0-350V 0.2A Bench Power Supply

[T3sl4co1l]

How will you use you power supply with 350V output if voltage drop on Mosfet pass transistor is only a few volts ? How will you detect a shorted pass transistor ?

Simple -- crowbar based on the error amplifier output.

If it's commanding a low output, yet the output is high, the pass device is cacked.

If the output is high, but it's commanding even higher, it's saturated.  (It might still be failed, who knows -- you'll find out when it comes out of saturation.)

You'd also delay this decision, and add an exception for low command * low output, to avoid nuisance trips.

It's just a comparator or two, a couple transistors, and a handful of resistors and a capacitor. 

Tim

[oldway]

The tubes are not dead, there are many applications where the tubes could never be replaced .... You use every day a microwave oven with a magnetron tube and you do not complain .

Just because you do not know anything about this technology does not mean you have to neglect it.

I know everything about this technology, from the underlying physics to the electrical characteristics of many standard parts.

I know better than anyone why tubes are nearly forgotten today, and why solid state has surpassed it in all but a few (electron-wave physics) applications.

Magnetrons are one, I gave another example (TWT) which I guess you didn't pick up on.  Particle accelerators use klystrons, and more broadly speaking, any particle accelerator is itself a large, very complicated tube filled with vacuum, a vacuum tube if you will.

They only remain useful in niche applications.  Everything from DC to UHF has been superseded.  I'm not sure if GaN FETs will ever be cheap enough to replace the microwave oven magnetron, but it'll be an interesting day if it comes.

A tube failing shorted (NB: between anode and cathode) ? .... it is extremely rare, I have never seen one in my life .... And on the tubes with connection of the anode on the top of the tube it's just impossible.

Er, well... screen and grid, let's say. Not a healthy situation for any of the circuits in this thread.

Cracked or melted glass also lets in gas, which acts to increase leakage or short out (sparking) electrodes.

I don't get you, man.  Just because you can't imagine it, it doesn't exist?  What kind of thought process is that?  That's an extremely narrow, unimaginative way to go through life.

Tim

I'm old enough to be your father and I learned electronics at a time when almost all devices used vacuum tubes.
I started repairing TVs at a time when color did not exist yet and whe were using 90 °  black and white cathode ray tubes with an ion trap.
I lived the time of the tubes, so do not try to give me lessons in this area, I have an experience you will never know.
The reality is that you are prejudiced against tubes simply because it is old technology.

You still have not posted any diagrams on your 0 to 350V 200mA power supply solution ..... you only criticize without bringing any practical solution.

At least, the solution I propose is simple and safe, which is perfectly suited to a realization by someone who, as he admitted, knows almost nothing in electronics.

[T3sl4co1l]

The reality is that you are prejudiced against tubes simply because it is old technology.

I get that you have lots of experience, but I'm not seeing a reason why that should be applicable to design.

You know how to repair any damn TV or radio, I get that.  But we're talking design here, and we're talking solid state -- both things that -- I guess you admit? -- are at best, on the edge of your scope of knowledge?

Your knee-jerk reaction about my experience is based on your prejudices, not mine; you could at least do me the respect of looking up some projects on my website, like this,
https://www.seventransistorlabs.com/Images/DistAmp2.jpg
or this,
https://www.seventransistorlabs.com/Radio_20m/
or even this, somewhat jokingly as you'll see from the design approach:
https://www.seventransistorlabs.com/Images/Borg.gif

You still have not posted any diagrams on your 0 to 350V 200mA power supply solution ..... you only criticize without bringing any practical solution.

A rejection of criticism is rather abhorrent to the ideals of this democratic process of development...

Several of the diagrams above are already close enough that I don't need to draw one up myself.  A reading of my posts shows this: I have noted that some are going in the wrong direction, while some are very nearly complete.  None have been exactly satisfactory, but there's no accounting for taste.

I'd gladly draw up a proper design -- simulation and BOM included -- if sponsored for it.

I welcome any constructive criticism of my ideas.

Tim

[oldway]

I'd gladly draw up a proper design -- simulation and BOM included -- if sponsored for it.

Ask this to 001 ..... good chances, ... he already complains for a few dollars difference between the price of a PL509 and an EL34 ..... 

[David Hess]

The crowbar circuit protects against a pass element short or control circuit failure where the output voltage rises all the way to the input voltage.

How will you use you power supply with 350V output if voltage drop on Mosfet pass transistor is only a few volts? How will you detect a shorted pass transistor?

In a high voltage power supply, I would expect more than just a few volts of difference at maximum output.  It is not like efficiency is at a premium so a minimum difference can be enforced to make a simple crowbar circuit work reliably.

There are alternative ways to detected a shorted pass element as T3sl4co1l suggested.

[Zero999]

How will you use you power supply with 350V output if voltage drop on Mosfet pass transistor is only a few volts ? How will you detect a shorted pass transistor ?

Simple -- crowbar based on the error amplifier output.

If it's commanding a low output, yet the output is high, the pass device is cacked.

If the output is high, but it's commanding even higher, it's saturated.  (It might still be failed, who knows -- you'll find out when it comes out of saturation.)

You'd also delay this decision, and add an exception for low command * low output, to avoid nuisance trips.

It's just a comparator or two, a couple transistors, and a handful of resistors and a capacitor. 

Tim

Yes a crowbar circuit could be added to the error amplifier, but that won't protect against the adjustment potentiometer failing open.

...Just about any semiconductor will fail short circuit. I don't know about valves.

Why is that a safety hazard? If the bench power supply is capable of giving a lethal voltage, then the best safety practice is to assume the output voltage has the potential to be lethal, even if it's set to a non-hazardous voltage. Never touch the output of a bench linear power supply which is powered by a lethal voltage on the other side of the pass element!

The same principle applies to a mains circuit switched of with a TRIAC or thyristor.

It's exactly the opposite ... The safety rules require that if a power supply is adjustable from 0 to 350V, it can be used with 12V output without any risk for the operator.
A well-designed 0 to 350V power supply meeting basic safety principles can not be more dangerous when set to 12V than a simple 12V power supply with 7812.

What safety rules?

If a power supply is adjustable, up to a hazardous voltage, then the operator should beware that it can fail, causing the output to rise to the maximum possible voltage setting, plus a bit more. As long as the operator is aware of this fact, then there's no additional risk, other than a few blown components.

By the way, the LM7812 can fail short circuit input-to-output too, although it's not a safety hazard, because the maximum rated input voltage is under 35V.

[Neomys Sapiens]

I'm not sure if there are no other solutions: my Heinzinger TNs300-120 does 300V 400mA with 2 transistors and a thyristor preregulator. And it is compact as well as totally reliable.

[floobydust]

Faintly tried a LTSPICE IV simulation to estimate how much current IRFP460's would experience and the 2N3904 current-limiter, transient under a short-circuit.
Before this thread exceeds its own SOA 

I find you really need the parasitics - lead inductance, ESR, ESL and even then I'm getting pretty much bullshit results.

At first it's what I expect... the 2N3904 seeing 1.7Apk discharging the MOSFETs' gate capacitance, and IRFP460 seeing around 3-4Apk drain current (1.3kWpk) each, for nanoseconds.
But changing output capacitance C2 from 0.22pF to 0.22uF drastically changes those values; 2N3904 seeing 0.039Apk discharging the MOSFET's gate capacitance, and IRFP460 seeing around 0.2Apk drain current, at C2=0.22uF

So the my simulation bullshit detector  went off and I stopped.
Perhaps someone can explain what is going on. Strange SPICE results, but I'm not expert-level with it.


I notice the current-limit setpoint (2N3904 vbe) is sloppy as it seems to be working a bit too hard here. I think Zetex parts are better suited.
Sims missing MOSFET lead, gate resistor and PCB inductance.

I drew up a design using my own SS power supply regulator scheme that works well for me, but hold I'll off until flames and smoke subside, lol.

[David Hess]

Yes a crowbar circuit could be added to the error amplifier, but that won't protect against the adjustment potentiometer failing open.

Always wire the potentiometer so that an open causes a low voltage or current fault.

[T3sl4co1l]

At first it's what I expect... the 2N3904 seeing 1.7Apk discharging the MOSFETs' gate capacitance, and IRFP460 seeing around 3-4Apk drain current (1.3kWpk) each, for nanoseconds.

Quite a lot better than 40A, wouldn't you say?   Well within SOA too, of course.

But changing output capacitance C2 from 0.22pF to 0.22uF drastically changes those values; 2N3904 seeing 0.039Apk discharging the MOSFET's gate capacitance, and IRFP460 seeing around 0.2Apk drain current, at C2=0.22uF

Well, if you're going to go beyond the semiconductor core and include bypass caps and stray inductance, you'll see a lot more.

Nothing that can't be calculated.  0.2uH + 0.22uF is about 1 ohm and 760kHz.  So the FET source will be pulled down in about 1/4 wave of that or 330ns.

The switch will see a peak current of around 350V / 1 ohm = 350A, but only for that fraction of a microsecond as the cap is discharged.  What do you expect, you put a cap across the output and of course it's going to spark when you short it out.

If you want a supply that's so fast and current limited that you could connect an LED to it without damage, well... you can't actually, because of drain capacitance.  With lots of series resistance, you might be able to get peak current * pulse width manageable, but anyway this sort of extremely cautious output characteristic is unusual for high voltage supplies, for this very reason, and not really very useful (most things you'll hook up to a power supply will have their own local bypass caps, so, who cares?..).

So the my simulation bullshit detector  went off and I stopped.
Perhaps someone can explain what is going on. Strange SPICE results, but I'm not expert-level with it.

Slower rise time suggests more time for the current limiter to respond, and less peak current in both 2N3904 and IRFP460.  Seems fine to me. 

Tim

[floobydust]

It does look like slowing down dV/dt on the (source) output is really lowering the stress on the MOSFETs and current-limiter.
I was out to lunch on the 40Apk by a factor of 10, not bad really but the instantaneous power blip can be extremely high.

I'm used to some output capacitance for regulator stability. It needs to be >0.1uF to slow things down, but adding >33uH inductor also works and gets rid of that sparking cap. It just adds some back-EMF and resonance to dampen.

On the SOA thing, it was also discussed in EEVblog #1023 - Rigol DL3021 Electronic Load Teardown.  The IRFP250N's Rigol using qty. 6 for 40A/150V, qty. 8 for 60A/150V and discussion about HP load. To have some idea of derating required in the absence of DC SOA specs.
Link in that thread of NASA (SOA related) MOSFET failures paper is quite a good read.

Everything is saying SOA curves are antiquated, fudged by many manufacturers, the semiconductor (failure) physics are not fully understood hence no accurate model exists, and the push to switching applications has made parts weaker in linear operation.

[Zero999]

Yes a crowbar circuit could be added to the error amplifier, but that won't protect against the adjustment potentiometer failing open.

Always wire the potentiometer so that an open causes a low voltage or current fault.

How? It's possible to connect a potentiometer like that, so it fails low, if the wiper contact fails, but not if there's a break in the track.

I suppose a good quality wire wound potentiometer could be used, which is extremely unlikely to go open circuit, if it's used at a fraction of its power rating.

[T3sl4co1l]

Yes a crowbar circuit could be added to the error amplifier, but that won't protect against the adjustment potentiometer failing open.

Always wire the potentiometer so that an open causes a low voltage or current fault.

How? It's possible to connect a potentiometer like that, so it fails low, if the wiper contact fails, but not if there's a break in the track.

I suppose a good quality wire wound potentiometer could be used, which is extremely unlikely to go open circuit, if it's used at a fraction of its power rating.

Potentiometer paranoia concerns the wiper going open -- which is the typical case when you have a pot going scratchy.  Scratchy pots are more common with DC bias, a prime concern here.

Usual method is simply a resistor from wiper to whichever side is safe.  In this case, wiper to VOUT.  The value is large enough that the pot dominates the adjustable range (so as not to noticeably distort the voltage(angle) characteristic), and with the divider values so that it pulls to a safe value when the wiper opens up (or any other part of the pot, if possible -- which should also be somewhat reasonable in this case).

Tim

[oldway]

I made it
It works
Thanx

I'm stunned, a masterpiece, you're a genius, an Einstein of electronics, you even invented a triode circuit with G1 grid floating .... 

I bow, you are too strong .... And the next invention, what will it be? A space ship, a trip back and forth on the moon? 

Let's be serious ! Have I not explained to you why you have to use a pentode and not a triode (nor a pentode connected in triode) to limit the current in the event of a short circuit at the output?

Have I not provided you with a schematics that can control the pass pentode by opto coupler so that all voltage regulation and current limiting can be done by operational amps, with great accuracy and stability?

How will you reduce the voltage below 200V if you have two zeners of 75V, or 150V, in the cathode of the driver tube?

What will happen if you short circuit the output terminals? Did you even try it?

What's this long tail diagram (differential amplifier) with the grid 1 of one of the two triodes "in the air"?

I am asking me why whe are loosing our time to try to help you..... 

[Ian.M]

Meanwhile I've been contemplating how to minimise the cost of the transformers for a 2x PL509 based HV PSU, 0-500V 0-250mA, without doing *STUPID* things like hoping that the insulation between two secondaries of a modern off-the-self low voltage transformer is good for 600V! 

I don't see much if any benefit going to a 6.3V heater tube like EL509 or EL84 for the pass device as then you'll need another transformer (or switching PSU) for the semiconductor control electronics.

Its going to need a 20-0-20V transformer for the heater supply, with its secondary  floating at the output voltage so it doesn't exceed the heater-cathode breakdown voltage. If you make the filament transformer 50VA, its got enough spare current to drive a 110V:18-0-18V 10VA transformer backwards to get your floating (cathode referenced) screen grid supply, and a bridge rectifier and a positive and negative regulator for a floating +/-15V supply for the control electronics.   Unfortnately you cant run grounded control electronics that way.

The negative grid bias required can be got by a charge pump off the main HT transformer and stabilised by a chain of 33V 1W Zeners,  with a pull-down resistor to the bias supply so the default pass device state is hard off,  with a cascode chain of KSA1156YS 400V PNPs for the control circuit to pull up the grid from down around -200V to the max output voltage of 500V.   There's probably something better (probably a MOSFET) that can do the grid drive in one device, but availability and leakage current are serious issues.  I wouldn't be adverse to opto-coupling the control signal down to the negative bias rail.

Not sorted out yet is tap-changing or thyristor preregulation to avoid the need for more than two 'bottles',  and the resulting specs for the HT transformer to give approx 600V unreg when the output is at 500V, though if possible I want to use a 220V:220V isolating transformer (for easy off-the-shelf availability) and a voltage doubling rectifier circuit. I also haven't done any significant work on the floating regulator control circuit and current limit.

[oldway]

Using PL509 instead of EL34 is nonsense for the following reasons:

- The PL509 was a flyback pentode for color tv, it is no longer manufactured and, at least in Europe, it is more expensive than an EL34.
EL34 is still manufactured for tube amplifiers and will continue to be manufactured for a long time.

Its anode / cathode isolation is sufficient since it was used in public address amplifiers with 700V of anode voltage, which means at least 1400V peak in operation

By using two tubes in series, a heating voltage of 12.6V can be used. In practice 12V is enough .... There are hundreds of 12V used transformers for sale for a few bucks because everyone changes the 12V halogen lights by leds ....

For the G1 polarization voltage and G2 power supply, just a second transformer 12V / 110 + 110V powered by the 12V output voltage of the first.

For the anode voltage, we provide 2 ranges, one from a 400V winding, the other from a 230V winding.
Range switching is manual.
We only use transformers with one secondary....isolation between one secondary and another does not apply.

Isolation between primary and secondary must be at least 2400V in every commercial transformer. It is fully safe !

Do not use a voltage doubler because the impedance is higher, so the voltage drop too .... Prefer a bridge rectification.

