly on your first try. You either have to use an (expensive!) fully-integrated module, or do a decent bit of math and part sourcing yourself. Neither option is exactly attractive to a hobbyist building a fairly simple one-off PCB.

quacksilver 2y ago
Linear regulation is also very good when you have situations where you want to avoid generating unwanted noise or stray RF. Cheap buck converters are very noisy and annoying if you are building an audio or radio related project, or have such things nearby.
- f1shy 2y ago
  In my experience, not only noise in the RF sense, but also audible. I put together a little audio amplifier, and the sound of the DC/DC makes it unusable in quiet situations. The 12kHz (coming physically from the converter, amplifier off) really hurts the ears!
  - arghwhat 2y ago
    That’s magnetostriction, components under switching load (caps, inductors) need to be secured in place with an appropriate glue/putty. Using a higher switching frequency can also help, plenty to choose from.
    - schoen 2y ago
      Can that also help with the emanations security issue where an adversary might be able to extract usable data from the audio produced by the electronic components?
      - mschuster91 2y ago
        TEMPEST and other side-channel hardening is hard to do if you lack access to anechoic/RF isolated chambers, sensitive scopes/microphones and knowledge.
      - kragen 2y ago
        yes, but the audio usually doesn't travel as far as the rf; you'd almost have to be in a situation where the adversary can't put equipment near you but has managed to subvert a microphone
      - foldr 2y ago
        The usable data would just be "DC-DC converter is on/off". In theory, if the converter uses a variable frequency or duty cycle, you might be able to extract some information about that too. But that's not very interesting.
        arghwhat 2y ago
        A DC-DC converter always uses a variable duty cycle to maintain the target output voltage (or for CC, current). Without it, the voltage would vary wildly depending on load. For something like an audio amplifier, obtaining precise power supply load would in turn give you a curve over amplifier load, which effectively gives you the speaker amplitude. Input caps and filtering will likely remove the high frequency components entirely, but you might be able to construct at least part of the played waveform.
- Animats 2y ago
  > Cheap buck converters are very noisy and annoying... Yes. This is why the good ones have more parts. It's a totally fixable problem, and the parts cost to fix it isn't high, but it takes extra engineering effort.
  - shiroiushi 2y ago
    All true, but for a hobbyist it probably isn't worth it if their goal is to just build some little audio project.
  - atoav 2y ago
    You are right. Yet, if you asked me how to get less noise on your audio circuit the LDO is the easier answer that will cost you less time to implement and likely give you the superior result. Especially for beginners without a ton of measuring equipment and experience having potentially bursty high frequency components in series can be an interesting way to not get the thing they were planning done, but instead have to deal with an entire new set of problems whose existence they didn't even know about. Technically you are correct, but "just slap a LDO on it" is probably the better advice.
    - Animats 2y ago
      Agreed. I had to learn a lot to build one. I had an application so unusual (driving antique teletype machines from a laptop) that I had to do a unique design. For most low-volume applications, it's not worth the trouble.
roaringraster 2y ago
And 3.3/5 is approximately 66% efficiency, which isn't too horrible. So even if you get your buck converter working, getting those 95%+ efficiency numbers you see in datasheets out of the circuit is not trivial.
_fizz_buzz_ 2y ago
If you have 5V and have to step down to 3.3V using an LDO is a very reasonable choice (at 100mA you have about 170mW losses). However if you have e.g. 24V and need to step down to 3.3V, an LDO can get annoyingly hot (at 100mA you now have over 2W losses). But I agree, this is really a "it depends" situation.
- klysm 2y ago
  Most LDOs straight up won’t tolerate those conditions without very careful thermal management
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michaelt 2y ago
> On the other hand, buck converters require you to actually do some actual engineering. You can't just haphazardly throw in a single IC and expect it to work flawlessly on your first try. It used to be a hassle a few years ago - but these days you can haphazardly throw in a R-78K3.3-0.5 - which has the pinout of a classic three-pin 3.3v linear regulator, but it's actually an 80% efficient DC-DC converter with 500mA output and an input range that goes up to 36v. That's enough current even if you've got something like an ESP32 that needs 250mA - and for any type of hobby project, the $2.40 is fine.

