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Old 21st September 2012, 09:30 PM   #61
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Default I'll have an error amplifier with that

Getting a little more complete:

Click the image to open in full size.
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Old 21st September 2012, 09:37 PM   #62
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add a low side current shunt and a comparator and you can interface constant current and constant voltage

next thing you know your assembling a steps generator

v4lve
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Old 22nd September 2012, 08:40 AM   #63
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by jackinnj View Post
Getting a little more complete:

Click the image to open in full size.
Nice, thanks. Wouldn't it better to swap the location of the pot. I.e. fixing the divider from the output and the potentiometer varies voltage from 5V reference instead?
Ale
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Old 22nd September 2012, 02:10 PM   #64
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Quote:
Originally Posted by mogliaa View Post
Nice, thanks. Wouldn't it better to swap the location of the pot. I.e. fixing the divider from the output and the potentiometer varies voltage from 5V reference instead?
Ale
The circuit I showed is essentially that appearing in "The Art of Electronics" without the compensation and current limiting. It's just an inverting amplifier.

For the microprocessor controlled HV supply reference was a 10-bit DAC with a 4.096 Volt reference and some level shifting to get the MOSFETs to respond linearly to the DAC. I stated my preference to NOT use a potentiometer in the HV voltage divider. Select a value which works and solder it in.

I only ran mine in a range of 0 - 400V, so ymmv.
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Old 23rd September 2012, 02:48 PM   #65
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Hi Ale
my 5 cents
valve power supply:
power supply _1.jpg power supply _2.jpg
Vyacheslav
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Old 7th October 2012, 07:57 AM   #66
mogliaa is offline mogliaa  United Kingdom
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Have reworked circuit to include the voltage feedback regulator as per latest suggestions.

Voltage regulation is now done on the +5V reference side. I have a nice 10T 100K pot that will help in smooth setting of the output voltage.

Q1 is now BUJ302A (1000V NPN) to ensure the pass section is bulletproof. Probably don't needed, but got it at hand and want to ensure that the IRFPG40 and BUJ302A survive any accidental output short.

I have added R12 and C3 to improve the HF response of the circuit.

This one simulates quite well, so will give it a go unless anyone spots anything to improve. I will need to etch a small PCB to fit this out.
Thanks for helping out so far!
Ale
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Old 13th October 2012, 10:33 AM   #67
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Hi Ale,

Yes, I can vouch for the Horowitz & Hill Circuit, too. Vouch for its operation, that is, but if you connect it to a 300BSE amplifier - it absolutely destroys the sound! As in - it removes every nuance from the music.

But for testing things it will be fine.

Some ideas:

M1 needs a gate stopper too. On high voltage supplies I prefer the value of stoppers to be 75 ohm to 220 ohm). connect nothing at all to the gate side of the stopper (it's there to isolate the antenna-effect of the gate wiring - adding zener leads etc at this point will defeat the stopper action, to some degree).

M1 gate can be protected with only one zener, cathode to gate.

Is M1 in a TO-247 package? for a variable supply, M1 might have 500V across it, and so you need all the power handling you can get. Also, TO-247 isolates HV better.

100pF Wima FKP2 across R7 may be helpful to prevent phase lag at the opamp feedback-input. We'll need a 12V zener across R8 to protect the opamp, especially if that cap is fitted.

- R9 will probably need to be lower than R10, to make sure the gain is below 1 at the frequency where phase-margin vanishes. Maybe 2,2K.

Even with the opamp gain controlled at high frequency, the gain of M2's stage is too high to be sure at HF. If you have a high-voltage RC series network across R2, you can tailor the response after building the circuit (test with pulsed loads, adjust the Zeros' RC values to get the neatest response.)

- normal resistors have limited voltage durability - even 2W types are only good for 500V. Making R7 out of 2x 2W series-connected parts
(if you need small size, the Welwyn LHVC resistors give 1600V durability for small size and price:

LHVC2010-1MFT4 - WELWYN - RESISTOR, 1M 1% 2010 | Farnell United Kingdom

using the same selection for R2, if the supply has a wide adjustment range.

I would be tempted to put a ferrite chip-bead in M2 drain, in case that decides to self-oscillate. Murata BLM21A600S maybe.

Somewhere, I may have the drawing of the version of this circuit that I made, including all the stability zero values - I'll look for it, if it's helpful.
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Old 13th October 2012, 11:24 AM   #68
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Another subtle feature of the Horowitz & Hill circuit is a diode (1N4148) between the output (anode) and M2 drain. This gives the regulator the ability to sink current - very useful for reactive loads!

Using a BZX55 zener in this position (Vf = 0,8V@20mA, Vishay data) may suffice - you can remove the other zener without loss of any performance.
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Old 13th October 2012, 11:34 AM   #69
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by Rod Coleman View Post

But for testing things it will be fine.
Hi Rod,
Yes, I will use this for testing stages and circuits, nothing else. I'm aware that these type of feedback regulators don't sound well.
Quote:
Originally Posted by Rod Coleman View Post
Some ideas:
M1 gate can be protected with only one zener, cathode to gate.
Is M1 in a TO-247 package? for a variable supply, M1 might have 500V across it, and so you need all the power handling you can get. Also, TO-247 isolates HV better.
Yes, it is. And I bought an insulation pad to cut it to fit the TO247 as couldn't find a proper insulator for this package

Quote:
Originally Posted by Rod Coleman View Post
100pF Wima FKP2 across R7 may be helpful to prevent phase lag at the opamp feedback-input. We'll need a 12V zener across R8 to protect the opamp, especially if that cap is fitted.
Yes, missed cap out despite seeing it on the text book!
Quote:
Originally Posted by Rod Coleman View Post
- R9 will probably need to be lower than R10, to make sure the gain is below 1 at the frequency where phase-margin vanishes. Maybe 2,2K.

Even with the opamp gain controlled at high frequency, the gain of M2's stage is too high to be sure at HF. If you have a high-voltage RC series network across R2, you can tailor the response after building the circuit (test with pulsed loads, adjust the Zeros' RC values to get the neatest response.)
What do you mean here, make provision in the PCB to fit an additional RC in parallel to R2?

I have amended circuit diagram incorporating your suggestions...
Thanks a lot!

Ale
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Old 14th October 2012, 10:45 AM   #70
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I did assemble and test a very similar circuit with the same topology in the past. Found out that it is better to use an NPN transistor (e.g. MPSA 42) instead of the mosfet in place of M2 because of its lower cut-off current, but then of course you are limited by the Vce max which is 400v.

Also, the diode bridging the source and drain of the series regulator should stand the B+ - Vout(min) difference, hence an 1N4148 is not suitable here. A regular 1N4007 will do fine.

Lastly, the choice of the opamp is quite critical if you want to be able to regulate the output close to the bottom rail. You must make sure that under no circumstance the opamp will latch (look for bottom rail common mode latch free types). Speed is not so much of a problem for most practical purposes. The popular LM358 if a good choice too.

You might also want to play with a "speed up capacitor" in parallel to the upper half of the voltage sampling resistive divider. It decreases ripple at the output, but at the same time slows down transient response. A multi-step simulation will easily reveal a good tradeoff value for it.

Cheers
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