Electronics > Projects, Designs, and Technical Stuff

High Voltage Bench Power Supply Design

<< < (13/17) > >>

cur8xgo:
This makes me wonder...

Is there:

-a reliable, robust method to parallel 100 TO247 or TO220 fets
-a fet which is cheap that could be paralleled using that method, to essentially create some giant combined fet that could handle linear mode dissipating 5KW or more no problem into a water cooled heatsink or what not?



H713:
Sure you could. The FQA8N90C-F109 that I linked earlier in the thread is probably one of your better options in terms of SOA/cost ratio. At least if you're going for high voltage. Where I work we have some power supplies that each have over 250 pass transistors (I believe they're BJTs) in parallel. Each one has a fuse and current sharing resistor. They are liquid cooled. So I can say that it's possible because I've seen it done. But the cost is... excessive. I mean, if you want to use this for a power supply I'd really look at designing a switcher. If you want to use it for a load... I mean, nothing is stopping you...

As for this power supply, I managed to track down the problem with my prototype. Dodgy breadboard connections. I also discovered that the 5534 doesn't really seem to work with this circuit. While dealing with the breadboard issues I realized that the TL071 that I had subbed in for troubleshooting purposes was still in. I tried switching it out for a 5534, and discovered that it doesn't really work. The circuit works perfectly fine with TL071 and OPA604 op-amps. My strong suspicion is that the relatively low input impedance of the 5534 is loading down the voltage divider. There are a few other updates as well that I will add to the next schematic revision.

Also... at this point I do not believe that the gain of Q1 is too high, as if I lower it much more I start to lose output voltage range. It was certainly too high in the first schematic versions, but I do believe that this has been corrected. In addition to that, things are really fairly stable. Not surprisingly, with the faster OPA604 it is a little less so, but with a 20uF output cap it seemed fine with both resistive and capacitive loads throughout the entire voltage range. I still need to do some more testing to make sure it is stable into any output load, but I'm going to wait until I get some boards and the actual parts in so that the testing is more relevant.

cur8xgo:

--- Quote from: H713 on June 25, 2019, 06:29:21 am ---Sure you could. The FQA8N90C-F109 that I linked earlier in the thread is probably one of your better options in terms of SOA/cost ratio. At least if you're going for high voltage. Where I work we have some power supplies that each have over 250 pass transistors (I believe they're BJTs) in parallel. Each one has a fuse and current sharing resistor. They are liquid cooled. So I can say that it's possible because I've seen it done. But the cost is... excessive. I mean, if you want to use this for a power supply I'd really look at designing a switcher. If you want to use it for a load... I mean, nothing is stopping you...

--- End quote ---

I'll keep this in mind for a rainy day!

Also what about changing things to where

MAINS 175VAC to (low voltage)VAC (24? 50?)

low voltage VAC gets rectified then linear regulation then switched into transformer which steps it up to 500V, feedback loop connectes to 500V but mosfets never see high voltage

H713:
Okay time for an update: Boards and parts from Mouser have arrived, I've done a bit of testing, and I have some not-so-good news. The good news is that the boards work. The bad news is that the FQA8N90C-F109 MOSFETs are not proving to be nearly as robust in linear operation than their SOA ratings would suggest. I did some lazy testing by setting my Eico 1030 to 410V and using it to power the test board. I then connected the output of the board to a 150R resistor and slowly brought up the output voltage over 15 minutes.

The FQA8N90C was good to maybe 170mA with about 350V across it, while the IRFP450 was good to about 210mA, but its 500V rating is a problem for a power supply that I want to output up to 500V (which necessitates a much higher input power than 500V). Not going to lie, the fact that the FQA8N90C transistors couldn't even handle half the current at 350V as their datasheet suggested was a bit frustrating and discouraging, especially since they were clamped to an oversize heatsink with a 10k RPM server fan blasting on it the whole time- they were gently warm when they blew. And no, they were not oscillating. I knew this could be a problem- but I'm still confident that this can be a good and functional design with a little work.

I ordered a some Fuji 2SK3675 parts to try, but we shall see how they perform. They are older, so that might bode well. Aside from the FETs not being nearly as tough as I was hoping, the circuit is working quite well. Attached is an updated schematic. I'll post pics of the unpopulated boards tomorrow, as of right now I can't be bothered.

I'm going to be a tad flexible on the max output voltage and current, but I'm wondering if anyone here is aware of any MOSFETs (at least 600V rated- more is better) that might actually take linear operation (aside from the very expensive IXYS linear-rated fets). Perhaps one of the older HEXFETs? It'd be great if someone smarter and more experienced with these parts than myself could give a list of parts to look at. The IRFPG50, IRFPE50 and IRFPF50 are interesting, but I've never used any of them and the datasheets do not have a DC SOA.




MagicSmoker:

--- Quote from: H713 on July 08, 2019, 05:03:39 am ---...The bad news is that the FQA8N90C-F109 MOSFETs are not proving to be nearly as robust in linear operation than their SOA ratings would suggest.
...
The FQA8N90C was good to maybe 170mA with about 350V across it, while the IRFP450 was good to about 210mA, but its 500V rating is a problem for a power supply that I want to output up to 500V (which necessitates a much higher input power than 500V).
...

--- End quote ---

Hmm... you aren't going to get an SOA of >200mA at 500V of drop in a single MOSFET (except, perhaps, if it's in a SOT-227b package, as that's 100W of Pd right there). You could, however, make a pass transistor capable of up to 500V and 300mA by wiring several MOSFETs in series.

Attached is an LTSpice file of a circuit ripped straight from an EDN article a few years ago that is a constant current source for charging capacitors but which can be modified to act as a voltage regulator easily enough.



Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod