Author Topic: High Voltage Bench Power Supply Design  (Read 15841 times)

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Offline Wolfgang

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Re: High Voltage Bench Power Supply Design
« Reply #50 on: June 19, 2019, 03:34:15 pm »
Never seen an IGBT with DC soar. Interesting ! Any types recommendable ?
 

Offline duak

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Re: High Voltage Bench Power Supply Design
« Reply #51 on: June 19, 2019, 05:42:48 pm »
H713, the pass transistor stage may be oscillating by itself.  Bipolar transistor emitter follower circuits can and do oscillate with capacitive loads.  The solution there is to add damping resistance in series with the base.  I expect that FET source followers could do the same.  You already have series gate resistors here of a reasonable value so I would try increasing the values of the source resistors R7 - R9 to a few ohms and see what happens.  As I mentioned before, larger resistors here will help balance the currents in the pass transistors.  You can also try a series RC damping network between the output terminals - try 10R and 100n and see what happens.  You've already found that electrolytic capacitors with their higher losses do not cause the trouble that the film caps do.

You've got a lot of voltage gain in this circuit, and it's distributed among a number of stages each of which introduces a delay - this is a recipe for an oscillator.  I would analyze the circuit to determine its frequency and phase response and do what I could to improve the phase margin.  Do you know about Bode plots or Root Locus analysis as applied to feedback systems?  (I used to know more at one time - if you don't use it, you lose it)
 

Offline H713Topic starter

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Re: High Voltage Bench Power Supply Design
« Reply #52 on: June 20, 2019, 06:55:30 am »
A little further testing revealed that smaller values of electrolytics do cause a small amount of oscillation, something like 30-40 millivolts. A series RC network did not cure the problem, nor did a gate resistor on Q1. I currently don't have a source resistor in the test sample, as there is only one pass element being used. I can try that, but I doubt it will cure the issue.

What did cure the issue is the brute-force solution of putting a 100uF electrolytic on the output. This sort of defeats the current limit protecting the DUT, though it still protects the pass transistors. While I personally would not have any problem using a supply like this (it's still less scary than a lot of the power supplies I work with), It's really a brute-force solution to the problem and makes this design considerably less useful for others.

Duak,

Bode plots and Root Locus analysis is a bit over my head, or at least the terminology is. With audio amplifiers, I occasionally do test the frequency response and phase shift to get an idea of where instabilities are coming from, though normally with well-designed tube amps it comes back to the output transformer, which there isn't a ton you can do about. That said, I'm really not sure of the best way to test a circuit like this. It's not like I can feed an AC signal to the input and look at it on the output.
 

Offline duak

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Re: High Voltage Bench Power Supply Design
« Reply #53 on: June 20, 2019, 05:18:49 pm »
H713, Many linear power supplies do indeed use an output capacitor for stability and it sure does render the current limiting moot for transient loads.  A sure fire way to get around the current transient is to design a supply that inherently current limits, but the voltage is varied by clamping it to the desired value.  The hp 6186C supply works this way.

Back to your circuit, consider it to be the positive going side of a Class-A audio amplifier.  By injecting an AC coupled square wave, say 100 Hz, into U1 pin 2 and then following the signal path you can see how the various stages respond and try various things.  Because the feedback loop is delaying the signal, you can try paralleling R13 with some small capacitance, say 1n0 with some series resistance.  This is called lead compensation.   Also, because you have gain to burn and the problem is that there is too much at high frequencies, reducing gain without adding delay can be helpful.  Try increasing the value of R3.
 

Offline H713Topic starter

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Re: High Voltage Bench Power Supply Design
« Reply #54 on: June 21, 2019, 06:21:11 am »
I didn't have a chance to do the frequency/phase analysis today, however I did experiment with a few of your suggestions. I was able to increase the value of R3 to about 2.5K if it is connected to the -18V supply rather than GND. This makes things better but doesn't kill the issue cold. Unsurprisingly, the power supply is less stable at lower output voltages. With this configuration, it is good down to about 75 volts with a 33uF output capacitor, but below that threshold and the 33uF capacitor causes an oscillation in it of itself. With a 100uF capacitor it is fine. If I have time tomorrow I'll try the 1nF cap across R13 and see what that brings about. As I understand it, this is essentially to roll off the frequency response (and lower the overshoot on square waves).

Quite honestly, I don't have a big issue with a big output capacitor in the 100uF range. It will take up considerable board space and add cost, but this isn't a production unit so that is not a big deal, especially when I have the parts in stock. I also don't have an issue with its impact on current limiting. The point of the current limit in this design is not to make this thing idiot-proof (no 500V power supply will ever be), but rather to avoid literally melting the DUT and/or blowing the pass elements. The main thing I don't like about it is the fact that I feel like it is a band-aid fix to avoid proper debugging and circuit modifications. With that said, it seems to be a fairly common solution to this problem. I'd like to get this circuit as stable as possible before adding the output capacitor, however.
 

Offline H713Topic starter

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Re: High Voltage Bench Power Supply Design
« Reply #55 on: June 24, 2019, 06:41:55 am »
Alright, the 1nF cap in parallel with R13 seems to have done the trick. I would do the square wave testing, but I successfully managed to properly nuke my proof-of-concept/prototype. :palm: I'm fairly confident that this is not a design issue, and more of a sketchy parts from who-knows-where issue. And no, the pass transistor did not short. I have yet to try this with the actual driver and pass transistors, so there likely will be some tweaking required. Square wave testing will be done then as well.

Meanwhile, I have an updated schematic and a version 1 PCB layout. Gerber files are attached and can be viewed in an online viewer. Feedback greatly appreciated, as my layout and routing skills are something I'd like to improve. I did try to keep the PCB a somewhat reasonable size whilst limiting myself to 2 layers for cost reasons.
 

Offline blackdog

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Re: High Voltage Bench Power Supply Design
« Reply #56 on: June 24, 2019, 05:09:14 pm »
Hi,


I have some remarks  ;)

Split R2 in 15K than 4.7uF to ground en 33K to the drain of Q1, this wil help the hum suppression this wil be a small 30dB.
Is the SOA enough for Q1?

Place a 8.2V or a 10V Zener over R14 to protect the opamp in case of short circuit, anode to ground.

If you want to spend a little money, replease the diode D5 with a 10V reference.
The minimum output voltage will be a bit higher, but the power supply will become more stable and will get less noise.
Adjust the value of R14 to get the right ratio again.

If the current value of C5 just keeps the power supply stable, then something's wrong...
The 10nF of the pin-2 and the 100nF over the compensation connection of the NE5534 is really ridiculous...  :-DD

Tip
If you come up with values for components that are far above the normal values during testing, then you know that something is not going right.
C5 is for normal use with gains lower than 3x about 22pF!
Above gains of 3x a capacitor in not necessary for the NE5534.

But...
This is not a normal circuit, you have extra gain in the loop, so compensation for a stable regulator wil be far more difficult.
Yes C7 helps :-) but that is nog good enough, and no do not make this capacitor higher in value!
You have to solve the problem where it arises and that is the gain or Q1.

Place a resistor and a capapcitor in series from the Drain van Q1 to ground, this wil lower the gain in the loop.
Try to find out for yourself which value's ar needed.
As a starting point you can take the value of R2 which is the dominant part of the impedance of the drain of Q2.

Q5 need a small base resistor, 100-Ohm or so.

You need a Zener to protect the Gate-Source voltage of the power MOSFets.

Place direct on the gate of Q1 en resistor, say 150-Ohm.

Kind regads,
Bram
Necessity is not an established fact, but an interpretation.
 

Offline H713Topic starter

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Re: High Voltage Bench Power Supply Design
« Reply #57 on: June 24, 2019, 05:34:17 pm »
If I remember right, the 100nF on the compensation pins was not really needed but I put it there anyways just so it made it onto the pcb in case I needed it later. 100nF is definitely ridiculous, same with the 10nF, I want to say that 1nF made it stable. I think I put the really large caps in just to see if it made it stable, and unsurprisingly it did not.

Good call on The gate protection zener, I totally forgot to draw that in.
 

