9 ms·

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 sp

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
[deleted]
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
- hakonjdjohnsen 2y ago
  This is known as a charge pump, and is the third concept described in the linked article. The article only mention one flying capacitor, but you can use more than one and connect them in series to get a higher multiple of the input voltage. Ben Eater also did a nice introduction to charge pumps by building a simple one on a breadboard: https://www.youtube.com/watch?v=4alV5LzHLE4&t=704s https://www.youtube.com/watch?v=4alV5LzHLE4&t=704s
- Animats 2y ago
  The high voltage version of that is called a Marx generator. The Museum of Science and Industry in Chicago used to have a million volt Marx generator made by General Electric. The parallel to series switching was mechanical. The "crack" when it fired echoed through the whole building. They've downsized to a Tesla coil.
  - jrockway 2y ago
    I know exactly what you're talking about! I used to go there so often as a kid and definitely remember the crack. (Also nearby was that exhibit where you could perform electrolysis on some water. It had a "push to start" button that was activated by touching some glass with a button outlined by a red decal. I still don't know how that works.) Something else stuck in my head from that museum was the Operation Lifesaver displays around the model train exhibit. I think my dad was a little freaked out by how many times I wanted to watch the video of the train hitting a car. Honestly, I'm a little disappointed every time I visit a museum because of how much fun I had at MSI as a kid. I think a lot of that was from being a kid, but ... good museum.
amelius 2y ago
I see you made a current limiter from a mosfet + resistor. I wonder if there are ready-made components that do the same, and also monitor overheating. Maybe not necessary in this case (because you're only limiting the inrush current, not a continuous current). There are current-limiting diodes but as far as I've seen they are only available for smaller currents.
- michaelt 2y ago
  You can get single-chip current limiters for LED driver applications. A CL2N8-G for example. In some applications you can also use almost any linear voltage regulator - put a resistor between your linear regulator's ground and output pins, and you'll get a constant current. Of course if your application involves the amount of power dissipation that requires a heatsink, you'll probably end up with a discrete component for that anyway :)
  - deleted 2y ago
    [deleted]
  - amelius 2y ago
    Let's say I have a voltage source of 48V, and I want to limit current in my system to 4.5A, precisely, and with overheating protection. I could be wrong but I don't think the led-driver and voltage regulator solutions would fall in this range. Also, a heatsink would not be required if the duration in which the current needs limiting is small.
    - namibj 2y ago
      You can use a current limiting buck set to pass through input voltage up until the current limit.
- Animats 2y ago
  Oh, you mean Q2-R5 in [1]? That was a late addition, and yes, it's a linear current regulator using a depletion-mode MOSFET. I did that because I didn't want to change the board layout much and it only took two components. Others have built this device, and having good protection circuitry means it works for them, not just me. Q2-R5 is not the inrush current limiter. The inrush current limiter is U2. That's an AP2553W6.[2] That's a part designed to solve a specific problem - plugging into a USB port. USB port ICs have overcurrent detection which will quickly turn off power from the port if you try to pull too much power. This keeps external devices from pulling down the +5 rail in a laptop or pad with limited power available. The port turns off until reset. (On Windows this used to take a reboot; Linux usually resets if you close and open the device.) So if you plug in something with a large filter capacitor, the inrush current as the capacitor charges can momentarily cause an overcurrent condition and shut the port down. The AP2553W6 has both a linear regulator and a switch. When everything is good, the switch is closed and power flows through. If there's a momentary overload, it current limits. If there's a steady overload, it cuts power. Devices which don't do this power startup properly are often the cause of problems mentioned in searches for "USB port stops working". Not doing this properly saves about $0.25 in parts cost in volume. Such devices will work fine plugged into a USB charger or a desktop machine, but may shut down a USB port on a laptop or tablet. (This is for USB-A. USB-C is more complicated.) [1] https://github.com/John-Nagle/ttyloopdriver/blob/master/board/images/schematic.png https://github.com/John-Nagle/ttyloopdriver/blob/master/boar... [2] https://www.diodes.com/assets/Datasheets/products_inactive_data/AP255x.pdf https://www.diodes.com/assets/Datasheets/products_inactive_d...
petertodd 2y ago
> 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 actually usually not true, as the vast majority of DC to DC converters are step-down converters: you do not want the voltage to spike. And in general, it isn't really a "spike". A better way to think about what is happening is that passing a current from a power supply through an inductor transfers energy into the magnetic field. When you stop doing that, the magnetic field diminishes, transferring energy back into current. But this time, you direct the current into the circuit. The trick is that by picking the timing and other parameters correctly, you can pick the voltage of the downstream current. Specifically, you can do this because the voltage across the inductor is a function of the slope of the strength of the magnetic field around the wire in the inductor. Pick a different slope, and you can pick a different voltage. Since you usually want a stable voltage, the graph of the magnetic field strength will be (roughly) a sawtooth, and the graph of the induced voltage will be (roughly) a square wave (I am simplifying here for understandability!). A sawtooth shape has a consistent current slope, which leads to a consistent voltage.
- Terretta 2y ago
  DISCLAIMER: Described for entertainment value only. Some details omitted. Don't try this at home! That energy transfer makes an “interesting” party trick. Get the step up/down winding transformer from an old CRT TV. Get rid of other components*, and wire it with a 9 volt battery on one side, and connect the other with + to conducting surface on three sides of a box with - to the three opposing sides. Put a switch on the underside that opens the circuit. To pick up a box generally requires touching two opposite sides. Opening the circuit dumps the field into the person picking it up who gets a momentary jolt. It's enough to run through multiple people: hold hands in a ring of 2 - 10 people, and have two people at ends of the ring each press an opposite side of the box and pick it up, the whole ring gets the jolt! As a grade school science experiment, have the experiment display say something along the lines of "Guess the weight" so people pick up the box and get a surprise. For more about retro transformer circuits, see: https://hackaday.com/2016/07/04/retrotechtacular-dc-to-dc-conversion-by-vibrator/ https://hackaday.com/2016/07/04/retrotechtacular-dc-to-dc-co... This is sort of a single vibe (the switch opening) of a vibrator-transformer-rectifier transformer, to collapse the magnetic field that dumps into the still "closed" side through the person picking it up. No rectifier since it's not AC, it's just C. So the same principle, without the rest of the parts. * WARNING: Don't look up the rest of the owl. Don't build this. Don't try this. Don't let anyone touch this.
kosma 2y ago
That voltage spike only applies to flyback converter. Your typical buck/boost converter doesn't do that - the current waveform is a sawtooth, and voltage ripple is designed to be in the mV range.
mindslight 2y ago
DC-DC converters are not a "dead short across the input for part of the cycle" in normal operation - rather the voltage is across the inductor. If the switch stays on too long and the inductor reaches its saturation current, or one of the many other (cascading) failure modes, then can you end up with effectively a short across the input. This can happen to many kinds of electronics (eg a simple tantalum decoupling cap, or an IC's SCR latchup), but designing the power topology is a good place to think about these failure modes. (Although going 5V->120V with USB as the power source, I can understand how "dead short" was a decent intuition)
- Animats 2y ago
  > DC-DC converters are not a "dead short across the input for part of the cycle" in normal operation - rather the voltage is across the inductor. If the switch stays on too long and the inductor reaches its saturation current, or one of the many other (cascading) failure modes, then can you end up with effectively a short across the input. Right. Which is why under-designed AC-line powered power supplies can catch fire. The failure mode of MOSFETS is usually to the "on" state, so the switch staying on is quite possible.
  - mindslight 2y ago
    Sure, I don't see how FETs having a common failure mode of passively conducting is exceptional though? You've got to design (and test) for likely failure modes. Like even if you use a linear regular, you should be thinking about what happens when there is a short downstream. The problem with fires isn't the type of circuit per se, but rather that anything connected to the mains can unleash a significant amount of power. And that designing/building for safety increases cost, for additional components or even things like larger PCB area for creepage. We basically take a lot of the work that goes into electrical safety for granted.
retrac 2y ago
I think of it as synthesizing a sine wave (AC power) with DC pulses, using an inductor or capacitor for smoothing. The result is then rectified back to DC.
eternityforest 2y ago
I don't think a dead short for part of the cycle is the right way to think about it. The inductor has a bunch of impedence unless the switch fails. You might be doing a short through some parasitics for a bit, but in the ideal case, you're not. The new chips typically do soft start on their own, the problem is you need input caps, which really are a dead short when first connected. Which wouldn't normally be an super big issue with small ceramic capacitors, except the cable inductance forms an accidental boost converter that wants to give your actual converter a voltage spike.
Saris 2y ago
>That's a classic auto ignition system. I just fixed up a 1972 Honda motorcycle, and when I took the cover off the ignition point it was fascinating looking at that whole system, so simple and easy to fix.

deleted 2y ago

[deleted]

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
hakonjdjohnsen 2y ago
This is known as a charge pump, and is the third concept described in the linked article. The article only mention one flying capacitor, but you can use more than one and connect them in series to get a higher multiple of the input voltage. Ben Eater also did a nice introduction to charge pumps by building a simple one on a breadboard: https://www.youtube.com/watch?v=4alV5LzHLE4&t=704s https://www.youtube.com/watch?v=4alV5LzHLE4&t=704s
Animats 2y ago
The high voltage version of that is called a Marx generator. The Museum of Science and Industry in Chicago used to have a million volt Marx generator made by General Electric. The parallel to series switching was mechanical. The "crack" when it fired echoed through the whole building. They've downsized to a Tesla coil.
- jrockway 2y ago
  I know exactly what you're talking about! I used to go there so often as a kid and definitely remember the crack. (Also nearby was that exhibit where you could perform electrolysis on some water. It had a "push to start" button that was activated by touching some glass with a button outlined by a red decal. I still don't know how that works.) Something else stuck in my head from that museum was the Operation Lifesaver displays around the model train exhibit. I think my dad was a little freaked out by how many times I wanted to watch the video of the train hitting a car. Honestly, I'm a little disappointed every time I visit a museum because of how much fun I had at MSI as a kid. I think a lot of that was from being a kid, but ... good museum.

amelius 2y ago

I see you made a current limiter from a mosfet + resistor. I wonder if there are ready-made components that do the same, and also monitor overheating. Maybe not necessary in this case (because you're only limiting the inrush current, not a continuous current). There are current-limiting diodes but as far as I've seen they are only available for smaller currents.

michaelt 2y ago
You can get single-chip current limiters for LED driver applications. A CL2N8-G for example. In some applications you can also use almost any linear voltage regulator - put a resistor between your linear regulator's ground and output pins, and you'll get a constant current. Of course if your application involves the amount of power dissipation that requires a heatsink, you'll probably end up with a discrete component for that anyway :)
- deleted 2y ago
  [deleted]
- amelius 2y ago
  Let's say I have a voltage source of 48V, and I want to limit current in my system to 4.5A, precisely, and with overheating protection. I could be wrong but I don't think the led-driver and voltage regulator solutions would fall in this range. Also, a heatsink would not be required if the duration in which the current needs limiting is small.
  - namibj 2y ago
    You can use a current limiting buck set to pass through input voltage up until the current limit.
Animats 2y ago
Oh, you mean Q2-R5 in [1]? That was a late addition, and yes, it's a linear current regulator using a depletion-mode MOSFET. I did that because I didn't want to change the board layout much and it only took two components. Others have built this device, and having good protection circuitry means it works for them, not just me. Q2-R5 is not the inrush current limiter. The inrush current limiter is U2. That's an AP2553W6.[2] That's a part designed to solve a specific problem - plugging into a USB port. USB port ICs have overcurrent detection which will quickly turn off power from the port if you try to pull too much power. This keeps external devices from pulling down the +5 rail in a laptop or pad with limited power available. The port turns off until reset. (On Windows this used to take a reboot; Linux usually resets if you close and open the device.) So if you plug in something with a large filter capacitor, the inrush current as the capacitor charges can momentarily cause an overcurrent condition and shut the port down. The AP2553W6 has both a linear regulator and a switch. When everything is good, the switch is closed and power flows through. If there's a momentary overload, it current limits. If there's a steady overload, it cuts power. Devices which don't do this power startup properly are often the cause of problems mentioned in searches for "USB port stops working". Not doing this properly saves about $0.25 in parts cost in volume. Such devices will work fine plugged into a USB charger or a desktop machine, but may shut down a USB port on a laptop or tablet. (This is for USB-A. USB-C is more complicated.) [1] https://github.com/John-Nagle/ttyloopdriver/blob/master/board/images/schematic.png https://github.com/John-Nagle/ttyloopdriver/blob/master/boar... [2] https://www.diodes.com/assets/Datasheets/products_inactive_data/AP255x.pdf https://www.diodes.com/assets/Datasheets/products_inactive_d...

petertodd 2y ago

> 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 actually usually not true, as the vast majority of DC to DC converters are step-down converters: you do not want the voltage to spike. And in general, it isn't really a "spike". A better way to think about what is happening is that passing a current from a power supply through an inductor transfers energy into the magnetic field. When you stop doing that, the magnetic field diminishes, transferring energy back into current. But this time, you direct the current into the circuit. The trick is that by picking the timing and other parameters correctly, you can pick the voltage of the downstream current. Specifically, you can do this because the voltage across the inductor is a function of the slope of the strength of the magnetic field around the wire in the inductor. Pick a different slope, and you can pick a different voltage. Since you usually want a stable voltage, the graph of the magnetic field strength will be (roughly) a sawtooth, and the graph of the induced voltage will be (roughly) a square wave (I am simplifying here for understandability!). A sawtooth shape has a consistent current slope, which leads to a consistent voltage.

