Low voltage op amp to control high voltage series pass transistor - Page 1
Forum section: electronics / projects

author: tree
author_rank: Regular Contributor
timestamp: September 12, 2013, 12:46:55 am
content: I am in the middle of designing a low drift and low ripple power supply for myself. To control the output voltage, which ranges from 0 to 20V, I am using an op amp whose output is fed into a Darlington pair of power bjts. The output of the Darlington pair is both fed back to the op amp's inverting input (i.e. the op amp is configured as a voltage follower) as well as to the load. The desired output voltage is present on the non-inverting input to the op amp as a reference. I have a transformer that produces +/- 40V rails so I am using those to power the op amp. I have a high voltage op amp but recently, the op amp was fried even though it can be powered from +/- 45V. As a result, I have decided to use a low voltage op amp to control the +/- 40V rails. The problem is, I have no idea how to go about doing this. I have tried using single transistor amplifiers to translate 0 -> 5V to 0 -> 20V but have had no luck. So the question, how do I use a low voltage op amp that is powered from +/- 5V to control +/- 40V rails so that my output ranges from 0 to 20V?

author: c4757p
author_rank: Super Contributor
timestamp: September 12, 2013, 12:50:28 am
content: PNP transistor regulates the rail. Op amp drives an NPN transistor rated for the voltage which sinks base current from the PNP transistor. Reverse PNP/NPN for the negative rail. Warning - the gain will be high, so it will be difficult to stabilize.Alternatively, you could look into a floating design like what is used in the LM317/LM337.

author: tree
author_rank: Regular Contributor
timestamp: September 12, 2013, 01:03:17 am
content: Quote from: c4757p on September 12, 2013, 12:50:28 amWow. Thank you. I can't believe I didn't see that despite trying a very similar connection.It worked in a simulation. Time to build it up and test and attempt to stabilize.

author: dannyf
author_rank: Super Contributor
timestamp: September 12, 2013, 01:04:36 am
content: You are essentially trying to implement a cheap 3-terminal regulator.QuoteI don't think you can do that.QuoteWhat kind of opamp is that?

author: tree
author_rank: Regular Contributor
timestamp: September 12, 2013, 01:06:07 am
content: Quote from: dannyf on September 12, 2013, 01:04:36 amWhy can't I do that?OPA445

author: c4757p
author_rank: Super Contributor
timestamp: September 12, 2013, 01:44:32 am
content: You can do that.Jesus Christ that is one overkill op amp for a power supply.

author: tree
author_rank: Regular Contributor
timestamp: September 12, 2013, 01:56:01 am
content: Quote from: c4757p on September 12, 2013, 01:44:32 amYes it is. Although the magic smoke escaped when I connected it to the +/- 40V rails.

author: c4757p
author_rank: Super Contributor
timestamp: September 12, 2013, 02:07:06 am
content: That's some expensive smoke! How'd it smell?

author: tree
author_rank: Regular Contributor
timestamp: September 12, 2013, 02:37:27 am
content: Quote from: c4757p on September 12, 2013, 02:07:06 amI smelled electrons!

author: edavid
author_rank: Super Contributor
timestamp: September 12, 2013, 02:41:46 am
content: If the output is only 20V, why not use an ordinary op amp with say +25V, -5V supplies?Why a voltage follower? Either the error amp or the reference is going to need some gain... why not the error amp?If you do end up with a discrete voltage gain stage, I suggest keeping the emitter follower output... NPN-NPN instead of NPN-PNP.

author: chrisbrown
author_rank: Contributor
timestamp: September 12, 2013, 02:50:40 am
content: Quote from: tree on September 12, 2013, 12:46:55 amDid you check the ripple on those rails? Your AC peaks could be well over 45V.

author: tree
author_rank: Regular Contributor
timestamp: September 12, 2013, 02:51:31 am
content: Quote from: edavid on September 12, 2013, 02:41:46 amI thought about doing that, but I only have one tap on my transformer and its output is 40V peak. I have a huge safety margin due to the fact that I want to allow for very high current draw.Quote from: edavid on September 12, 2013, 02:41:46 amBecause I can. I don't need gain if the reference is within my output range.Quote from: edavid on September 12, 2013, 02:41:46 amI am not using discrete voltage gain stages. I want to use an op amp for gain but some power bjts for large current output.

author: tree
author_rank: Regular Contributor
timestamp: September 12, 2013, 02:52:21 am
content: Quote from: chrisbrown on September 12, 2013, 02:50:40 amWhen I said +/- 40V rail, I meant peak, not RMS.

