Electronics > Beginners
A good homebrew 500V 1A power supply
<< < (3/3)
Wolfgang:
Unfortunately there is a pitfall using even powerful MOSFETs in linear mode.
They become thermally unstable and burn out even when the power level is way below their dissipation limit, similar to the second breakdown of bipolars.

Googling "MOSFET linear mode" or "Spirito Effect" gets you the info you need.

IXYS can sell their stuff only because its needed in linear power supplies, electronic loads, current regulators or other stuff where MOSFETs are not running in
saturated mode. Nobody would buy them if the cheap switching stuff would survive this, too.

They cost a bit more than switching FETs, but their half-life is so much longer it pays off.   >:D
In a HV Supply, a punched pass transistor could immediately kill your load. That makes the cost of a safe pass device well spent.


David Hess:

--- Quote from: Dr.Krieger on September 06, 2018, 09:09:43 pm ---Why go with that (low-Vce?) "BJT cascode" thing instead of using a non-IXYS, cheap 1000V Transistor?
I see a BUF420AW 1000V 30A in Mouser for about $10, that's not so bad if you could use one for the job, right?
--- End quote ---

Wolfgang identified the problem.

The temperature coefficient of threshold voltage reverses at high drain-to-source voltage so the MOSFET cells suffer from current hogging like a bipolar transistor would suffer from current crowding under the same conditions leading to secondary breakdown.

IXYS and some others make special linear rated power MOSFETs which have better safe operating area curves at high voltages.  MOSFETs intended for switching applications often do not even show this in their specifications.
floobydust:
A HV power supply like this needs to incorporate at least dual-slope SOA protection, instead of just hoping the pass transistors' are tough enough with simple current limiting.

Output current and voltage across the pass transistors can be looked at, this is common in audio power amplifiers for example.
Michael Kiwanuka: SOA Protection for Audio Power Amplifiers is a good read.
Navigation
Message Index
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod