4 ms·

A relevant part that changed my view of charge pumps is the LTC7820 [0]. This is an inductorless charge pump that can be used as a an unregulated voltage double

by addaon 11mo ago

A relevant part that changed my view of charge pumps is the LTC7820 [0]. This is an inductorless charge pump that can be used as a an unregulated voltage doubler or halver... at 500+ W and 98%+ efficiency. I used to think of charge pumps as designed for generating bias voltages where the actual power is quite small... but this shows that they scale quite well. (There's also the LTC7821 that combines the unreglated inductorless halver of the '7820 with a regulated, nominally-2:1 buck to give a regulated 48V -> 12V converter with some impressive efficiency numbers.) [0] https://www.analog.com/media/en/technical-documentation/data-sheets/7820fc.pdf https://www.analog.com/media/en/technical-documentation/data...

bsder 11mo ago
But, why use those parts? These circuits take a lot of parts to do a job that you can do with modern high frequency stuff with a lot lower cost and parts count. The normal point of a capacitive doubler is either to give you a voltage you need without a lot of extra parts count (often negative) or to generate a very high voltage.
- addaon 11mo ago
  Inductors are large, expensive, hot, and unreliable.
  - bsder 11mo ago
    Not on modern high-frequency switchers. GaN stuff can be 99%+ efficiency. The frequencies are multiple MHz which shrinks the inductors significantly--sometimes allowing PCB based coils (See Anker 120W teardowns).
    - throwaway31131 11mo ago
      I’m with you, I’m not sure the volume or cost would be less once you factor in capacitors that are high enough quality for the application. The datasheet mentions low profile a lot. That does make sense as one can make a flat, high quality, capacitor. Making a flat high quality inductor is harder and probably more expensive and likely consumes more volume overall. I can imagine some applications where being flat is important, like the back of a panel.
    - addaon 11mo ago
      I'm a huge fan of GaN (see the current front page discussion of LiDAR), but 99% at 500 W at 12 V is hard. 3 milliohm in the inductor will eat up 100% of your budget, without accounting for any switching losses. Like most Linear parts, the LTC7820 is impressive, fits a niche, and is too expensive for most applications -- but when you need it, it's good to know it's there.
    - namibj 11mo ago
      Yeah 120W is table stakes. Also Anker wall adapters are insulated that's worse for efficiency. Integer ratios can be done with GaN and class 1 ceramics and stray inductance to charge the GaN drain, so they only experience ZVS (they don't need ZCS). Use a resonant gate drive. Efficiencies around 99.8% are feasible for a doubler/halver based on the "3L FC buck" topology, in the 10~30 MHz range.
- labcomputer 11mo ago
  The app notes and data sheets of related parts suggest that the target application is large conversion ratios, where the duty cycle in an inductive converter is close to 0 (or 1). That forces tradeoffs, like a lower switching frequency (lower efficiency), a larger inductor (more weight and/or cost) or very short T_on for one of the FETs (lower efficiency because transition times become important). So you can use the charge pump as the first stage of a hybrid converter to get a higher system efficiency.
  - exmadscientist 11mo ago
    My recommendation to anyone who finds themself stuck in this corner of the design space is to consider a tapped inductor converter. I did a tapped inductor boost last year to take 3V input to 80V output (at not-much output current, I forget exactly what it was but it was mostly a bias voltage; also, the actual output voltage was DAC-set and could be quite low, so the loop dynamics were unpleasant). It was definitely annoying to wrap my head around, and very annoying to select the inductor (Würth has a nice OTS series, at the usual Würth prices; HVM would likely want a custom or semicustom design) but it just plain worked the first try and continued working through the usual stress tests and also the unusual stress tests of the Very Expensive Load™ getting itself Very Expensively Killed™ (for non-power-supply reasons). I was really happy with that converter, that kind of step-up ratio isn't easy and it just worked.
    - bsder 11mo ago
      Links or refs? That sounds very interesting. A basic search coughs up a bunch of papers from academic paper mills, and I don't see obvious links to an OTS series from Wurth Elektronik.
      - exmadscientist 11mo ago
        My refs were written into the schematic so I dug it up and might as well post the whole thing (SEE WARNINGS BELOW): https://i.ibb.co/zVXZWxHg/tapped-boost.png https://i.ibb.co/zVXZWxHg/tapped-boost.png (forgive me, I have no idea on the state of image hosting in 2025 so I hope that link holds up) I misremembered what design I actually ended up building. It was nominal 5V to 75V but was tested at a wider range (including the mentioned 3V to 80V). The variable voltage was implemented later, in a linear stage, which did in fact have a number of issues. ADI app note AN-1126 was very helpful in this design. It pushes for a different topology, yet compares with this one and others in some detail. Its arguments against tapped inductor designs are threefold: (1) sometimes EMI issues on the switching node (fair, and a real concern, but not interesting for what I needed to do), (2) the inductor is annoying to source (very true), and (3) the main technical objection is that demands on the output rectifier are high and might require you to use something crappier than a Schottky. That last one is true in general but for this design in this decade, I was comfortably within medium voltage Schottky territory, and so their main objection was a complete nonissue. That looked good to me so I went ahead and built it, and was not disappointed. The inductor was 744889030330 (say that three times fast) from the WE-MTCI family which worked a dream. I don't remember why this exact switcher chip was chosen but I do remember having a lot of candidates and so the choice was kind of arbitrary. The control scheme type it uses is important, though. I don't think the zener did anything but being paranoid I wanted to have the footprint there for the prototype build rather than not have it. THE PREVIOUSLY PROMISED WARNINGS WHICH ARE ACTUALLY REALLY IMPORTANT SO I AM USING CAPS: 1. 80V can kill you. Really, it can. Use caution! 2. This is the design I sent off for fab (and perhaps not even the final version, if the folder notes are any guide). It is not the redlined, working version. I believe this stage was OK. Maybe it needed a bit more output capacitance? Certainly I know the following stage, not shown here, had severe issues (most of them stupid and obvious once noticed). So treat this as good inspiration, not ground truth. 3. Did I mention that 80V can kill you? USE CAUTION.
- hulitu 11mo ago
  > you can do with modern high frequency stuff EMC
- namibj 11mo ago
  ZVS (not ZCS) topologies that use planned stray inductance and WBG transistors and class 1 ceramics can operate at 10s of MHz with less than 1% of loss per voltage halving. They have insane power density and lack of wound inductors means there's nothing that causes problems as you push into the kilovolts. But really they're just more efficient (integer ratios, where `n-1` not prime) than classical Buck/boost topologies.
em3rgent0rdr 11mo ago
The '7660 is good for low-power and is my go-to DIP-8 part when I need a half or double voltage supply on a breadboard.

by addaon 11mo ago

bsder 11mo ago

But, why use those parts? These circuits take a lot of parts to do a job that you can do with modern high frequency stuff with a lot lower cost and parts count. The normal point of a capacitive doubler is either to give you a voltage you need without a lot of extra parts count (often negative) or to generate a very high voltage.

addaon 11mo ago
Inductors are large, expensive, hot, and unreliable.
- bsder 11mo ago
  Not on modern high-frequency switchers. GaN stuff can be 99%+ efficiency. The frequencies are multiple MHz which shrinks the inductors significantly--sometimes allowing PCB based coils (See Anker 120W teardowns).
  - throwaway31131 11mo ago
    I’m with you, I’m not sure the volume or cost would be less once you factor in capacitors that are high enough quality for the application. The datasheet mentions low profile a lot. That does make sense as one can make a flat, high quality, capacitor. Making a flat high quality inductor is harder and probably more expensive and likely consumes more volume overall. I can imagine some applications where being flat is important, like the back of a panel.
  - addaon 11mo ago
    I'm a huge fan of GaN (see the current front page discussion of LiDAR), but 99% at 500 W at 12 V is hard. 3 milliohm in the inductor will eat up 100% of your budget, without accounting for any switching losses. Like most Linear parts, the LTC7820 is impressive, fits a niche, and is too expensive for most applications -- but when you need it, it's good to know it's there.
  - namibj 11mo ago
    Yeah 120W is table stakes. Also Anker wall adapters are insulated that's worse for efficiency. Integer ratios can be done with GaN and class 1 ceramics and stray inductance to charge the GaN drain, so they only experience ZVS (they don't need ZCS). Use a resonant gate drive. Efficiencies around 99.8% are feasible for a doubler/halver based on the "3L FC buck" topology, in the 10~30 MHz range.
labcomputer 11mo ago
The app notes and data sheets of related parts suggest that the target application is large conversion ratios, where the duty cycle in an inductive converter is close to 0 (or 1). That forces tradeoffs, like a lower switching frequency (lower efficiency), a larger inductor (more weight and/or cost) or very short T_on for one of the FETs (lower efficiency because transition times become important). So you can use the charge pump as the first stage of a hybrid converter to get a higher system efficiency.
- exmadscientist 11mo ago
  My recommendation to anyone who finds themself stuck in this corner of the design space is to consider a tapped inductor converter. I did a tapped inductor boost last year to take 3V input to 80V output (at not-much output current, I forget exactly what it was but it was mostly a bias voltage; also, the actual output voltage was DAC-set and could be quite low, so the loop dynamics were unpleasant). It was definitely annoying to wrap my head around, and very annoying to select the inductor (Würth has a nice OTS series, at the usual Würth prices; HVM would likely want a custom or semicustom design) but it just plain worked the first try and continued working through the usual stress tests and also the unusual stress tests of the Very Expensive Load™ getting itself Very Expensively Killed™ (for non-power-supply reasons). I was really happy with that converter, that kind of step-up ratio isn't easy and it just worked.
  - bsder 11mo ago
    Links or refs? That sounds very interesting. A basic search coughs up a bunch of papers from academic paper mills, and I don't see obvious links to an OTS series from Wurth Elektronik.
    - exmadscientist 11mo ago
      My refs were written into the schematic so I dug it up and might as well post the whole thing (SEE WARNINGS BELOW): https://i.ibb.co/zVXZWxHg/tapped-boost.png https://i.ibb.co/zVXZWxHg/tapped-boost.png (forgive me, I have no idea on the state of image hosting in 2025 so I hope that link holds up) I misremembered what design I actually ended up building. It was nominal 5V to 75V but was tested at a wider range (including the mentioned 3V to 80V). The variable voltage was implemented later, in a linear stage, which did in fact have a number of issues. ADI app note AN-1126 was very helpful in this design. It pushes for a different topology, yet compares with this one and others in some detail. Its arguments against tapped inductor designs are threefold: (1) sometimes EMI issues on the switching node (fair, and a real concern, but not interesting for what I needed to do), (2) the inductor is annoying to source (very true), and (3) the main technical objection is that demands on the output rectifier are high and might require you to use something crappier than a Schottky. That last one is true in general but for this design in this decade, I was comfortably within medium voltage Schottky territory, and so their main objection was a complete nonissue. That looked good to me so I went ahead and built it, and was not disappointed. The inductor was 744889030330 (say that three times fast) from the WE-MTCI family which worked a dream. I don't remember why this exact switcher chip was chosen but I do remember having a lot of candidates and so the choice was kind of arbitrary. The control scheme type it uses is important, though. I don't think the zener did anything but being paranoid I wanted to have the footprint there for the prototype build rather than not have it. THE PREVIOUSLY PROMISED WARNINGS WHICH ARE ACTUALLY REALLY IMPORTANT SO I AM USING CAPS: 1. 80V can kill you. Really, it can. Use caution! 2. This is the design I sent off for fab (and perhaps not even the final version, if the folder notes are any guide). It is not the redlined, working version. I believe this stage was OK. Maybe it needed a bit more output capacitance? Certainly I know the following stage, not shown here, had severe issues (most of them stupid and obvious once noticed). So treat this as good inspiration, not ground truth. 3. Did I mention that 80V can kill you? USE CAUTION.
hulitu 11mo ago
> you can do with modern high frequency stuff EMC
namibj 11mo ago
ZVS (not ZCS) topologies that use planned stray inductance and WBG transistors and class 1 ceramics can operate at 10s of MHz with less than 1% of loss per voltage halving. They have insane power density and lack of wound inductors means there's nothing that causes problems as you push into the kilovolts. But really they're just more efficient (integer ratios, where `n-1` not prime) than classical Buck/boost topologies.

em3rgent0rdr 11mo ago

The '7660 is good for low-power and is my go-to DIP-8 part when I need a half or double voltage supply on a breadboard.