Offline duak

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Re: High Voltage Bench Power Supply Design
« Reply #58 on: June 24, 2019, 06:18:05 pm »
H713, since you're going to layout may I suggest splitting R13 into two or three parts?  I calculate that with a 500 V output, R13 would have to dissipate 2.5 W.  It'll be easier to find smaller precise and stable resistors than one larger one.

When the voltage crowbar circuit trips, triac D10 will have to dissipate the 7.2 Joules of energy stored in C14 and C15.  I doubt it will survive and will most likely short out.  If the pass transistors weren't initially damaged, they'll probably overheat and short out too.  A fuse in the +Vout line between C14 & D9 to limit the current should minimize damage.  Note that the fuse should be rated for breaking the circuit voltage - these parts are usually sand filled to extinguish the arc that occurs at a higher voltage.
 

Offline H713Topic starter

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Re: High Voltage Bench Power Supply Design
« Reply #59 on: June 24, 2019, 09:37:56 pm »
You aren't wrong with the crowbar, and I wasn't really expecting the triac to survive. It is a last resort to stop the full DC bus from being dumped into the load if a pass element shorts. I was going to have a small fuse in series with the output of the capacitor bank (separate to this board), so if the triac shorts I don't expect the pass transistors to blow. I already found a suitable 600v rated fuse holder and fuses on mouser.
 

Offline cur8xgo

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Re: High Voltage Bench Power Supply Design
« Reply #60 on: June 25, 2019, 02:56:39 am »
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?



 

Offline H713Topic starter

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Re: High Voltage Bench Power Supply Design
« Reply #61 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...

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.

 

Offline cur8xgo

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Re: High Voltage Bench Power Supply Design
« Reply #62 on: June 25, 2019, 04:19:26 pm »
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...

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

 

Offline H713Topic starter

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Re: High Voltage Bench Power Supply Design
« Reply #63 on: July 08, 2019, 05:03:39 am »
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.




 

Offline MagicSmoker

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Re: High Voltage Bench Power Supply Design
« Reply #64 on: July 08, 2019, 10:11:47 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).
...

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.



 

Offline Wolfgang

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Re: High Voltage Bench Power Supply Design
« Reply #65 on: July 08, 2019, 03:38:12 pm »
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?

lets see ...

- even the linear rated IXYS FETs have a max. dissipation of ca. 100W per chip in TO247 case in linear mode
- that makes 50 FETs a 10$ each, if your need 5kW,.
- and, you have to make sure that they are equally loaded. Meaning the balancing resistor drop is
several time the variation in threshold voltage.
- Then, your total gate input capacitance is in the range of a microfarad, and it must be driven from a really low impedance.

Plan B: Make 5 driver modules a 10Fets, each with a separate driver. You could stack this scheme until you have the current capability you need.

Another thing you could consider at 5kW is a tube. The circuit will be a lot simpler, the mechanics not so much.

have fun, play safe
   Wolfgang


-
 

Offline duak

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Re: High Voltage Bench Power Supply Design
« Reply #66 on: July 08, 2019, 04:54:19 pm »
I've posted a reply in this thread earlier on.  To re-iterate, MOSFETs have a problem called "electro-thermal instability" when used as pass transistors when they have to support a high drain to source voltage at low currents. (see 1st attachment).  The other papers talk about it in more detail.  Basically, MOSFETs have multiple cells and each one has a slightly different threshold and transfer function.  This means that they don't share the current equally and then localized hot spots develop.  At some point, a cell will fail, short out and the device is pretty much useless.

What can one do about it?
- IXYS linear FETs are better, but they still exhibit the problem.  I found that out when testing a high voltage electronic load. 
- older, high voltage FETs are better than new ones that are optimized for switching.
- the FQA8N90C-F109 shouldn't be too bad because of their 900 V Vds rating.  However, it uses a newer cell design so it likely was optimized for switching.
- use larger source resistors to better distribute the current among the FETs. I would say something that develops 5-10 V at full current.
- use a foldback current limiter to reduce output current at lower output voltages.  This can be done with a high valued resistor from the drains of the FETs to the base of current limiter transistor.  It will need a resistor between the base of the transistor and the current sense resistor.  However, the extra current from this resistor may increase the minimum output voltage slightly because it is bypassing current around the pass transistors.

