3 ms·

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 sta

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=...
- moffkalast 2y ago
  > current range 0 ~ 6.5A > Fuse recommended (5A) I see they are very confident about going to 6A. These ratings are often just "yeah it can technically do that but it will reach 100 degrees during it", for any kind of stable continuous draw you just have to halve the rating to be safe.
  - michaelt 2y ago
    If you want you can buy the next size up [1] giving you 11A at 5V with the same 92% efficiency. Of course, it'll still dissipate the same amount of power, because 8% of 25W is still 2W. [1] https://www.meanwellusa.com/webapp/product/search.aspx?prod=NID100 https://www.meanwellusa.com/webapp/product/search.aspx?prod=...
    - moffkalast 2y ago
      That looks more like it (8A fuse), and relatively cheap on Mouser too interestingly enough. Thanks for the heads up I'll have to order and try one. It does puzzle me why they went with the 2.54mm pin layout though, those are rated for 3A max I think? So even if the draw is perfectly split between the two vout pairs they give it'll be melting at 6A already, probably more like 5 if not.
      - michaelt 2y ago
        I know the pin looks small compared to the cable you'd run to a wall socket for a 10A current - but those cables are a lot longer these copper pins, and cables are sized with the assumption the copper will be coated in insulation then coated in another layer of insulation then installed into a wall full of insulation. A 10mm copper pin measuring 0.6mm x 0.6mm pin would have half a milliohm of resistance. Even if you ran your entire 5A load through a single pin, it would only have to dissipate 13 milliwatts. I'd be more worried about the PCB traces if I were you - 2oz copper is only 0.07mm thick :)

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=...

moffkalast 2y ago
> current range 0 ~ 6.5A > Fuse recommended (5A) I see they are very confident about going to 6A. These ratings are often just "yeah it can technically do that but it will reach 100 degrees during it", for any kind of stable continuous draw you just have to halve the rating to be safe.
- michaelt 2y ago
  If you want you can buy the next size up [1] giving you 11A at 5V with the same 92% efficiency. Of course, it'll still dissipate the same amount of power, because 8% of 25W is still 2W. [1] https://www.meanwellusa.com/webapp/product/search.aspx?prod=NID100 https://www.meanwellusa.com/webapp/product/search.aspx?prod=...
  - moffkalast 2y ago
    That looks more like it (8A fuse), and relatively cheap on Mouser too interestingly enough. Thanks for the heads up I'll have to order and try one. It does puzzle me why they went with the 2.54mm pin layout though, those are rated for 3A max I think? So even if the draw is perfectly split between the two vout pairs they give it'll be melting at 6A already, probably more like 5 if not.
    - michaelt 2y ago
      I know the pin looks small compared to the cable you'd run to a wall socket for a 10A current - but those cables are a lot longer these copper pins, and cables are sized with the assumption the copper will be coated in insulation then coated in another layer of insulation then installed into a wall full of insulation. A 10mm copper pin measuring 0.6mm x 0.6mm pin would have half a milliohm of resistance. Even if you ran your entire 5A load through a single pin, it would only have to dissipate 13 milliwatts. I'd be more worried about the PCB traces if I were you - 2oz copper is only 0.07mm thick :)