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Old 14th Jun 2020, 3:14 pm   #1
Diabolical Artificer
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Default High voltage regulator circuit.

I've built a high voltage regulator to supply screen grids with 300v, see attached. It's a simple circuit that uses a mosfet as the series pass device and a bjt as a current regulator.

My circuit implementation is different in that there's no bridge rectifier, the circuit is fed by HT which is 420v. I'm using a zener string instead of the pot to give a voltage reference on the gate,I'm using a 50k preset pot to adjust current to the zeners for fine adjustment. Am using a 2SK2545 instead of the IRF740, no problem there AFAIK, the mosfet is rated at 600v/40w, but I'm wondering whether Vds & Vgs are too great. Vds will be 120v Vgs a tad under that.

I'm also wondering if there's a better protection circuit, I've used this circuit quite a few times and the mosfet's dies pretty easily if there's too much current draw.

TFL, Andy.
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Old 14th Jun 2020, 3:49 pm   #2
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Default Re: High voltage regulator circuit.

Surely Vgs can only be the few volts required to turn on the MOSFET? As in Vdg being a few volts less than Vds.....
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Old 14th Jun 2020, 4:29 pm   #3
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Default Re: High voltage regulator circuit.

Quote:
Originally Posted by Diabolical Artificer View Post
... I'm also wondering if there's a better protection circuit, I've used this circuit quite a few times and the mosfet's dies pretty easily if there's too much current draw ...
I think protection can be hard to implement simply, particularly if you want to protect against very sudden over-current, as in a flash-over inside a downstream valve. You have to worry about charge stored in capacitances which can hold the gate voltage at a high level while either the drain or source goes out of range.

I have a rule of thumb concerning audio amps/pre-amps which turn up here for repair. If there is a MOSFET HV regulator in them and if the kit actually has any fault at all then the fault will include a blown MOSFET HV regulator. So far the rule has proved very reliable indeed. Much, much more reliable than solid-state devices trying to deal with badly-behaved thermionic ones.

EDIT: You could look up 'Maida Regulator' as an introduction to the world of solid-state HV regulation.

Cheers,

GJ
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Old 15th Jun 2020, 6:35 am   #4
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Default Re: High voltage regulator circuit.

"Surely Vgs can only be the few volts required to turn on the MOSFET? As in Vdg being a few volts less than Vds..... " right Chris, brain fart there.

Thanks G. That doesn't bode well though at present the fet runs cool. I've looked at the Maida reg in the past I think and numerous other HV reg circuits, would probably be better with a valve regulator but with a big valve for the series pass it has a large footprint.

Is the gate of fets their Achilles heel, would a BJT be less prone to faults? Also should i be looking for a fet with a low gate capacitance or would using a beefier device help, something like a TO247 package?

Andy.
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Old 15th Jun 2020, 7:42 am   #5
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Default Re: High voltage regulator circuit.

Have you looked at the supplies used in the Sussex valve tester?
This uses an LR8 with Mosfet current boost and incorporates over current protection.

Peter
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Old 15th Jun 2020, 8:43 am   #6
GrimJosef
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Default Re: High voltage regulator circuit.

Quote:
Originally Posted by Diabolical Artificer View Post
... Is the gate of fets their Achilles heel, would a BJT be less prone to faults? Also should i be looking for a fet with a low gate capacitance or would using a beefier device help, something like a TO247 package? ...
I confess I'm starting to get out of my depth now. I have seen some fairly huge FETs blown up in kit which comes in, so I suspect it is the fragile gate insulator which is the weak point. The fact that this is still a problem despite large numbers of people having tried to solve it for 30+ years rather suggests that there isn't an easy solution.

Buried deep in nearly 30 pages (yup !) of posts on a DIYAudio thread on the subject was the advice (I'm remembering what I can) that:

Protecting against limited overcurrent can be quite straightforward. But that's very different from protecting against an instantaneous hard short with a large reservoir capacitor behind the FET. If you want to build a genuinely bomb-proof lab-grade supply then study the circuits developed by HP and incorporate the tricks that they built in.

At that point I found room for a 6080.

Cheers,

GJ
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Old 15th Jun 2020, 10:16 am   #7
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Default Re: High voltage regulator circuit.

Aaaah, ....6080s!
The mainstay of 60s and 70s 250 and 300 V regulated supplies.
We had hundreds of these very forgiving brutes quietly working away.
Virtually bullet-proof.
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Old 15th Jun 2020, 3:07 pm   #8
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Default Re: High voltage regulator circuit.

Quote:
Originally Posted by Diabolical Artificer View Post
I the mosfet is rated at 600v/40w
Maximum ratings are never in combination so it is safe to say 600V OR 40W. For any particular device the truth lies somewhere in between and is usually contained in a small table (sometimes not) hidden at the end of data sheet. Safe Operating Area (SOA) chart for 2SK2545 suggests that under DC operating conditions with VDS at 420V maximum current is 80mA.

Looking at MOSFET operation within the circuit attached to OP but instead with 420V supply, current limiting starts to occur at approximately 120mA and any further current load will settle at, say 180mA. At this point the poor old FET is destroying itself trying to dissipate about 76Watts (absolute max = 40W) in constant current mode! Due to three thermal time constants between the die and heatsink the dangerous rise in temperature will permanently damage the silicon. Any protection circuit would have to detect and make safe in <1ms.

There is also another caveat with using a MOSFET in constant current mode, that is the rise of RDS bulk resistance with temperature. 2SK2545 RDS increases approximately 2.5 times and this results in runaway power dissipation and is a product of square of the current multiplied by resistance.

Rich
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Old 15th Jun 2020, 4:05 pm   #9
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Default Re: High voltage regulator circuit.

"Have you looked at the supplies used in the Sussex..." No, I will do though, thanks for the heads up Peter.

Thanks G, yep, I've tried to trawl through it too, after a while I zoned out, that's a good summation. There's another protection circuit that cuts power quickly if a valve starts pulling too much current, so hopefully that'll cover that type of fault, however valve regulation looks tempting, no room unfortunately.

I did and do look at SOA graph's when using trannies, but is Vds 420v? Vd = 420v but Vs = 300v so surely Vds is 120v? I was going to tweak this circuit by changing R2 after measuring Ig2. thanks for the info on CC operation, I find the thermal info on datasheets hard to get to grips with. I suspect a bigger fet is going to be needed for this job, there's eight 807 screen grids to supply.

Thanks for all your IP, Andy.
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Old 15th Jun 2020, 6:21 pm   #10
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Default Re: High voltage regulator circuit.

To give an idea of how to make something bullet proof I designed a 450V 10A constant current source, this had to cope with a dead short. Final design had 96 TO220 MOSFETS in parallel. Six where made 15 years ago and not one has blown up.
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Old 16th Jun 2020, 10:56 am   #11
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Default Re: High voltage regulator circuit.

By the time your over-current protection operates, into a short, ~200mA (?) the mosfet is sinking ~80W. You could try a device capable of dissipating that, maybe a 250W device with a monster fan cooled heat-sink, but it is then rather over-engineered

Better to go for a 'fold-back current limit' - in this case the current limit goes DOWN as the output voltage goes DOWN so limiting the max power in the series mosfet. Plenty of ideas on the web !

In reality you don't need a PSU like this to supply 100mA at 1volt so it would still be usable.

dc
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Old 17th Jun 2020, 5:35 am   #12
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Default Re: High voltage regulator circuit.

Thanks Dave. I found an interesting protection circuit whilst reading an old book, it used a "voltage dependant relay" which was in a BJT PSU, from memory it was a resistor on the base of the series pass device, the went to one end of the relay coil, the other end being fed by Vcc+. The SW shorted the rails, blowing a fuse, I guess a VDR is a normal relay + resistor.

Andy.
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Old 17th Jun 2020, 6:25 am   #13
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Default Re: High voltage regulator circuit.

Quote:
Originally Posted by Electronpusher0 View Post
Have you looked at the supplies used in the Sussex valve tester?
This uses an LR8 with Mosfet current boost and incorporates over current protection.

Peter
I agree, the variable HV supply in the Steve Bench and Sussex valve testers are just about identical, they work well and have adjustable current limit, I've shorted mine to earth, just to see, with no ill effects.
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Old 17th Jun 2020, 6:32 am   #14
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Default Re: High voltage regulator circuit.

Just one observation, the original circuit, back at the start of the thread, isn't actually a regulator. It's a pot with a current booster and a current limiter. Any variation of the input voltage comes through proportionately onto the output.

Mosfets can be reliable, but you can't take any shortcuts with them. Their destruction is fast, very fast. You can't abuse them for even a short time and think 'well, it's not for long'

A common misconception is that their gate-to-source resistance is immense, and so you can use very large drive impedances. You can, but only at pure DC. Power Mosfets have surprisingly large input capacitances into the nanofarads region and this gets forgotten when you have your DC goggles on. This can make reaction of regulator devices slow to transients and surges, and by the time the device reacts, something has gone outside its capabilities and has been damaged. If you want them to be responsive, you have to have fairly low impedance driver circuits.

Mosfet gate oxide is fragile, ratings are in the 8-20v range for power devices.

Bipolar transistors are also very fragile. Reverse bias of the base-emitter junction is the easiest way to kill one. Reverse b-e ratings are typically not much more than 5 volts. In this direction it's a reverse biased diode, so the resistance is very high, and the capacitance is very small compared to the mosfet. Th what extent the abused device lives depends on the current available when the junction avalanches. Low currents do damage, reduction in gain and Ft happens the transistor will never be the same again. Multiple events do cululative damage. Higher currents will do immediate and total destruction.

You can use the b-e junction reverse biased at low current avalanche as a noise generator. With an RF transistor the capacitance can be low and the output can go to UHF. It's a common trick in amateur radio circles for assessing the noise figure of receivers, but the transistors used this way are ruined for normal use and their noise level changes with use as well.