author: chrisbrown
author_rank: Contributor
timestamp: September 12, 2013, 03:01:18 am
content: Just checkin'. Sorry for your loss.

author: tree
author_rank: Regular Contributor
timestamp: September 12, 2013, 03:18:42 am
content: Quote from: chrisbrown on September 12, 2013, 03:01:18 amThank you. You're welcome to come to the funeral.

author: Kremmen
author_rank: Super Contributor
timestamp: September 12, 2013, 05:51:09 am
content: There have been a number of suggestions but i haven't spotted one that has been discussed on this forum previously. As far as memory serves, in the context of a circuit originally created by a forum member, but i fail to recall who. Anyway,The PNP pass element suffers from high output impedance and a les than optimal pole-zero placement. It would be better to use an NPN pass element as usual. To avoid high voltage output requirement in the controlling opamp, you can emulate the 3 component resistor-zener-transistor circuit.- supply the pass transistor base current from the collector using a resistor,- divert excess current to ground using another NPN BJT, collector to pass transistor base and emitter to GND- now control this second transistor using an opamp. The opamp output never needs to rise beyond ~0.7 V or whatever you decide based on the base resistor.You need to invert the sense of the control signal but that's about it. All the normal caveats regarding loop stability apply, but now you have a "normal" PSU control loop.The only drawback of this circuit is that uoy will constantly waste the max base current of the pass element, either to supply the base or by diverting excess to ground. This can be mitigated using high hfe transistors.

author: dannyf
author_rank: Super Contributor
timestamp: September 12, 2013, 11:16:42 am
content: QuoteIt is not possible that 1) the opamp is a follower; and 2) the input comes in on the inverting terminal.

author: planet12
author_rank: Regular Contributor
timestamp: September 12, 2013, 11:34:58 am
content: Not sure what package you were using the OPA in, but if you look at the data sheet, on +-40V rails it'll dissipate over 300mW when idle. When actually doing something, much much more - eg. short circuit current at that voltage will, if I'm counting on my fingers right, end up with over 2W dissipation. That's... gonna smoke.

author: planet12
author_rank: Regular Contributor
timestamp: September 12, 2013, 11:36:30 am
content: ^^^ I'm a twit, I missed the fact there was a second page before I hit "reply."

author: baljemmett
author_rank: Unknown
timestamp: September 12, 2013, 01:44:25 pm
content: Quote from: dannyf on September 12, 2013, 11:16:42 amHe didn't say it did - it was the feedback (from the output side of the Darlington pair) that was being brought back to the inverting input, with the control input arriving at the non-inverting.

author: tszaboo
author_rank: Super Contributor
timestamp: September 12, 2013, 02:33:32 pm
content: You can also bootstrap opamps to increase their power supply voltage. Read after that and face the consequences of using this technique.

author: tree
author_rank: Regular Contributor
timestamp: September 12, 2013, 02:44:20 pm
content: I had absolutely no load on the op amp. I simply connected the rails and the magic smoke escaped. It was in a DIP package and it almost caught on fire, almost like a complete short.I will have to read up on bootstrapping.I have also attached my latest schematic.

author: Dave
author_rank: Super Contributor
timestamp: September 12, 2013, 04:10:17 pm
content: U2 gets 19V on the inputs with this configuration, well beyond the supply voltage. No wonder it burst into flames.

author: tree
author_rank: Regular Contributor
timestamp: September 12, 2013, 04:19:01 pm
content: Quote from: Dave on September 12, 2013, 04:10:17 pmI don't have U2 in the physical circuit. That small subcircuit simulates the behavior of a current sense amplifier with a gain of 50

author: dannyf
author_rank: Super Contributor
timestamp: September 12, 2013, 09:51:16 pm
content: QuoteThere shouldn't be any DC issues to cause the opamps to burn out. AC stability may be an issue.QuoteU2 requires a special opamp whose common mode voltage range goes beyond its supplies - many instrumentation opamps or high side current sense amplifiers do that.

author: tree
author_rank: Regular Contributor
timestamp: September 12, 2013, 11:33:37 pm
content: Quote from: dannyf on September 12, 2013, 09:51:16 pmMy latest schematic is in the simulation phase. I have not built it yet. My first schematic, in which I used an OPA445, a high voltage op amp, is the one that you should refer to when I say the op amp got fried.I will be using a MAX4372, which is a high side current sense amplifier whose common mode is at 28V max, which is more than sufficient. However, since I don't have a model of it in LTSpice, I am trying to simulate the gain using the configuration you see in the latest schematic.