Low dropout power regulator - or is it an oscillator - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: April 10, 2018, 09:23:28 am
content: Some time ago I designed a low dropout voltage regulator. There was a topic about this and I was advised it was a bad idea due to the risk of oscillation. Well unfortunately there is no option as switch-mode is out as this has to be a very quiet design. This is not meant as a perfect regulator as it's main purpose is to simply protect a load on a vehicle from receiving more than 30ish volts. So most of the time 20-30V would be supplied (24-27 expected in reality) so the circuit would not actually regulate but should 30V be exceeded (typical auto surges are supposed to be up to 40V for 500ms but yea customers....) then regulation should kick in.So up to 30V no problem. After 30V the output is a little wobbly but quite OK (400mVpp at 5.9A) however looking at the output of the opamp There is significant oscillation with it pulsing from 0-6V. So yes it would appear that I have created an oscillator. There is also significant ripple before I2 so it would seem that a lot of the clean output is due to the LC filter. Obviously this is not what was intended. The maximum output is to be 10A.Now I suspect that in my attempt to stop oscillation by slowing things down I have caused instability in any case. The opamp is designed to not oscillate and that is probably what is making the circuit sort of useable at the moment.On advice I put in R28 to remove the gain that could cause oscillation, and R27, something to do with high frequency bypassing.I put C12 in in the hope of "calming down" the circuits response which has clearly not worked. The idea of the circuit was to act as an "active voltage clamp" rather than a fine tuned regulator.Any suggestions?

author: capt bullshot
author_rank: Super Contributor
timestamp: April 10, 2018, 09:29:49 am
content: C12 is at the wrong place to slow the circuit down. It does so, but within the output stage around the OpAmp, this causes the oscillation.As a general rule, the OpAmp should be slower than the output stage for this kind of circuit. You'd achieve this by placing a capacitor from the OpAmps output to it's negative input (or maybe a series R and C).

author: Zero999
author_rank: Super Contributor
timestamp: April 10, 2018, 10:01:33 am
content: Don't say we didn't warn you.Switched mode regulators aren't noisy, if designed properly.The problem with this design is it uses a P-type output device, which has a high voltage gain, that is dependant on the load, so will be very difficult to stabilise. Even if you manage to get it to work on the bench, there's a huge risk it could oscillate in the field, where the temperature, input/output voltage and load are different.Your best bet is not a switched mode regulator, but a linear regulator with an N-type output device, which will have a voltage gain of just under one and therefore be much easier to stabilise. Unfortunately, for a low drop-out voltage, a slightly higher voltage, than the input voltage, is required to bias the output device fully on. A charge pump can be used to generate the bias voltage, which doesn't have to be noisy, if done properly.

author: Simon
author_rank: Unknown
timestamp: April 10, 2018, 10:10:37 am
content: Quote from: capt bullshot on April 10, 2018, 09:29:49 amOK so considering I have a PCB of this now I should just remove C12 as it's obviously not helping. Any idea on the sorts of values I should have between the output and the negative input ?

author: Simon
author_rank: Unknown
timestamp: April 10, 2018, 10:21:33 am
content: Quote from: Hero999 on April 10, 2018, 10:01:33 amI know, but I like a challenge and it's good to learn on the jobYes if I was redesigning it which I need to do at some point I'd look at a charge pump with NPN BJT's as I really can't afford the heat generated by a regular design and don't want the voltage drop under normal running.Switch mode will be unlikely as the company is now fearful of them and I'm not sure I can do a 20A SMPS myself and get it past mil spec EMC tests first go but yes I'm sure that in the right environment one can be designed, where I work where electronics has only just stopped being a massive pain we would rather not have and is just becoming accepted as part of the design process I would never have time or backing for putting a lot of work into a SMPS. I did one once but it was low power (50mA) and never got a complaint 20A is a bit different.

author: Simon
author_rank: Unknown
timestamp: April 10, 2018, 10:22:45 am
content: Quote from: blueskull on April 10, 2018, 10:12:01 amSo 10nF ? I'm going to remove it first and see what happens.

author: Zero999
author_rank: Super Contributor
timestamp: April 10, 2018, 10:24:54 am
content: Quote from: Simon on April 10, 2018, 10:10:37 amOh dear. Did the prototype work, before you went to a PCB? Perhaps the layout isn't helping?The output stage has far too much gain. Try reducing R28 to 2k7 (a capacitor in parallel may also help) and adding a base-emitter resistor to Q1, say 560R.

author: Simon
author_rank: Unknown
timestamp: April 10, 2018, 10:35:03 am
content: Quote from: Hero999 on April 10, 2018, 10:24:54 amThis is the prototype.....no time for sensible stuff like bread boarding which is not possible here so a small PCB would have been in order to test it out. Ultimately it's not a problem as these are prototype builds and under normal conditions the circuit does not enter regulation and just sits wide open but it would be nice to figure this out.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 10, 2018, 11:29:12 am
content: C12 as in the original plan makes things worse. If at all a small cap in parallel to the other resistor could help a little.The first point would to slow down the OP with an capacitor output to the inverting input.It would be a good idea to test with a spice simulation before soldering or making a board.With currents in the 10 A range and relatively high voltages that poor single transistor would get a bit too much heat and the driver probably can not provide it. The circuit looks more like good for 10 mA, maybe 0.5 A if lucky.

author: Simon
author_rank: Unknown
timestamp: April 10, 2018, 02:35:35 pm
content: Quote from: Kleinstein on April 10, 2018, 11:29:12 amWhy? it is a 300W transistor that is well heatsinked. I'm not sure what you mean about the driving of it. It is a darlington so the base current is well within the capability of the drive transistor.I have removed C12 and things have improved although still there is oscillation. At this point I am not bothered as the actual output is quite smooth after the filtering and closed loop regulation only happens in case of over voltage. There is an input LC filter as well so maybe I have too much filtering and that is causing oscillation. Maybe I should remove the output filter.I need to move on to other parts of the circuit now so will leave any dabbling until later. I will need to make a second and third board so could omit the filter on the next one and see what happens.Ultimately the thing needs redesigning anyway as I need to integrate an ECU into the same PCB which will greatly alter the layout and allow for a better layout. At this point I'd use an NPN based regulator with a bootstrap to give the higher control voltage required.

author: Zero999
author_rank: Super Contributor
timestamp: April 10, 2018, 03:13:35 pm
content: Quote from: Simon on April 10, 2018, 02:35:35 pmThe symbol is for an ordinary BJT, rather than a Darlington pair, which confuses matters.One disadvantage of using a Darlington pair is it has a much higher on voltage, therefore a greater drop-out. The data sheet lists a worst case saturation voltage of 2.5V, at 25A, with a base current of 250mA.QuoteWhat's the input voltage & load current step response like?

author: CopperCone
author_rank: Super Contributor
timestamp: April 10, 2018, 03:54:08 pm
content: Well if you want transient blocking ltc sells a chip that will function as a dcdc buck converyer if thr voltage goes above a certain threshhold but is normally fully on and low noise.If you have voltage spikes and shit it means that your already dealing with something else in the electrical systemfucking up so who cares?Do emc tests even cover this stuff? Never did milspec.This kind of hybrid solution may be more reliable due to much lower thermals. If youhave to design a 2iv system thay can handle 40 thats alot of stress or oversized components.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 10, 2018, 05:26:02 pm
content: Quote from: CopperCone on April 10, 2018, 03:54:08 pmLoad dump is very rare, and yeah, failed equipment is not necessarily an unacceptable condition.QuoteMil? SAE J1113 and ISO 7637-2 are standard for anything on an automotive bus.If you must, there's MIL-STD-1275 Rev. E which specifies an acceptable failure envelope for possible transient amplitudes and durations, on a nominal 28VDC bus.Tim

author: CopperCone
author_rank: Super Contributor
timestamp: April 10, 2018, 06:59:18 pm
content: I thought simons post was the thread starter

author: Simon
author_rank: Unknown
timestamp: April 10, 2018, 07:16:22 pm
content: Quote from: Hero999 on April 10, 2018, 03:13:35 pmNot got there yet, the load won't be changing so it is just about input steps, I'll use an arduino to control the power supply that will accept a change every 250ms so I would end up with at most a 500ms surge which is what is in the spec.

author: Simon
author_rank: Unknown
timestamp: April 10, 2018, 07:19:44 pm
content: Quote from: T3sl4co1l on April 10, 2018, 05:26:02 pmNo my job is to take care of other people's shit. As things stand my load is not auto rated so needs something and yes I have an envelope as per MIL-STD-1275 Rev. E that i have to survive and being better than that is not a bad thing when your customer always thinks they are right and proving they are not is hard work.

author: David Hess
author_rank: Super Contributor
timestamp: April 10, 2018, 11:58:37 pm
content: C12 is completely in the wrong place; while it does lower the gain, it also adds phase lag. This capacitor should go between the output and inverting input of the operational amplifier. A smaller capacitor might go across R2 to add phase lead for better stability. These capacitors might actually be two capacitor one resistor networks. Type 2? I always forget.You *can* stability your existing loop by increasing the value of C12 a lot to provide dominant pole compensation but line and load regulation will suffer.Q1 needs a base-emitter shunt resistor to remove remove charge from the base and cancel leakage current. Without it, the control loop can turn it on quickly but only shut it off slowly. I usually set the shunt current to double the required base drive to start off but that is very conservative.R28 is clever to control the voltage gain of the pass transistor if it works.If you invert the function of Q2 by using the operational amplifier to drive the emitter as a common base stage, then a bypass capacitor from the output of the operational amplifier to the base of Q1 could allow direct control of the charge in Q1's base. You might need to add a PNP emitter follower to the output of the operational amplifier to restore DC current gain. Really fast linear pulse amplifiers do this. On the other hand, it is a lot of extra complexity which should not be needed and voltage regulators do not need that kind of performance.I would split the value of C9 and put half of it on the other side of the filter inductor. The collector of Q1 really needs to see a low AC impedance to keep control of its voltage gain.

author: Cerebus
author_rank: Super Contributor
timestamp: April 11, 2018, 02:09:11 am
content: Just for fun I ran this up in LTspice, ran a simulation and plotted the loop gain:That is one fugly bode plot. May I suggest that next time you undertake this step yourself,beforecommitting to a PCB?Edited to add: I forget the gain margin figure: -56dB, yes minus 56dB @ 1.06 kHz. So everybody who said "too much gain" award yourself a gold star.

author: Simon
author_rank: Unknown
timestamp: April 12, 2018, 11:51:36 am
content: My next move is to try a base to emitter resistor and remove the inductor.

author: Simon
author_rank: Unknown
timestamp: April 16, 2018, 11:51:30 am
content: A 180ohm base emitter resistor calms it down a bit but by passing the inductor makes matters worse, I guess 3x 1uF ceramics have very very low ESR. I'm thinking of adding a few uF of electrolytic in front of the inductor but I'm happy with it for the moment as a prototype run.

author: David Hess
author_rank: Super Contributor
timestamp: April 17, 2018, 04:23:27 pm
content: Quote from: Simon on April 16, 2018, 11:51:30 amThe output capacitor is an integral part of the frequency compensation and adds phase lag. This has less importance when the output impedance is low as with an emitter follower but low dropout regulators are common emitter with a much higher output impedance so this becomes important. The higher ESR of a solid tantalum or aluminum electrolytic capacitor introduces a breakpoint where phase lead it added preserving stability. If you want to use film or ceramic output capacitors, especially with a low dropout regulator, then extra care is needed. Sometimes a low value resistor is added in series with the output capacitor for a minimum ESR.