Back to Mosfets, the original circuit can be modified to be more robust. Firstly it needs a big enough device or bank of devices (with current sharing resistors) to handle the fault condition. The drive resistors and potentiometer resistance will need to come down in value. You need to also recognise that ordinary zeners are not fast. Quite sluggish and quite high capacitance. One trick is to use a properly fast diode to a point which is regulated by a zener, where the zener has a bit of current applied to it to set it up ready for action.

I have tens of thousands of mosfet transmitters out in the field doing pulsed operation approaching 500W dissipation during the pulse. I cannot for a picosecond let any voltage rating be exceeded, or any current rating. The devices have power rating specially for this application. Average dissipation is just a couple of watts. Very little heatsinking is needed, but the devices include enough thermal mass to handle the pulse. Should anything go wrong and the pulse last too long, the device fries. VERY expensively. But, with care, it can be done and millions of device-hours done so far without any design issues. Oh, and they have to survive induced lightning surge tests The pulse modulator mosfet is chopping 50v at up to 12A in nanoseconds. The drive circuit has an impedance of only a few tens of ohms.

So rule number 1: Don't even think about short cuts or 'keeping it simple'

Rule number 2: Mosfet gates are high resistance items, they are NOT high impedance.

Rule number 3: Ordinary Zeners are too thick and slow as bouncers.

Rule number 4: Mosfets have infinite gain at true DC, but this comes crashing down when there are AC components and transients on the loose, so circuits behave quite differently to how we look at them with our DC goggles on.

David
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Old 17th Jun 2020, 7:07 am   #15
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Default Re: High voltage regulator circuit.

And simulate with ltSpice or the like, not perfect but a good “understanding generator”. Can save you from schoolboy errors, saved me many times.
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Old 17th Jun 2020, 4:11 pm   #16
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Default Re: High voltage regulator circuit.

Quote:
Originally Posted by Diabolical Artificer View Post
I'm wondering whether Vds & Vgs are too great. Vds will be 120v Vgs a tad under that.
Vgs will be whatever it needs to be, probably 4 to 5V. It is protected by the Zener, ZD1. You didn't say how much load current you are expecting to pull...

Quote:
I'm also wondering if there's a better protection circuit,
It's OK except for something missing: there needs to be a 1k resistor in series with the base of Q2. Without it, it is possible to blow the base of the transistor, leaving the MOSFET unprotected. But if you add the resistor, your MOSFET will be safe from momentary output shorts. It is fast acting, no need to worry about capacitance or Zener sluggishness, that's just leading you down the garden path.
But it will not surivive *sustained* shorts. If you want to survive sustained shorts then add a 100mA fuse and don't be clumsy.
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Old 17th Jun 2020, 4:45 pm   #17
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Default Re: High voltage regulator circuit.

Use a voltage doubler instead of a bridge rectifier.

You then have some intrinsic current limiting so the MOSFET does not have to work so hard in short circuit situations.
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Old 17th Jun 2020, 6:01 pm   #18
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Default Re: High voltage regulator circuit.

Quote:
Originally Posted by GMB View Post
Use a voltage doubler instead of a bridge rectifier.
Or just add a resistor.
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Old 17th Jun 2020, 6:04 pm   #19
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Default Re: High voltage regulator circuit.

Quote:
Originally Posted by daviddeakin View Post
... If you want to survive sustained shorts then add a 100mA fuse ...
Stating the obvious probably, but I'll mention it anyway. A 100mA fuse will be resistive enough to drop several volts when running at its rated current. So put it before the regulator. And have a think about its rated voltage. Almost all the ones we can source easily and cheaply are rated for 250 VAC. Depending on your raw supply voltage that might or might not be high enough to break reliably.

I speak from experience. I once worked on a large power amp (150 W/ch) which had an HT voltage of ~580V if I remember rightly. The manufacturer had very properly fitted an HT fuse rated for a higher voltage than this. It was about the size of the last two joints of my little finger and each one cost more than a fiver. The fault that the amp came in with, of course, caused it to blow these fuses. To try to keep costs under control I temporarily replaced the fuseholder with a 0.25" x 1.25" open one and fitted a glass-bodied fuse. The first one failed explosively (very exciting !). So I boxed the assembly to catch the flying glass. After a few had gone bang one tracked, allowing the fault current to continue flowing via an arc on the inside of the glass which this time hadn't shattered. "Every day's a school day" as they say.

Cheers,

GJ
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Old 18th Jun 2020, 12:00 pm   #20
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Default Re: High voltage regulator circuit.

AFAIK, the above circuit can be turned into a 'foldback' limiter with just 1 more resistor!

Increase the sense resistor to 68R and connect a 62K resistor from Q2 base to GND.
With the output shorted, 10mA through the sense resistor gives 0.68V which will will turn on Q2.
At a 400V output, 100mA through the sense resistor will give 6.8V but this is divided by 1K/62K turning on Q2.

I didn't build it !
Nor did I sim, YMMV

dc
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