Electronics > Projects, Designs, and Technical Stuff
DC high-current power supply-using PNP pass elements.
<< < (2/2)
Ian.M:
There's a better 2N6051 datasheet with SOA & derating curves here:
https://4donline.ihs.com/images/VipMasterIC/IC/HRIS/HRISD005/HRISD005-2-73.pdf?hkey=EF798316E3902B6ED9A73243A3159BB0

You could replace the diode D in the schematic with a Vbe multiplier using any non-darlington reasonably high gain power   transistor that can handle the regulator IC's worst case short circuit current (before its SOA & thermal limiting kicks in) which would allow you to trim the current at which the external  pass transistors start to 'kick in'. 
Xnke:
Thanks, Ian. That sheet shows that a better operating point (at least a known safe operating point) would be anything less than 10A at 10Vc-e, so long as I have enough heatsink to keep the suckers are room temp (I don't). It also calls out a linear deration per degree C over 25C, for current-so some napkin scratching later, if I can keep the cases at 75C, each device can handle 7.5A.

That's stinking hot. I'd prefer to keep them around 40C, where each device can handle 9.5A or so. As far as heatsinking goes, a 400mm x 200mm section of heatsink with 80mm tall fins spaced 10mm apart with a 10mm thick base is available, and was used in the 14kW linear VFD that I pulled those fuji transistors from. It did have a 220v fan blowing on it at that level, though.

Ian.M:
I wouldn't go over 80% of the derated SOA if you want reliability.  Also you have to design for worst case ambient temperatures which can be quite high in an equipment rack in a summer heatwave (assuming you don't have server room grade air conditioning in your shack), so 40 deg C case temperature is unrealistic.   As the transformer is good for 20A DC continuous, IMHO you'll probably need three of the power transistors, operating at 60 degC max case temperature (which derates them to approx 8.3A Ic_max).   

I'm betting you'll need active cooling, though you could use 12V DC and PC fans so you can easily make them variable speed and temperature controlled, shutting off below 50 deg C heatsink temperature to minimise the annoyance factor when its lightly loaded or the environment is cool enough.  As you want the main output to be as clean as possible, its probably worth using a separate transformer or PSU for the fans and system monitoring.
David Hess:
My big 50 amp Astron uses 8 output transistors under similar conditions without active cooling but the heat sinks and tops of the TO-3 transistors are exposed on the sides.

The one I built used a 20 watt heat sink with a fan mounted directly over it and an operational amplifier (probably an OP-90) based temperature controller with a 1N4148 epoxied to the heat sink for temperature control.
Xnke:
Alright, quick topology check here, gotta start with something. Only working out voltage regulation at the moment, the adjustable current foldback will come later. (when I figure out how the heck to do that.)

Transformer with the chunky stud diodes and all the capacitance I can bolt on it (339,990uF) puts out about 13.5VDC loaded at 40A output, coming up to 15.5VDC at 30A, and 16.2VDC at 20A. It has a few taps that I can work with to move the voltage around, but it will definitely need active cooling on the transformer if I pull more than 20A continuous.

Here, my load resistor is the 690R on the far right-I should have labeled it better but missed it while watching the hockey game (Preds-Blackhawks)



I've definitely made some mistakes in the basic topology here, and none of the components are anything but representitive. I'm probably going to be using something like an LM324 or NE5532 op amp, I've got a LOT of the latter pulled from old broken audio gear. (Lots of the old Realistic brand CD players with dead transports, all had 8 NE5532's and all of them were socketed)

Pick that apart and let me know what's gotta change. If anyone has and good methods of applying an adjustable (preferably with a 10K pot) current limiting, that behaves such that when the limit is hit, the power supply just folds back and the voltage drops, it is much appreciated.
Navigation
Message Index
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod