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Old 9th Jul 2020, 10:23 am   #41
dave cox
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Default Re: High voltage regulator circuit.

Andy,
Q5/6 is a long tailed pair (LTP). Q3/4 a current mirror with Q2 a 'current through-er' (cascode)

Effectively, the current source I1 is a collector load for half the LTP. The mirror / cascode are really only there to steer the control current to the right place and avoid issues with the poor performance of high voltage transistors!

dc
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Old 10th Jul 2020, 8:35 am   #42
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Default Re: High voltage regulator circuit.

Ahhh, 1 transistor less and better regulation - replace Q3 with 430R
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Old 11th Jul 2020, 3:14 pm   #43
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Default Re: High voltage regulator circuit.

Thanks Dave.

A.
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Old 13th Jul 2020, 12:10 pm   #44
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Default Re: High voltage regulator circuit.

Thanks Martin for the later Sussex HT/Screen grid supply, however am unsure what those "pads" labelled 1 , 2 & 3 under "JAN8W" (if that is what it is, hard to see), in other words where does the mosfet gate go, through that 1m resistor to...? On the old version it goes to a potential divider, which sets V out, so I presume it's a pot in this case.

I'm pootling away at this in the background, will post again when i have something to report, Andy.
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Old 13th Jul 2020, 8:05 pm   #45
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Default Re: High voltage regulator circuit.

Same confusion here Andy !

I would guess the regulator feeds the divider chain of switched resistors with the gate being connected to the tap (or a pot, as you suggest). In that case the major error contributions will be :
(1) The LR8 regulator (but with a nearly fixed load)
(2) Gate/source voltage drop, at delivered current / temp / device
(3) Current sense series R (I can see 2 but I don't know which is which)

Better, but still not a regulator !

dc
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Old 14th Jul 2020, 7:23 am   #46
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Default Re: High voltage regulator circuit.

Quote:
Originally Posted by Diabolical Artificer View Post
Thanks Martin for the later Sussex HT/Screen grid supply, however am unsure what those "pads" labelled 1 , 2 & 3 under "JAN8W" (if that is what it is, hard to see), in other words where does the mosfet gate go, through that 1m resistor to...? On the old version it goes to a potential divider, which sets V out, so I presume it's a pot in this case.

I'm pootling away at this in the background, will post again when i have something to report, Andy.


The pads 123 go the original resistor chain of the Sussex, the LR8 just replaces the zener chain.

When I built my version I dumped the resistor chain and connected the 1M from the Mosfet gate to the LR8 output. I then used the pot in the LR8 circuit to set the output volts

Its Peter by the way, not Martin.......
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Old 14th Jul 2020, 1:44 pm   #47
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Default Re: High voltage regulator circuit.

Updated design : including TL431 ref, current mirror replaced by common emitter & MOSFET model / output. LTSpice file included.

dc
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Old 20th Jul 2020, 6:29 am   #48
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Default Re: High voltage regulator circuit.

Thanks again Dave, now the amp is finished I'll have some room ont bench to fettle this.

Andy.
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Old 21st Jul 2020, 10:58 am   #49
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Default Re: High voltage regulator circuit.

Hi Dave, I'm breadboarding this circuit today, a few questions. Re V2, why does the base of Q6 have to be at 2.5v, and do I really have to use a TL431, I hate these bl**dy devices with a passion. The resistors are always ones I'll never have so far I calculated 1r and 3.119r with a 2k2 supply resistor, can't I just use a diode instead? A blue diode gives me 2.65v, then i can just tweek R6, yes?

Re D2, does that read 24v?

Re I3, that can just be a resistor yes? I've worked this out to 93.k, so could just use a preset and fixed R. I've sussed I4, I'm using a MJE350 CCS.

Lastly as i understand the circuit if Q5's base sees a change in current, Q6 will go the other way switching on Q4 harder (or softer)that changes V on the gate of the fet. Q2/Q7 regulate Vgs.

At present I'll run this at 100v, then when it's stopped smoking, crank Vin up.

Andy.
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Old 21st Jul 2020, 3:16 pm   #50
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Default Re: High voltage regulator circuit.

Andy, IIRC the TL431 is 2.5V without any resistors!
But yes, an LED would work and adjust the R3 a bit on the other side, it will have the same voltage across it.
I choose R6 to balance the current in the 2 legs of the LTP Q5/6 but its not critical.

I specified D2 at 30V but 24 will be fine. Monitor it in any case, it probably wont be that stable but it should not matter too much !

Yea, if the output voltage goes up too much, Q5 turns off, Q6 on then Q4 on (throught Q2) pulling the mosfets gate down by pulling current through R7.
Note, current can also be pulled through R7 by Q7 if the load is getting too much current.

Good luck Andy !

dc

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Old 21st Jul 2020, 3:30 pm   #51
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Default Re: High voltage regulator circuit.

I think I3 should be about 9K ?

EDIT
Ooops 9K it too high ...
You need to get something like 2mA down each leg of the LTP when the regulator is regulating !
R6 will have ~0.7V across so R=0.7V/2mA so 330R or 390R - ish
'I3' - this has the zener voltage at one end (eg 30V) and the reference voltage (plus a BE junction) at the other and should give 4mA
30V-(2V5+0V7) = 26V8 the R=26V8/4mA so R ~ 6K8 (not 9K although it would probably work)

dc

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Old 21st Jul 2020, 4:27 pm   #52
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Default Re: High voltage regulator circuit.

Well, i've built the circuit, but it doesn't work, Vop sits at around 30v and won't budge. I can't see how adjusting R2 changes Vgate. Is the circuit round R1, R5 Q7 right? As i read it the base of Q7 is fixed by R4/5 divider.

These are the changes and values i came up with, see attached schematic. As mentioned I've had no luck with TL431's so used a blue LED to give 2.5v. I3 I worked out should be 93.3k so used a 100k. At first I powered this up with a 35v PSU limited to 10mA to check that I'd made no connection mistakes. I used a 10k 10 turn pot, and did see some minor adjustment. When i powered it up to 400v I swopped this 10k pot for a fixed 150k and 100k pot, it being the nearest I have to 300k.

Andy.
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Old 21st Jul 2020, 4:51 pm   #53
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Default Re: High voltage regulator circuit.

Andy, 23.1V on 100K is not 2.3mA its 0.23mA!

But I don't think that's the only problem!

It looks like Q4 is off so Q2 should be off as well so what is pulling current through R7 ?
If no current was passing through R7 it should pull the gate right up to the power rail so why isn't it ?

You could try removing Q2, the output should go to within a few volts of the rail.
If it doesn't the Q7 could be turning on to limit the current ...

BTW the MPSA42 can only stand 300V !!
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Old 22nd Jul 2020, 6:32 am   #54
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Default Re: High voltage regulator circuit.

Thanks Dave, right, 230uA, sorry, long day slaving away at the bench : ) Am also aware that the MPSA is only a 300v part, the only other thing I have are 500v mosfet's in a TO47 package, doubt they'll be rated for much current though. Really I could do with dropping 100v ish before it gets to the reg, a tap on the HT winding would have been nice, wish I thought of it when i wound the it, peanut!

Will have another crack at this again today.... is my reading of the circuit about right? Not sure how Q2 & 7 operate, I presume D4 is protection but what's D3 doing? Also wouldn't it be a good idea to bung a 100r on the fet gate, i thought this is good practice with mosfet's.

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Old 22nd Jul 2020, 12:08 pm   #55
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Default Re: High voltage regulator circuit.

Hi Andy,
Starting with Q7 - this is the current limiting transistor. Any output current develops some voltage across R1. eventually this voltage will be high enough so that the Vbe on Q7 makes it turn on. The current through Q7 will pull the gate of M1 (blame LTSpice!) toward ground, hopefully reducing the load current.

It's difficult to follow the effect of R4/R5 in my head, so some simplification is in order ...

Imagine shorting the output. The emitter of Q7 in now clamped to GND. The load current into the short will develop some voltage on R1 and most of that voltage ( actually 180K/(180K+3K) or 98% ) will appear as Vbe on Q7. At somewhere around 14mA into R1 Q7 will be conducting strongly dragging the gate voltage down. So, into a short circuit the current will be limited at this level.

At higher output voltages the current limit will also be higher, this is the REAL purpose R4/R5 (so called fold-back) ...

Now let the output be at 300V and work out the current limit. The emitter of Q7 is at 300V. To limit the current Q7's base needs to get to ~300.7V. Now workout the voltage needed at the junction of R1/R5, this is 300.7*(180K+3K)/180K=305.7V. So one end or R1 is at 300V and the other 305.7V which occurs ~ 83mA

At you say, D4 is protection. For little or no load current, at a high output voltage, R4/R5 can reverse bias BE junction of Q7, this is very destructive so I popped in a diode. I only spotted this via the simulation

dc
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Old 22nd Jul 2020, 12:34 pm   #56
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Default Re: High voltage regulator circuit.

MOSFET's ...
I've not really used these so much, and then almost entirely at sub 30V eg buck convertors, so my practical advice is very limited!! I regularly see gate protection with zener diodes but some people seem skeptical that they would be anywhere near fast enough to offer real protection from a sudden excursion of voltage. On the other hand, the sheer capacitance of the gate to D / S makes this less likely. I don't recall if the Sussex PSU design uses zener protection.

A gate resistor is probably a good idea, as close as is physically possible. What you really don't want here is any inductance.

The same argument could be applied to Q2's base. Common base amplifiers can also oscillate in a similar way and can be cured in the same way with a few tens of ohms to kill any Q. Something to watch out for.

dc
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Old 22nd Jul 2020, 12:42 pm   #57
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Default Re: High voltage regulator circuit.

D3.
If the load is a short circuit, Q7 will turn off M1 by dragging its gate toward GND. Q2's base is stuck at 30V (or whatever) so Q2 gets its BC junction forward biased (abnormal) so D3 stops any current flow. Again, only spotted in the simulation!

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Old 22nd Jul 2020, 1:50 pm   #58
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Default Re: High voltage regulator circuit.

Thank you very much Dave, i now have a better idea of how things work, much appreciated.

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Old 22nd Jul 2020, 2:19 pm   #59
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Default Re: High voltage regulator circuit.

Q2, not sure if I explained this one before ...

It's operating in the 'common base' configuration which can also be called cascode (*). Current fed into the emitter simple appears at the collector (well, most of it). Put like that it seems kind of pointless but the usual purpose is that it mitigates the 'miller effect' of the transistor feeding in current to the emitter since the voltage there barely changes. The cascode transistor itself still gets effected by its BC capacitance (its collector voltage is changing) but it is not being multiplied by gain so its a win-win.

In this 'design' that aspect is probably not necessary. But, if you look at the specification for HV transistors, you will see the gain is very poor (a reason why people like valves for HV?). For this design I was trying to mitigate poor gain by using a low voltage / high gain and high voltage / low gain pair to get around it. Time will tell if that works out

dc


NOTE * Yes cascode and most definitely not cascade. I once found myself joining a lecture given by a really good Czech electronics engineer to 1st year UG's. At the end of the lecture I noticed them scribbling away and mostly looking very puzzled so I inquired what was so puzzling - I thought he was very clear. Turns out that, thanks to his slight accent, they thought he said cascade when he had actually said cascode - which they had never heard of! It was funnier at the time!

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Old 23rd Jul 2020, 8:21 am   #60
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Default Re: High voltage regulator circuit.

The cascode circuit is a very valuable trick to have up your sleeve.

It works with valves, bipolars, FETs.

You can use it to break the Miller capacitive feedback path and increase bandwidth.

You can use a high voltage part on top of a low voltage part. You get the voltage rating of the top, and the better gain of the bottom transistor.

You can have endless fun mixing FETs and bipolars.

You can also 'fold' cascodes.

David
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