Terretta 2y ago
DISCLAIMER: Described for entertainment value only. Some details omitted. Don't try this at home! That energy transfer makes an “interesting” party trick. Get the step up/down winding transformer from an old CRT TV. Get rid of other components*, and wire it with a 9 volt battery on one side, and connect the other with + to conducting surface on three sides of a box with - to the three opposing sides. Put a switch on the underside that opens the circuit. To pick up a box generally requires touching two opposite sides. Opening the circuit dumps the field into the person picking it up who gets a momentary jolt. It's enough to run through multiple people: hold hands in a ring of 2 - 10 people, and have two people at ends of the ring each press an opposite side of the box and pick it up, the whole ring gets the jolt! As a grade school science experiment, have the experiment display say something along the lines of "Guess the weight" so people pick up the box and get a surprise. For more about retro transformer circuits, see: https://hackaday.com/2016/07/04/retrotechtacular-dc-to-dc-conversion-by-vibrator/ https://hackaday.com/2016/07/04/retrotechtacular-dc-to-dc-co... This is sort of a single vibe (the switch opening) of a vibrator-transformer-rectifier transformer, to collapse the magnetic field that dumps into the still "closed" side through the person picking it up. No rectifier since it's not AC, it's just C. So the same principle, without the rest of the parts. * WARNING: Don't look up the rest of the owl. Don't build this. Don't try this. Don't let anyone touch this.

kosma 2y ago

That voltage spike only applies to flyback converter. Your typical buck/boost converter doesn't do that - the current waveform is a sawtooth, and voltage ripple is designed to be in the mV range.

mindslight 2y ago

DC-DC converters are not a "dead short across the input for part of the cycle" in normal operation - rather the voltage is across the inductor. If the switch stays on too long and the inductor reaches its saturation current, or one of the many other (cascading) failure modes, then can you end up with effectively a short across the input. This can happen to many kinds of electronics (eg a simple tantalum decoupling cap, or an IC's SCR latchup), but designing the power topology is a good place to think about these failure modes. (Although going 5V->120V with USB as the power source, I can understand how "dead short" was a decent intuition)

Animats 2y ago
> DC-DC converters are not a "dead short across the input for part of the cycle" in normal operation - rather the voltage is across the inductor. If the switch stays on too long and the inductor reaches its saturation current, or one of the many other (cascading) failure modes, then can you end up with effectively a short across the input. Right. Which is why under-designed AC-line powered power supplies can catch fire. The failure mode of MOSFETS is usually to the "on" state, so the switch staying on is quite possible.
- mindslight 2y ago
  Sure, I don't see how FETs having a common failure mode of passively conducting is exceptional though? You've got to design (and test) for likely failure modes. Like even if you use a linear regular, you should be thinking about what happens when there is a short downstream. The problem with fires isn't the type of circuit per se, but rather that anything connected to the mains can unleash a significant amount of power. And that designing/building for safety increases cost, for additional components or even things like larger PCB area for creepage. We basically take a lot of the work that goes into electrical safety for granted.

retrac 2y ago

I think of it as synthesizing a sine wave (AC power) with DC pulses, using an inductor or capacitor for smoothing. The result is then rectified back to DC.

eternityforest 2y ago

I don't think a dead short for part of the cycle is the right way to think about it. The inductor has a bunch of impedence unless the switch fails. You might be doing a short through some parasitics for a bit, but in the ideal case, you're not. The new chips typically do soft start on their own, the problem is you need input caps, which really are a dead short when first connected. Which wouldn't normally be an super big issue with small ceramic capacitors, except the cable inductance forms an accidental boost converter that wants to give your actual converter a voltage spike.

Saris 2y ago

>That's a classic auto ignition system. I just fixed up a 1972 Honda motorcycle, and when I took the cover off the ignition point it was fascinating looking at that whole system, so simple and easy to fix.