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Old 14th February 2011, 07:52 PM   #1
tomchr is offline tomchr  United States
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Default High Voltage Regulators (Maida or zener)

Folks,

I'm having an interesting issue with my high voltage, positive Maida regulator. For schematic see attachment to this post.

In the schematic you'll notice a positive and a negative Maida regulator. The negative works fine. It delivers a rock solid -160 V as adjusted.

But the positive regulator has a few issues as built. In its original state (as drawn in the schematic), it has trouble regulating above 380~385 V at light load. I figured it needed more current to regulate properly. I also needed to change the output voltage to 470 V, so I needed to make changes anyway...

To get 470 V out, I use an Antek AS-4T430. That gives me 600 V rectified DC (no load). All the caps have been replaced by two caps in series with a 680k in parallel with each cap to allow the voltages across the caps to split evenly.

I used 100 kOhm for the resistor from ADJ to ground. This should allow 4.7 mA to flow through the regulator. According to the National LM317 datasheet, this should be plenty. However, the regulator failed to regulate. I had to increase the current to over 15 mA before it would regulate properly. I'm wondering if the high resistances cause the LM317 to run out of loop gain or something... Anyway. I finally got the stinkin' thing to regulate.

I plugged it into my amp (see this thread for a rough schematic). And the silicon in the regulator went *poof*. I'm not sure it likes starting up into a 100 uF "short circuit".

My question is two-fold:
1) Is the Maida regulator suited for these high voltages at all? 600 V in, 470 V out. Is it normally capable of starting up into a relatively high capacitive load (100 uF) at these voltages?
2) If I'm required to burn 10 W in the ADJ-ground resistor to make the Maida regulator work, I might as well use a zener regulator. I'm thinking a 10x 47 V stack, each diode with its own 1200 uF/63 V cap (because I have a bunch) and a source follower on top. Are there any common pitfalls to this solution?

I will give the Maida another try. I'll replace the 6.2 V zener with a 15 V to give the LM317 more headroom. We'll see where this leads...

~Tom
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Old 14th February 2011, 08:15 PM   #2
SY is offline SY  United States
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You're on the right track. D1 should be at least 12V, especially with a Darlington. The output cap can be larger than 10u if you have the series resistor that you show. It seems weird given the commodity nature of the 317, but I've run into fakes- it might be worth double-checking. And you might put a protection diode from the output to the input. Scope the output to make sure you're not oscillating. And for layout, you want that 220R and the pot very tight to the output pin.

You do NOT want that 100uF load on the reg! It will fail 100% of the time. The regulator replaces that big cap.
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Old 14th February 2011, 08:52 PM   #3
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Tom, The circuit looks mostly OK to me, but my suggestions would be:

- Is the problem in the Darlington follower? Since the load seen by the darlington is a regulator, you could get oscillation during transients [of the Real part of the input impedance goes negative kind]. Especially with only 27 ohm in the emitter. I think that adding 120 ohm between the zener and the Darlington base might improve the margin on that.

- During charging of the 100uF, does the SOA of the follower transistor get violated, or does it avalanche? It might see nearly 600V and the peak charging current simultaneously. At high voltages, the peak power handling for a BJT is much reduced.

- Regulators like the 317 are not always happy working into low ESR capacitors. The high ESR of ordinary electrolytics forms an ultrasonic zero that prevents loop gain falling to one too soon and provoking oscillation. 1.0 or 2.2 ohms at the output of the regulator can help, if low ESR caps are used, or if MKP/MKT or other caps are used as a bypass.

Personally, I think that the risk of destruction during transients makes a 40V part somewhat a liability in a 600V circuit.

Note also that the output impedance of a 317 rises quickly with frequency (ie it looks inductive), and the slope of this effect varies with load current in a nonlinear way. All this plays havoc with the supply **when used with impulsive loads**, and can lead to noise whose spectrum bounces around with the music programme.

The Zener power follower presents no real problems in use, but a Power FET is really needed for 600V, for SOA margin. One sure precaution is 120 ohms right on the gate, and a G-S zener of 12V. With 600V, this stopper resistor should be lower max. value than in low voltage applications.
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Old 14th February 2011, 09:07 PM   #4
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Quote:
Originally Posted by Rod Coleman View Post
Personally, I think that the risk of destruction during transients makes a 40V part somewhat a liability in a 600V circuit.

Note also that the output impedance of a 317 rises quickly with frequency (ie it looks inductive), and the slope of this effect varies with load current in a nonlinear way. All this plays havoc with the supply **when used with impulsive loads**, and can lead to noise whose spectrum bounces around with the music programme.
FWIW, I've never lost one in day to day use (fooling around in the lab, sure, burned plenty), and I've built close to a hundred of them. Used them in preamps and power amps, 200-600V.

The whole point of the output RC is to null the inductance; with 4-5 ohms and 40-50u, the output impedance is low (for this use) and flat to well above where the output transformer has given up.

A 100u load with no series resistance is the #1 culprit here, guaranteed failure mode. If Tom pulls that out and beefs his 10u cab up to 47u or so, all will be copacetic.
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Old 14th February 2011, 09:50 PM   #5
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SY: Picture this (or have a quick look at the schematic): SET output stage, Loftin-White(ish), with a 47 uF from the cathode of the tube to B+. Two channels, 47 uF each = 100 uF (give or take). But they do have a 660 ohm resistor in series, so the total impedance seen by the power supply on start-up should be [100 uF + 330 ohm]. 330 ohm at 470 V.... That's over an amp. I can't really do anything about that as I'm not willing to change the Loftin-White so another topology. And the regulator didn't have issues at 375 V so it should be possible to make it work at higher voltages as well.

Rod: It does seem like the BJT has failed. I'm not sure about the LM317. I'll need to wire it up in a low-voltage circuit to verify. The original Maida had a darlington pair in there as well. You could use a single BJT, but at 100~200 mA output currents, you're looking at several mA in base current for a typical power BJT. This will lead to significant power burn in the base-collector resistor (series resistor for the zener). I'm not sure if it has avalanched or not. It seems to be the B-E junction that's fused to a short. I'll need to double-check. The argument that a 40 V device should not be used in a 600 V supply doesn't really hold water, though, I do understand where you're coming from. That's one thing I like about the zener regulator. All devices are rated for operation at Vin(max) or more. But on the other hand.... If low voltage devices shouldn't be used in a high-voltage design, we shouldn't use LEDs for bias either...

I'll give it another whirl tomorrow. We'll see where it goes. I'll keep you posted. Thank you very much for the initial sanity check.

~Tom
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Old 14th February 2011, 10:03 PM   #6
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Tom, B-E short sounds like a possible B-E reverse overvoltage event. Turn-OFF transient event?
I wonder if an antiparallel diode might be worth including.
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Old 14th February 2011, 10:07 PM   #7
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Rod, I'd agree about the diode. And in this case, a nice 1kV MOSFET as a pass device might be a good idea if the bipolar isn't standing up to the stress.
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Old 14th February 2011, 10:22 PM   #8
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Hey All,

Is there a high voltage regulator PC board or kit that would work with a power amp? I don't know from transistors but from what I've seen they don't look like they would work point to point. I've seen schematics for Salas and Maida. But a PC board would make it much easier.

Kevin
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Old 14th February 2011, 10:24 PM   #9
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Jan Didden had an excellent one- you might look over at Linear Audio to see if they're still available.
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Old 15th February 2011, 12:34 AM   #10
tomchr is offline tomchr  United States
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Quote:
Originally Posted by SY View Post
Rod, I'd agree about the diode. And in this case, a nice 1kV MOSFET as a pass device might be a good idea if the bipolar isn't standing up to the stress.
Hmmm..... Typical pinout: B-C-E for power BJT; G-D-S for MOS. One could drop one in there and try it out. No base current to worry about then.

I have a diode connected in reverse across the output but not one that goes from output to input of the entire regulator (anode at the output). I should be able to get away with just one diode from out to in rather than one across the BJT and one across the LM317, correct?

I saw your comments about the layout earlier. It's pretty tight, actually. The total trace length from OUT to ADJ is on the order of 20 mm if that. It's dominated by the passives in the path.

~Tom
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