Oscillation on opamp output with P-mosfet - Page 1
Forum section: electronics / beginners

author: gavron04
author_rank: Contributor
timestamp: November 02, 2020, 05:48:36 pm
content: OPA1611 without C15 1nF - between IN- and OUT of OPAMP, measured in "TP-PRZEWOD"Schematic1Schematic2 - supplyOPA1611 + 1nF between IN- and OUT of OPAMP, measured in "TP-PRZEWOD"PCB layoutOutput of OPAMPI have tested:- 3 differents OPAMPs - TL071; OPA277; OPA1611; uA741- tested differents capacitor between IN- and OUT of opamp (22pF;33pF;47pF;100pF;1nF;10nF;100nF)- tested mosfet driven directly by OPAMP and via NPN transistor - there are no differences between circuits. Always oscillations,- tested R2 1k; 10k- R5 - tested 47R, 100RAbout circuit. Its tool for testing laboratory cables (banana plug).X1 - its connector for power supply. 5V 16A. Aimted DC/DC 5-12/12 for opamp supply.C14 - 1000uFCSR-2.0-R010 its shunt resistor - 10mΩ.PMosfet - IXTH120P065T.R_I - selectable resistor 10R;50R;80R;100R - 1A;5A;8A;10A testing current.PR1 - calibration 1mA across T1.Its problem with OPAMP, layout or...?Have a good dayOPA1611 + 1nF between IN- and OUT of opamp. Measured in TP-PRZEWODOPA1611 + 1nF between IN- and OUT of opamp. Measured in TP-PRZEWOD

author: Wimberleytech
author_rank: Super Contributor
timestamp: November 02, 2020, 10:08:33 pm
content: Maybe the additional phase shift through the emitter follower plus the source follower in a unity gain configuration is enough to cause oscillation.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: November 02, 2020, 10:14:39 pm
content: And does the oscillation stop with a resistive load. ?If so It's probably stray inductance in the leads you're testing (typically a few uH/meter), current sources tend to have a hard time with primarily inductive loads sinceinductance resists any change in current --> creates a phase lag in a current controlled feedback loop --> you have an oscillator .One solution is a damping network across the inductance as you already have here with C11 and R34. I would say the values are wrong though for the typeof loads your testing (only leads with banana plugs -> low resistance and primarily inductive above some frequency ). Try values around 4u7 for C11 and 2r2 for R34,for unconditional stability best R and C values depends on MOSFET Cgd and a few other variables (more Cgd means larger C lower R damping values generally)with those present then R2/C15 can be reduced.Regards

author: xavier60
author_rank: Super Contributor
timestamp: November 03, 2020, 04:33:47 am
content: Oscillations don't usually look like that. There could be an external source.

author: gavron04
author_rank: Contributor
timestamp: November 03, 2020, 09:36:44 am
content: @Kevin.DTry values around 4u7 for C11 and 2r2 for R34>> tried, didnt helpAnd does the oscillation stop with a resistive load. ?>> never stop, only when I disconnect testing cables from banana connector. Then opamp on output has 12V.@xavier60 - maybe you are right. See photo.1) Power supply is turned ON, output - NO, so my pcb has no power2) On osciloscope you can see same oscillation which I have on output of opamp and on TP-przewod.Probe is connected on shunt resistor.On this video you can see what power supply can do - output is off. Probe of osciloscope is connected to TP-przewod:https://youtu.be/WpYM72BdQmoI have disassembly aimtec dc/dc converter and pluged linear power supply on 317/337. Its too much better than on aimtec.Amplitude is now about 400uV, before on aimtec was 7mV!Also now scale is 1mV/div.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 03, 2020, 12:53:22 pm
content: What is a control voltage looks like, at opamp input, or at T1-R3 connection?

author: gavron04
author_rank: Contributor
timestamp: November 03, 2020, 01:30:44 pm
content: //now Im using linear power supply for opamp its better than aimtec//I have changed shunt resistor for 0R22, also for 1A test I have changed RI valueU=I*R 1A*0,22R=0,22VR=U/I , 0,22V/0,001A = 220Rso for 1A, RI has 220R.In attachments:- simulation circuit from LTSpice- osciloscope probe in TP-Przewod- osciloscope probe in output of OPAMP- and T1-R3

author: gavron04
author_rank: Contributor
timestamp: November 03, 2020, 02:10:56 pm
content: Here you can see effect of: LED light; KSGER T12(youtube links isnt recognized on forum??)here - power supply output is off, Im powering on/off AC voltage:here testing power supply:I think that is not oscillation, only quality of my electrical supply.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 03, 2020, 07:54:13 pm
content: You have a good possibility to dive into a world of common and differential mode noises.

author: gavron04
author_rank: Contributor
timestamp: November 04, 2020, 06:41:41 am
content: But for me it isnt desirable

author: homebrew
author_rank: Frequent Contributor
timestamp: November 04, 2020, 07:07:43 am
content: Maybe a detail ...Why do you have a split ground plane like that? Especially with respect to C14. Its current must pass entirely around the split to reach the other pin of X1.

author: gavron04
author_rank: Contributor
timestamp: November 04, 2020, 07:57:32 am
content: My friend told me that it will better.I have tested C14 connected to X1 - nothing changeI have tested to connect GND of banana laboratory cable to X1 GND pin - nothing change.So its not problem of layout

author: xavier60
author_rank: Super Contributor
timestamp: November 04, 2020, 08:28:44 am
content: A useful test is to see how much noise is seen when the DSO's probe is touch to the same place as the probe's ground lead is connected.

author: gavron04
author_rank: Contributor
timestamp: November 04, 2020, 08:34:49 am
content: I have tested this. Probe and probe ground is connected to R26 (shunt resistor) from +5V andaimtecon PCB.

author: xavier60
author_rank: Super Contributor
timestamp: November 04, 2020, 08:44:08 am
content: If that's 1mV/div, there is nothing to worry about.

author: homebrew
author_rank: Frequent Contributor
timestamp: November 04, 2020, 12:24:28 pm
content: I'm just a hobbyist, but I see so many things in that schematic that raise my eyebrows ...- Why use a precision opamp without its offset compensation and with a questionable reference voltage source?- Your whole control loop seems to depend on the stability of the supply voltage.- Why use a P-MOSFET and go though all the troubles with negative voltages?- What happens when you switch the ranges? Could the wiper loose contact at any point in time? If so, your PMOS transistor would fully open and fry everything! At least it blows the fuse ...- Read the datasheet of your DC-DC converter (if you REALLY need it). It makes special EMC recommendations.- What is the reason for R35?- Why have this transistor inverter around T2?- Layout - As noted before, you did create yourself an unnecessary huge current loop running the entire width of the board. You do cut ground planes to control where the current is flowing, not to make matters worse.I think that it is MUCH too complex for that simple task. If I understand correctly, you just want to run 1,2,...,10A through a cable an measure the voltage drop reliably ...

author: gavron04
author_rank: Contributor
timestamp: November 04, 2020, 01:16:59 pm
content: Thanks for your reply.1) Thats good question. I have forget to control offset of OPAMP. Why used precision OPAMP? Better performance - better opamp, but as I see - not this time. Now Im using OPA1611.2) Probably yes.3) In LTSpice all looks good, but on real layout it isnt. Maybe better option will be BJT transistor like 2SC52004) Im using BBM rotary switch - there is no problem when Im changing current (1;5;8;10A). CK1051. Should be MMB, I fu*k up this. Now I see.5)http://www.aimtec.com/method-to-reduce-the-output-ripple-noise-of-power-suppliesits better shown than datasheet. But im staying with linear power supply for opamp6) Its my fault - on PCB I havent soldered this resistor.7) T2 was for test, but situation was same. Mosfet driven by opamp (directly via 47R) vs Mosfet driven by NPN transitor. I have disassembled T2, now Im driving mosfet directly by opamp, via 1k resistor.I will jumper this cut. But as I wrote. I have connected directly GND banana lead to X1, and nothing happen.9) you just want to run 1,2,...,10A through a cable an measure the voltage drop reliably ... - in simple words, yes it is.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 04, 2020, 11:27:09 pm
content: It seems to me now I get a part of your problem. We have 'a trap for a young players situation' here:You should turn on 20 MHz bandwidth limit on your scope.We usually do things with power supplies with this '20BW' bandwith mode. (And all parameters normally are stated for 20MHz BW). And don't turn on a 'Full BW' mode until you really need a full bandwith.And you should usually work with a 10:1 divider switched on (otherwise you will not have good bandwidth). Again, work with 1:1 mode onlly when you really need it.So if a lowest voltage range was 1 mV/div you'll have 10 mV/div, and thus much lower noise on a screen.

author: gavron04
author_rank: Contributor
timestamp: November 05, 2020, 06:53:05 am
content: Thats a good point..Thanks for your reply. Today I will post new measurement.Have a good day