[chris_leyson]

Well done 001, hey I like the floating grid I'm still going to have a stab at the T-reg with a pair of EL34s with screen grid supply and not wired as triodes, found a little note on that by the way http://www.pmillett.com/hv_bench_supply.htm and no, I'm not going to build it ). Found a 350V 200mA transformer intended for the Maplin 4-20 amplifier so will use that and the 6.3V 5A winding will go to the front panel. Just need a small 6V heater transformer for the EL34s and another for the variable C- supply. BTW is there any harm in using a 6V rather than 6.3V transformer for the EL34 heaters, will a 10% drop in power matter ? Will fix the current limit at 150mA just in case. Would be nice if I could find a single transformer to do the job but I don't see that happening, it's getting harder to find harware for tube projects these days. It's been an interesting topic.

[Ian.M]

Using PL509 instead of EL34 is nonsense for the following reasons:

- The PL509 was a flyback pentode for color tv, it is no longer manufactured and, at least in Europe, it is more expensive than an EL34.
EL34 is still manufactured for tube amplifiers and will continue to be manufactured for a long time.

Unfortunately the EL84 iis only rated for 12W max anode dissipation and 65ma cathide current.  As an EL509 or PL509 is rated for 30W max anode dissipation and 500mA max cathode current, as long as one can get an adequate supply of N.O.S tubes without paying silly prices, designing in a pair of PL509 tubes is far from nonsense.  Yes they need higher bias voltages, but it will take five EL84s to do the same job.

If you can suggest an affordable beast of a power tube that's still in production as an alternative to the 509s, I'm all ears, but please don't bug me unless its rated for 600V, 25W Pa and 150mA Ik.

[oldway]

EL34, not EL84......

[Ian.M]

Thanks, I *was* being stupid,    EL84 EL34 design center ratings of  800V, 25W and 150mA are much more what's needed.

[oldway]

EL34 or 6CA7 ratings.

[Ian.M]

Yes. I grabbed the Mullard 1962 EL34 datasheet, so I think we are on the same page at last. 

What would you suggest for optimum  Vg2-k bias?   A nominal 110V RMS winding as you suggested earlier, would give peak 155V so around 140-150V after filtering it to smooth it.

What sort of G2 current is that likely to give?
Also how much headroom on Va-k does it need to prevent it going into G2 toaster mode?

[floobydust]

Bigger tubes presently in production equipment like guitar amps, might be easier towards a single power tube.
KT-90 or KT-120; ; 6550 (KT-88) is a bit under spec.
SPICE models of some here: http://tdsl.duncanamps.com/dcigna/tubes/spice/index.html

[oldway]

Yes. I grabbed the Mullard 1962 EL34 datasheet, so I think we are on the same page at last. 

What would you suggest for optimum  Vg2-k bias?   A nominal 110V RMS winding as you suggested earlier, would give peak 155V so around 140-150V after filtering it to smooth it.

What sort of G2 current is that likely to give?
Also how much headroom on Va-k does it need to prevent it going into G2 toaster mode?

I'm planning a G2 current of about 5mA/tube.

II do not have anode current / anode - cathode voltage curves for a G2 voltage equal to 140V, only for G2 voltages of 250V and 350V.
But I expect an anode current limited to about 200 mA with G1 at 0V and with a G2 voltage of 140V.

The minimum expected cathode / anode voltage (headroom) is 100V, so there is no risk of G2 overheating.
In addition, a current limiting resistor of 120 to 180 ohms is provided in series with each G2.

About power tubes, there are a lot who can be choosed.....the lower cost and most easily available are: 6V6, 6L6, EL34, KT88

NB: It is recommended to use 12V heating voltage (two tubes in serie) as 12V transformers are cheap and easily available.

KT120 is nonsense....It cost more than 100 $US ... An EL34 cost only about 15 $US

[CopperCone]

http://www.ixys.com/Documents/Articles/Article_Linear_Power_MOSFETs.pdf

ixys knows power mosfets pretty good

check out IXYS Linear L2™ series.

I would not buy ebay power components