Food for thought: I've been thinking that dithering the drive to the FETs might be a solution.  This is where a high frequency signal is applied to a system that has a granular response and the output is filtered to remove extraneous noise.  An example of this is the 50 - 100 KHz bias used in analog magnetic recording to reduce distortion.
 
« Last Edit: July 08, 2019, 10:06:14 pm by duak »
 

Offline Wolfgang

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Re: High Voltage Bench Power Supply Design
« Reply #67 on: July 08, 2019, 05:14:29 pm »
Hi,

I personally have never seen an IXYS MOSFET fail in linear mode. I run them at max. 70% or their max. dissipation spec in  linear mode, and with 80% of rated voltage, so no extraordinary precautions.

Dithering does not help much, if you do that you could resort to a switching regulator right from a start. The ripple removal effort will be similar, and then normal switching MOSFETs could do the job.

regards
  Wolfgang

 

Offline H713Topic starter

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Re: High Voltage Bench Power Supply Design
« Reply #68 on: July 08, 2019, 06:04:52 pm »
Where did the 5kW number come from? 500v * .5A is a maximum of 250W. If that .5A got reduced to 350mA I could live with that so long as I have a reasonable safety margin.

If I could find a FET that could reliably handle 600V at 200mA that would be fine as there will be 4 in parallel. The other thing I can try is putting the FETs in series rather than in parallel, though I don't expect that the improvement will be drastic.
 

Offline MagicSmoker

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Re: High Voltage Bench Power Supply Design
« Reply #69 on: July 08, 2019, 09:10:20 pm »
If I could find a FET that could reliably handle 600V at 200mA that would be fine as there will be 4 in parallel. The other thing I can try is putting the FETs in series rather than in parallel, though I don't expect that the improvement will be drastic.

Parallel operation of MOSFETs in linear mode is perilous business because their gate threshold voltages have a negative tempco and they are also prone to vicious oscillations. I would expect to derate to no more than 25W Pd for a TO-247 package and they absolutely must be on a common heatsink for good thermal coupling. So, plan on requiring 8-12 in parallel, not 4.

If you can get the Vds drop down to 100V or less via series wiring, however, each MOSFET will be able to handle the full 500mA so only 4-6 should be necessary. The MOSFETs are much less prone to oscillation when in series and the negative threshold voltage tempco is also not a problem (just results in unequal voltage sharing). It's a bit trickier to wrap your head around how a series cascade works, and there will be a larger minimum dropout voltage, but that is usually not a problem in a HV regulated power supply.



 

Offline H713Topic starter

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Re: High Voltage Bench Power Supply Design
« Reply #70 on: July 09, 2019, 04:59:06 am »
So I simulated and redrew the schematic for series operation (this what you had in mind for the configuration?)- seems to simulate well. In addition to stability, the other big advantage of this is that I can use slightly lower voltage FETs- which means that IRFP460s or other old MOSFETs will work for this design- even in a shorted output, no device will have to drop more than 170 volts. I think I will add one or two more FETs just to improve my safety factor. Tomorrow I'll try to test series operation to see how it behaves before redrawing the board. Not thrilled about needing to do so, but a bit of reading suggests that other people have had lots of trouble paralleling HV FETs. Oh well, the old boards will be useful for something or other.
 

Offline duak

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Re: High Voltage Bench Power Supply Design
« Reply #71 on: July 09, 2019, 04:25:57 pm »
H713, may I suggest adding a 10K resistor between the junction of U4-2 & the 1n capacitor and the wiper of RV1?  This will present a more constant resistance to the RC network formed with the 1n capacitor and make the feedback more predictable.

You may also find that Q1 may not completely shut off when U4-6 goes to maximum -Vout.  The bottom end of R3 should probably go to GND.
 

Offline H713Topic starter

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Re: High Voltage Bench Power Supply Design
« Reply #72 on: July 13, 2019, 05:59:41 am »
I spent some time today testing IRFP450s at the voltages they would see when used in series. Even with 175 volts across it I was able to draw about 850mA.  I can only imagine that IRFP460s will perform even better. Interestingly, the 2SK3675 performed quite poorly. Good thing I only payed $1.50 each. I think that the 500V Hexfets are probably going to be the ticket for this, though I still have more testing to do. If I figure that the FETs are good to ~800mA, that gives me a safety factor of around 1.7. That goes up if I go from using four series devices to five or six. Even with four pass elements, the safety factor of 1.7 is a worst-case-scenario number for a power supply set to 500V with the output shorted and with perfect load regulation on the DC bus- in practice it will be better. I still have to do a bit more testing before I feel confident in the design, but so far things look good. 

Again, no FET will have to drop more than 150V across in in the worst case scenario with 4 devices. In fact, they will never see even that because with a shorted output the supply will current limit at 500mA (or whatever it ends up being), and at that current the DC bus will drop by about 100V- that's another issue I need to address is the load regulation, and that may involve trying to find a transformer with lower copper losses. I've dumped the idea of a voltage doubler (not necessary with the control transformer I have now), and I'm going to implement range switching on the primary side so as to avoid the need for expensive high-voltage relays.

One other thing to consider- the only time the FETs should ever see 120V across them is in the case of a shorted output when the supply is set to its maximum voltage. This is (or should be) a rare occurrence. If the output is shorted while the supply is set to ~250V or so, then the FETs won't have to drop more than about 75 volts each.

I may also move the crowbar circuit off this board and onto the control PCB to save some space.

I sure would like to see the test setup where they came up with the SOA graph for some of these transistors. :bullshit: In the case of the FQA8N90C, the DC curves are optimistic by about 200%, and I had them clamped (with very good thermal compound) to a large block of aluminum- when they failed, they were gently warm. This is quite a shift from the tube world where many tubes can be pushed to 200% their absolute maximum ratings without much ill-effect.

I do hope that I can come up with a viable solution, since I'm not the first one to want a solid-state high-voltage linear bench power supply. I've read through the few threads that actually got passed the brainstorming phase. In many cases, everything simulated well, but once testing ensued the optimistic SOA ratings, current sharing and oscillation issues rendered the design impractical. Hopefully the series operation will fix these issues.
 

Offline duak

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Re: High Voltage Bench Power Supply Design
« Reply #73 on: July 13, 2019, 05:27:05 pm »
Back in my design days, some parts would require 50% derating for reliability.  The corollary is that the parts' datasheet promise 100% more than you'll ever get. eg., Motorola used to make the MJE802/4502 complementary power transistors for audio amplifiers and servos.  They were in aluminum TO-3 packages and if the environment was quiet enough you could hear them sing at the input signal frequency - very much like "needle talk" with records.  I was integrating systems using tape drives that used these transistors in the reel and capstan servos.  These transistors were failing in the field after a couple of years even though they weren't anywhere near their limits.  The common heat sink never got above 50 C.  The tape drive manufacturer changed to different devices of similar ratings but in steel cases and the failure rate dropped to nothing.  Turned out the transistor dice would partially detach from the aluminum case because of the different thermal expansion rates.

I'd like to see your final schematic when done.  I have an old Kepco DC supply with one vacuum tube in it for the pass device.  If it expires, it'd be nice to replace it with semiconductors of some sort.  I have tubes of IRFP360s (400 V Vdsmax) so putting them in series may work well.
« Last Edit: July 13, 2019, 07:28:46 pm by duak »
 

Offline H713Topic starter

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Re: High Voltage Bench Power Supply Design
« Reply #74 on: July 14, 2019, 04:12:02 am »
Here's the current schematic. There are a few improvements I'd like to make and some more stability testing to do. I'd like to make the current limiting more adjustable (maybe without requiring an expensive 20 ohm rheostat?) and I have yet to run a test with 6 IRFP460s in series- the most I've done is 4 FQA8N90Cs in series, but I have no reason to believe it won't work. Oscillation is not a big issue with this circuit. It would be nice to improve the ripple rejection a little, though with a well-filtered DC bus it shouldn't be a huge issue.
 


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