progbits 2y ago

I can highly recommend the MIT 6.622 Power Electronics course recently released on OCW: https://youtube.com/playlist?list=PLUl4u3cNGP62UTc77mJoubhDELSC8lfR0 https://youtube.com/playlist?list=PLUl4u3cNGP62UTc77mJoubhDE... https://ocw.mit.edu/courses/6-622-power-electronics-spring-2023/ https://ocw.mit.edu/courses/6-622-power-electronics-spring-2... Prof. David Perreault is excellent. While the course gets into pretty advanced topics that simply won't matter unless you are designing multi-kW systems, it covers all the fundamentals and builds understanding from ground up so you will know what makes sense to use and when.

kleiba 2y ago
Thank you! I was just going to ask about recommended resources for getting into electronics. I've never been able to find anything that I personally found useful - often times, introductury courses are too basic and slow to keep me focused, or they lack exercises or are too theoretical, etc. There are many hobbyists who have learned all that stuff and can design and implement their own circuits (say, audiophiles or model train enthusiasts), so obviously they have all been able to get there. But I have never managed to learn anything about electronics, although I would really like to.
- progbits 2y ago
  Yeah it can be hard. As a self-taught hobbyist I've found a mix of university courses (not whole curriculum, just pick and choose and don't feel bad fast-forwarding over some of the math theory), books (art of electronics, practical electronics for inventors), and high quality youtube channels (eevblog, phil's lab, robert feranec, microtype engineering) to be a good way to learn. Also eevblog forums are great. I don't post much but just reading through the discussions you get a lot. My greatest annoyance is the flood of very low quality Arduino tutorials everywhere that polute the search results. Not to be ungrateful, Arduino got me into the hobby, but if you just learned about resistors last week the world doesn't need your blogpost on how to connect it to a breadboard.
  - Max-q 2y ago

hcfman 2y ago

I've been working with audio recently and found so many of the devices that convert 3.7V to 5V for example inject noise into the rail that make's it in the microphone input source. The battery support from pisource does this terribly. But so do many battery sources. It's not just microphones that get affected, but also other sensitive sensors like accelerometers. I hope that other people making DC-DC convertors put some effort into making sure the supply is so clean so as to prevent this in future.

euroderf 2y ago
Isn't this like 95% fixable with a capacitor ? Aren't there cables with small embedded caps ?
- dragontamer 2y ago
  Oh hell no. I think a lot of people are overly cautious of DC-DC conversion in this topic, but you've gone full-tilt in the opposite direction and are severely underestimating the problems that occur. 1. Its not "power-conversion" that's hard per se, its EMC that's very hard and not taught very well at a bachelor's level. 2. DC-DC Voltage Converters usually handle the entirety of your board's power, meaning they are the highest power component. 3. High power and high-frequency is a difficult EMC problem. This means that a bad design will absolutely send your electrons / energy out and radiate out like an antenna. And if things on the same board pick it up, it will be called crosstalk. And if things off-board pick it up, its called electromagnetic interference which almost certainly leads to a compliance problem. --------------- 1. Hobbyists don't care about compliance. So bam. We are already dealing with the biggest problem by simply not caring about it. (Maybe you can care and go into deeper studies, but... if you're a beginner just don't care. Learn this very difficult stuff later). 2. Prevent crosstalk by following good board design rules: have a 4-layer board. Use Power+Signal / GND / GND / Power+Signal stackup. Use two vias (one for signal-1 to signal-4 traversals), and a 2nd via for GND2 to GND3 traversal of the return current). Thinking of both the forward current and a tightly bound reverse current is basically all you need to do to avoid difficult crosstalk problems on board. Done. Point#2 requires deeper studies than is typical in bachelor's level electrical engineering. But it truly isn't very difficult once you learn the theory. Tight ground-planes reduce crosstalk (and EMI problems), and furthermore thinking of the return-current explicitly prevents problems. Now you could have some truly difficult "ringing" from trace inductance and other such nasty problems... but that tends to occur beyond 100MHz. I'm thinking most beginners are going to be under 20MHz for most of their designs and thus never deal with those advanced "PDN" / Power Delivery Network problems. Though if you do go into PDNs, its obviously a tough subject with huge amounts of study and reading involved. But most of the problems truly are at very high frequencies and/or at EMI compliance. Beginner Hobbyists avoid the most difficult issues entirely by nature of beginner (aka: low-speed) and hobbyist (and therefore don't have to follow regulators). ---------- I'm not a professional. But my understanding is that top-level EEs who work on PDNs will simulate the circuit-board itself to figure out trace inductances / capacitances in the board itself. (Closer planes of ground/power will create more capacitance. Long traces tend to increase trace inductance, etc. etc.). And tight simulations are the only way to truly understand the PCB and how it interacts at high frequencies with high-power. But such methodologies are gross overkill for a 1MHz boost converter with a pre-made PCB Layout, and a list of capacitors + inductors already picked out for you. (ex: https://www.microchip.com/en-us/product/mcp1640 https://www.microchip.com/en-us/product/mcp1640) Seriously: Page 17 (https://ww1.microchip.com/downloads/aemDocuments/documents/APID/ProductDocuments/DataSheets/MCP1640-Family-Data-Sheet-DS20002234E.pdf https://ww1.microchip.com/downloads/aemDocuments/documents/A...) already gives you the PCB-layout you need for this, with recommended components. Don't overthink it, just copy the design from the document.

exar0815 2y ago

I do work in automotive EMC testing and it's nearly always the voltage conversion at fault when you fail tests or influence other devices. Buck-Boost converters are a noisy and finicky thing, and not easy to debug if you use a monolithic IC from the cheapest vendor. Quite annoying discussions.

Animats 2y ago

DC-DC converters are hard, but fun. The basic concept is that when you put current through an inductor for a while, then disconnect it, you get a big voltage spike. That's a classic auto ignition system. You can put that spike through a diode and use it to charge a capacitor to get DC out. The neat thing about switching power supplies is that there's very little resistance in the power path. That's why the efficiencies are so good. The not-neat thing is that they are a dead short across the input for part of the cycle, which is why failures can cause fires and why you may need an inrush current limiter and/or a fuse. There are boost converters, buck converters, and ones with transformers. With a transformer you can isolate the input from the output, which is mandatory for safety if you're driving the thing from the AC power line. Here's one of mine. USB 5VDC in, 120 VDC out, to operate antique teletype machines that need 60mA 120VDC.[1] The basic circuit is simple, but there are multiple surface mount ferrite beads and small capacitors to keep the spikes from coming out via the input USB, output, or as RF. LTspice simulation was needed to pick the values for those, so as to minimize noise in both voltage and current. [1] https://github.com/John-Nagle/ttyloopdriver/blob/master/board/images/schematic.png https://github.com/John-Nagle/ttyloopdriver/blob/master/boar...

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rkagerer 2y ago
Can't you also charge up capacitors then slam them together in series? Is there a name for that kind of supply?
- caf 2y ago
  Yes, it's called a charge pump. There's one specific sub-type called a Cockroft-Walton voltage amplifier.
  - pfdietz 2y ago
    They won the Nobel prize using this invention (which wasn't theirs). https://circuitcellar.com/resources/quickbits/cockcroft-walton-voltage-multiplier-2/ https://circuitcellar.com/resources/quickbits/cockcroft-walt... Cockcroft went on to great acclaim for "Cockcroft's Folly". https://www.bbc.com/news/uk-england-cumbria-29803990 https://www.bbc.com/news/uk-england-cumbria-29803990

3dGrabber 2y ago

There exists an interesting connection between Boost Converters and Hydraulic Rams [1]. A Hydraulic Ram is device that can pump water from a stream to a higher location by harnessing the kinetic energy of the stream, no other power source required. The equations for the two devices are essentially the same, only the units change. 1 https://en.wikipedia.org/wiki/Hydraulic_ram https://en.wikipedia.org/wiki/Hydraulic_ram

agumonkey 2y ago
I love analogies between fields like this.
- deleted 2y ago
  [deleted]
- nraynaud 2y ago
  There is a whole area of multi-domain simulation, where the simulator seamlessly jumps from one form of energy to another as long as the units match. I have always loved that.
  - agumonkey 2y ago
    oh nice
  - 3dGrabber 2y ago
    Modelica comes to mind. https://media.springernature.com/lw685/springer-static/image/prt%3A978-3-030-44184-5%2F12/MediaObjects/978-3-030-44184-5_12_Part_Fig1-140_HTML.png https://media.springernature.com/lw685/springer-static/image... https://en.wikipedia.org/wiki/Modelica https://en.wikipedia.org/wiki/Modelica
- SoftTalker 2y ago
  Water flows in pipes, valves, etc. concepts transfer to a lot of basic electrical circuits and concepts. E.g. voltage is analogous to pressure. Current is analogous to the volume of water flowing. Bigger pipe (wire) can carry more current. Valves are like switches or resistors. It works to de-mystify concepts for kids who have no concept of what electricity is but can think about water flowing in a pipe.

mikewarot 2y ago

I've learned that the magic search word for 150ish volt boost converters is "Nixie". My friend needed that voltage for a Geiger counter B+ battery replacement.

moffkalast 2y ago

96% efficiency sounds great on paper for synchronous converters, but as SBC current draw just keeps increasing and BLDC motors can run at higher voltages it starts to create a major heating problem when you have to supply both from the same source. Something like 12V down to 5V at 5A creates a managable amount of heat, but going higher, 20V, 30V on the battery side and things start to melt all around from heat losses from that large a drop. In some cases I've had to resort to using cascaded rails, stepping first down to 24, then 24 to 12 and then 12 to 5 just to keep the heating spread between different buck converters even if it multiplies losses. Would love to hear what the expert solution is to this that isn't just a massive heatsink.

mschuster91 2y ago
> Would love to hear what the expert solution is to this that isn't just a massive heatsink. A smaller heatsink with active cooling and parallel MOSFETs. At a certain power level, it's just physically impossible to rely on convection cooling alone - just look at audio amps or your average CPU/GPU... banks of MOSFETs, caps and inductors it is. While the BOM part count may be higher, you need lower-capability parts. The danger is, you need to carefully grade and match the MOSFETs, otherwise you risk them failing sequentially in a very short time if you're operating too close to their rated current - one burns out, the load distributes to the others, and then they fail because they cannot handle the additional load (or one fails into dead short instead of open, which instantly kills all of the others).
posterboy 2y ago
you forgot to mention it should fit under a thumbnail, probably
- moffkalast 2y ago
  It would be a nice plus :P Honestly the size isn't such a big deal, as long as it doesn't weigh as much as two African elephants like the average mains PSU of this amperage.
michaelt 2y ago
You can get 48V DC -> 5V DC 6.5A converters that are 92% efficient [1] You're dissipating 25W from your SBC already. You can dissipate the 2W from your DC-DC converter the same way. [1] https://www.meanwellusa.com/webapp/product/search.aspx?prod=NID65#1 https://www.meanwellusa.com/webapp/product/search.aspx?prod=...

ziofill 2y ago

I'm a theoretical physicist and I swear electrical stuff is so hard to understand! I have a lot of respect for electrical engineers ^^' (and electricians)

mglz 2y ago

For beginners it is super annoying that many tutorials say "there is a magical switch or oscillator here which is integral to the function of the boost converter, but we will not tell you how to actually realize it". Additionally, that needs to work at the voltage level you are starting out from and in many cases should be galvanically isolated from the converter. This is a lot to keep in mind and it is actually not trivial. The answer here is usually to find an IC that works at your desired input voltage or to have a linear regulator provide a small amount of power for the PWM generator. Also be wary of just running with an AI generated answer. Claude 3.5 Sonnet suggest you connect an Arduino straight to 230V and after some back and forth generates circuits which contain strange elements like "antiparallel diodes" which makes no sense.

posterboy 2y ago
sounds like a spherical cow on a frictionless plane.
- mglz 2y ago
  It is a very hairy cow, which likes to bite and is stuck in the mud. Also it has a wierd high-frequency response. There is a description of tractors to get it out, but we'll skip how the controls work for now.
awjlogan 2y ago
The TI Power Designer[0] is a great resource. Obviously it will only show you TI parts, but it's very helpful to get a base design. You can filter by complexity (roughly BoM count), size, cost etc based on the parameters (input voltage range, output voltage range, power etc). The designs usually have a reference layout as well. 0: https://webench.ti.com/power-designer/ https://webench.ti.com/power-designer/
- mglz 2y ago
  Very convenient, thank you!

saltserv 2y ago

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londons_explore 2y ago

I have often wondered if ideas from a buck/boost converter could be applied to a mechanical gearbox. Voltage and current in electrical circuits (where voltage x current = power) is completely analogous to torque and speed in mechanical shafts (where torque * speed = power). Every electrical component has a physical counterpart. Spring = capacitor. Inductor = mass with momentum. Resistor = friction brake. The goal would be a variable ratio gearbox using a fully mechanical system, using a spring and a hammer type mechanism to convert one torque/speed to another torque/speed. This is already done in impact wrenches, but I would hope that rather than having an impact rate of say 5 Hz, you have an impact rate of 50 kHz or more, allowing a smooth conversion from one speed to another. Obviously, the difficulty is in the details - designing parts to withstand 50k hammers per second for years of operating without failing from fatigue. Various other mechanical things already operate at high mechanical frequencies. SAW filters vibrate things mechanically at Ghz and don't suffer fatigue failures.

hwillis 2y ago
You're overcomplicating it; you only need a single clutch and in/out springs[1] to do this. If you're spinning at 4000 rpm and your springs cover 6 degrees of rotation, then your clutch needs to be able to actuate at 4000 Hz. When the clutch is engaged, the engine-side springs compress to supply the torque and match the speed difference. When it's disengaged, the springs expand back out as it returns to engine speed. The obvious problem is that clutches do not smoothly click on and off like a transistor. However there are more specialized devices that use stick-slip dynamics like piezo actuators. Since there is a much more rapid transition between "on"/"off", they can be very efficient and allow relatively weak devices to exert very large forces. They're just only able to take very small steps. [1] Labeled 4 here: https://haynes.com/en-gb/sites/default/files/styles/blog_landing_header_687/public/info_anatomy_clutch.HEAD_.png?itok=ktUUuTqa https://haynes.com/en-gb/sites/default/files/styles/blog_lan...
- londons_explore 2y ago
  > When it's disengaged, the springs expand back out as it returns to engine speed. What is it? I think you need an intermediate flywheel, with springs and clutches on each side. The intermediate flywheel's mass is tiny, so might be formed by just the masses of the springs and clutch mechanism.

kbouck 2y ago

I want to power my 12V devices with USB PD. Looks like 12V is optional in the spec and is supported only by some devices (eg. UGREEN), and not by others (eg. Anker) Given a USB PD power supply which supports 15V but not 12V, and a usb-c/barrel-jack cable configured to negotiate for 15V, what would be the simplest (yet safe) circuit i could add via barrel jack to regulate the to voltage down to safe/consistent 12V? is a simple linear voltage regulator (LM7812) sufficient? would i need capacitors to smooth it out?

klysm 2y ago
A 3V drop over an LDO is usually reasonable with low enough currents. Some LDOs require capacitors to be stable, and it’s usually a good idea to have some capacitance on your power rails anyway.
15155 2y ago
The one important thing missing from your query here is: How much current do you need? If you need, say, >100A, the possible architecture looks very different than ~1A or less.
_Microft 2y ago
It might be cheaper to get a power supply that supports later PD standards? E.g. IKEA is selling some cheap here for either 8€ (Sjöss, 1 USB-C port (max. 30W, up to 3A)) or 15€ (Sjöss, 2 USB-C port (combined power output of 45W, up to 3A, also on a single port)). Both support PD 3.0 and PPS (that's the fanciest PD standard that implements requesting arbitrary voltages from the power supply) They also stock nice and cheap USB-C cables. These power supplies work fine with USB-PD trigger boards set to 12V.
a96 2y ago
Linear loss obeys Kirchoff's and Ohm's and power laws pretty simply. If you need to lose a certain voltage at a given (max) current, the power lost is P = U * I. For example, 3 V * 0.1 A = 0.3 W. If your LM7812 can run at that overhead (yes), take the voltages and currents (yes) and dissipate the 0.3 W as heat (almost certainly yes) then it'll work. Caps will depend on how fast the response needs to be. If your load jumps too much too fast and there's no reservoir, you'll get a voltage drop. If it feeds too much back, you'll get a voltage climb. Caps may help and are pretty cheap and IIRC 78xx chips are very stable, but the datasheet will have limits and recommendations. All the information will be in the data sheet and making some test circuits is easy for these, especially if you have some measuring equipment. Anything below 1A will probably be easy, anything above might be risky, as a rule of thumb. Very small or very large voltage differences will also be tricky or impossible.

cushychicken 2y ago

A decent article, but there’s a ton of misunderstanding in the comment section. For one thing: LDOs can be more efficient than buck converters, especially at very low current consumptions. If you’re drawing sub 1 mA, like a battery powered system, an LDO is going to be a more efficient step down converter, because it doesn’t have switching losses. Bucks are only better choices for stepping down voltage at higher currents because the switching losses become negligible. Second: a ton of people here are vastly exaggerating the difficulty of designing a step down buck converter. Integrated designs from TI or analog devices will tell you all the compensating components, output capacitor values, inductor values, etc. for common step down output voltages. Most will include reference layouts with a four layer six layer or even two layer stack up for optimal performance. It’s really not that hard to get a one spin win out of most common buck designs. Don’t be afraid. Just follow the manual. You’ll be fine.

klysm 2y ago
The challenges I’ve had with buck or boost conversion is on mixed signal boards where I have very sensitive analog circuits. The ripple and switching noise on the output can make you lose bits on ADCs, or show up on a DAC quite easily. It’s easy to get a buck converter doing the right thing without horrible EMC/EMI if you’re careful and follow the manual, but it’s a lot harder to optimize for something like low noise without utilizing LDOs
- cushychicken 2y ago
  True, but also not the whole story. For one: most LDOs don’t have a high enough PSRR bandwidth to completely eliminate buck switching noise. Most LDO PSRRs roll off sharply in the low hundreds or high tens of kHz. That’s generally below the switching frequency (and noise frequency) of most buck regulators. If you’re dealing with audio that’s generally fine. RF, however, is a separate problem. Second: there are more cost effective and wide bandwidth solutions for noise reduction. Capacitance multipliers are one that spring to mind. Ferrite beads are another great means for tamping down high frequency noise. Third: layout and current return paths are often just as much of a problem as the buck itself. Couple a high current return path into an audio chain with a shared return, and you’re gonna have a problem no matter which way you slice it.

minkles 2y ago

Lots of things in here which kill me a little: 1. You don't get a voltage spike when you disconnect an inductor. The field collapses and induces a current. If you measure it across a high impedance then it looks like a voltage spike. If you measure it across a low impedance then it's not necessarily much of a spike. Ergo depends on load impedance. 2. SMPS designs are not necessarily noisier than linear power supplies. It's always a design trade off. In fact you see SMPS in all modern RF test gear which is generally far more sensitive and has far more bandwidth than anything back when linear supplies were common. Also there is a lot of noise coming off the diodes in a basic bridge rectifier as well! Noise is a whole-system design consideration that has to be made. 3. Don't use any LLMs for designing circuits. Please go read a book on it designed by experts, not stuff scraped from thousands of idiots. I've seen some horrible stuff out there. 4. I'm sure I'll come up with more over time.

hwillis 2y ago
> If you measure it across a high impedance then it looks like a voltage spike. If you measure it across a low impedance then it's not necessarily much of a spike. "Disconnect" implies an open circuit and high impedance.
- klysm 2y ago
  No, it’s the input that’s disconnected
  - hwillis 2y ago
    Irrelevant. If it's not an open circuit, then the inductor is connected to things in parallel, and the impedance increase creates a voltage spike. If the load impedance is significantly lower than the thing being disconnected, then you're just disconnecting something that doesn't matter to the circuit and it's silly to be that pedantic about an irrelevant situation. You're bending the statement from "disconnecting an inductor" to "disconnecting something from an inductor (while something else is still connected)"
    - 2y ago

marcodiego 2y ago

A commonly used alternative in the microcontroller world is to simply stack a few diodes. Very simple alternative which I have seen being used a few times.

shellback3 2y ago

Interesting, but I had expected to see a comparison of generating DC voltages using tubes, which were used in my university Electronics course, with solid state. In those days to generate a DC voltage from another DC voltage required generating an AC voltage from the DC and then rectifying it.

stonethrowaway 2y ago

Friendly warning to people who aren’t electronics savvy: this blog post is written in a “now draw the owl” sort of way. I’m not sure who the audience is. Anyone who can read this stuff at the level presented inherently knows most of this and then some. Everyone else will need a book and that book will cover this material as it’s fairly fundamental and will derive equations used in here as well so you can make sense of it.

anthomtb 2y ago
Maybe the target audience is those with lapsed circuit theory knowledge from undergrad and no hobby or professional power electronics experience afterwards? Describing myself, of course (most of icamtuf's stuff is up my alley, fwiw). One would think the title including "DC-DC voltage conversion" is enough of a squirrel-catcher to stop folks who either 1) Know nothing about what it means or 2) know exactly how to do it, from reading the article.

dgacmu 2y ago

I've moved a lot of my home computing to home-brewed 12V UPSes using these. LFP charger --> Battery --> 12V or 5V DC-DC buck or boost/buck regulator --> device. Most UPSes are designed for high wattage, short runtime, but things like my firewall or small proxmox box for SDN+DNS benefit from low-wattage, long-runtime, and getting the inverter out of the picture substantially improves runtime. Said proxmox box uses under 10W and gets about 20h of runtime from a $50 battery.

kijiki 2y ago
What charger and battery did you use?
- dgacmu 2y ago
  For the moment some random cheap ones from Amazon - I haven't run it long enough to see if it will last more than a few months of full-time use. The battery was a freebie, so I don't actually recommend it; the charger is an "ULTRAPOWER 4a". We'll see. What it's augmenting is a pretty cheap AIMS power brand modified sine-wave inverter/charger (meh, should have gone pure sine, it causes some of the power supplies to make funny noises) hooked up to a Litime 2kWh LFP battery (very happy with the big battery). TODO on the experiment is swapping out the inverter part with direct 12V conversion for some of the things on the inverter, but I wanted to test the regulator approach with a non-critical component for a month or two first before hooking it to something that can deliver 100A, obviously with a fuse. :-)
thehappypm 2y ago
Does it outperform a $50 UPS?
- dgacmu 2y ago
  Kinda depends what you're asking. There are IP camera 12V UPSes you can get that are ok -- I have a few TalentCell 98Wh units -- but they don't actually deliver regulated power, they just tap directly off of the li-ion cells. Those are about $65. With an inverter UPS, $50 gets you a unit with a single 12V / 8ah or so lead-acid cell; 90 or so watt-hours _could_ be reasonable except that the inverter draws upwards of 10-15W while running, so these units actually won't run even a router for more than a couple of hours. Also, lead-acid dies after about 3 years, and then you're in for another $15 or and a bit of a pain replacing it. I'm really sick of doing surgery on old UPSes at this point and trying to move entirely to LFP for the 10 year lifespan alone. If you want really long runtime, you basically have to DIY, but it's fortunately quite straightforward. Larger LFP batteries are about $250 per kWh, sometimes less. DIY'ing it, you can pick your capacity/runtime independent of your wattage, vs pre-packaged UPSes that frequently couple the two. (I have a big pile of APC UPSes of various sorts, some with external expansion battery packs, and it's quite a hassle to get truly extended runtime from any of them -- I don't want to have a generator, I want 20 hours of extended runtime. I realize I'm a little weird, but I like to hope I'm just ahead of the curve a little on switching. :-)

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the__alchemist 2y ago

I suppose the downside is the complexity in circuit design. If the datasheet has a good reference schematic and instructions, or if you are good with electronics fundamentals, it's not a big deal. If not... rolling the dice if it will work. These circuits have been one of the biggest sources of me having to respin boards. Requires a lot of attention to detail to get the passives correct in terms of values, net connections etc, compared to an LDO. Then you may go down rabbit holes like "The one at the switching frequency I want went out of stock. Can I use the one that switches slower? Can I substitute this capacitor in instead of the nonstandard value the ref diagram recommended?" They should IMO be the default for most designs, but double-check everything!

coding123 2y ago

Many people use DC-DC to down-regulate a 48 volt battery system to 12-volt for their RV where they remove the original battery pack and replace it with Lithium. Not everyone does that, some use a 12-volt lithium system. Less 4/0 cables needed when adding inverters and other fun stuff to the RV.