Switched-mode power supply feedback loop problem - Page 1
Forum section: electronics / projects

author: Sebastian
author_rank: Regular Contributor
timestamp: January 29, 2014, 01:30:24 am
content: Hi,I am working on a smps and it seems like the feedback loop is not working properly. It is my first smps so I haven't got any experience which I could use to troubleshoot this thing.The controller I am using is a TOP267VG from Power Integrations. It features a mode where it tries to start up and then stops again if the output is shorted or the feedback loop is open. Because the output is not shorted, the problem has to be the feedback loop. I used the supplied software from Power Integrations to design the circuit. It has two rails, 5V and 12V.As I said, I am not experienced with power supplys but the feedback circuit looks kind of wired to me. For example I don't get what the 1k resistor in parallel with the led of the optocoupler should be good for.I appreciate any kind of help or ideas, I am troubleshooting that for quiet some time now and I am out of ideas.

author: nickm
author_rank: Regular Contributor
timestamp: January 29, 2014, 03:49:02 am
content: What does the output voltage look like when it starts up?You don\t label which output is which but it looks like you are regulating of the 5V rail. If that's the case R9 and R10 should be the same value because the 431 reference voltage is 2.5V. With your values it will try to regulate around 10V which would explain why overvoltage protection kicks in.If it is regulating correctly then the problem might be overshoot on startup. The compensation is very limited so that is another area that could be fixed.R8 is there to provide bias current to the 431 so that it regulates properly when U2 isn\t conducting.

author: jman
author_rank: Contributor
timestamp: January 29, 2014, 11:19:00 am
content: try using to use a transformer with a axillary winding it will eliminate the use of optocoupler.

author: Sebastian
author_rank: Regular Contributor
timestamp: January 29, 2014, 01:13:10 pm
content: Yes, the lower output is 5V. I swapped R9 to 10k2 so R9 and R10 are the same value, but it is still not working.I attached some plots of the output, which is CH2 and the Voltage at the control pin of the TOP267VG which is CH1.It does exactly what it is supposed to do if the output is shorted or the feedback loop open: The Voltage at the control pin goes up and down because a current is applied to it by the controller that charges the capacitor to a threshold and then turns back off. If the feedback is not working properly, the cap discharges until it reaches a lower threshold and the current is applied again. The output is only enabled every 16 cycles to save power. You can see that on they attached images. The switching you see when the output turns on is at exactly 20kHz which I think is correct because the controller should increase the frequency during startup and don't go straight to the 132kHz. But as you can see the voltage on the control pin is also 20kHz but as far as I know it should be DC.BTW: I have a 60 ohm resistor attached to both outputs because otherwise the output voltage charges the output caps up to over 30V which is more than their rated voltage.@jmanI want to avoid starting over from scratch because I don't have that much time. If I can't get it running I will consider doing that, but I guess it has to have some downsides too, because otherwise everybody would do it like that.

author: nickm
author_rank: Regular Contributor
timestamp: January 29, 2014, 03:09:07 pm
content: Whats the schematic of your transformer look like?Here's an easy way to check if it is regulating properly. Put a resistor on the drain connection of your controller and connect the top of the resistor to a power supply. Make the resistor large enough so it won't burn up if full voltage is applied continuously to it. Connect an adjustable power supply to the regulated output. Adjust the output voltage around what the output voltage should be regulating to and watch the drain output of your controller. What you want to see is maximum duty cycle when you set the output voltage below the regulated voltage, and minimum duty cycle when the output voltage is set above the regulated voltage. There should be a small range where the duty cycle will change linearly. If you can't get this test to work something is wrong.

author: Sebastian
author_rank: Regular Contributor
timestamp: January 29, 2014, 05:52:50 pm
content: I attached the schematic and mechanical diagram of the transmormer.I wanted to try what you wrote but I don't know where I should connect the ground of the power supply that is connected to the resistor on drain.But I played around with it, tried this and that and noticed that I can get it kind of working if I disconnect the load and even if I reconnect it again. It regulates at almost spot on 3x the correct output voltage, both channels obviously.I was wondering why I did not notice that before and realized that I changed R9. So I changed it back and it wasn't working anymore. I changed it back to 10k2 again and made a screenshot.(500Ohm load)The noise on, what should be the 5V output, is about 4Vpp. I hope that is just because the output filtering is designed for a lower voltage.

author: nickm
author_rank: Regular Contributor
timestamp: January 30, 2014, 03:34:57 am
content: Hmm. There should be no noise on the output. It looks like it is still putting out power while you're doing this test.The idea of this test is to make sure its regulating without having to deal with passing power through the transformer. Apply power to the primary side but disconnect the transformer so it can't send power to the output. Normally you would simply apply power to the regulators Vcc but since your regulator gets powered from the drain its not as easy. Put a resistor to bypass the transformer which will power the regulator. Monitor the voltage at the drain, it should be the PWM pulses. Then put a second power supply on the regulated output. Vary the second power supply and check the pulses on the drain.

author: jman
author_rank: Contributor
timestamp: January 30, 2014, 05:26:38 am
content: can you post your VDS waveform?

author: jashugan
author_rank: Regular Contributor
timestamp: January 30, 2014, 09:28:02 am
content: Is C13 a 115 nF capacitor? It's an unusual, and more important, a big value. Moreover, you take the reference after the 2nd order LPF.Try this: first reduce C13 to a smaller value (10 nF for example); in case it is't enough, try to move the pin of R9 before L2.

author: Psi
author_rank: Super Contributor
timestamp: January 30, 2014, 09:46:01 am
content: I would test the feedback system by disconnecting the transformers AC output to the 5V rail and feeding the circuit some DC instead (with respect to output GND)Check the voltage across the opto LED is correctly going above and below ~1.1V as you adjust the DC above/below ~5.3V.Keep the mains disconnected while testing, it's not neededThe 1.1V is because IR leds have a forward voltage between 1.0 - 1.2V.I say 5.3V for DC input (instead of 5V) because you will lose 0.2-0.4V across diode V3 in order to get 5V at the outputThe actual voltages you get maybe a bit different than my figures, depending on the specs of your opto and diode, but check that the opto led voltage makes sense for turning on/off at an output around 5V.

author: Sebastian
author_rank: Regular Contributor
timestamp: January 30, 2014, 04:56:24 pm
content: @nickmI removed the transformer and put in a 100k Resistor instead of the primary winding. I plugged in the power supply and measured the voltage between Drain and Source. What I have measured does not look like PWM to me, but just have look at the attached pictures. The first two are without the power supply connected to the 5V output. the third one is with 5V and the fourth one with 10V.But what I noticed right now while I am writing this is that I think the main problem with this test is that the bias winding is missing. Should I connect a power supply instead of the bias winding? And what voltage should I set it to?@jashuganIt is even 150nF, but that is just a typo on the schematic I posted. I changed it to 10nF and It did not change anything in the test above. But as I wrote I think the test can't work like that.@PsiI did what you said and here are the results:Vout/V VLED/V4.90 0.3734.95 0.4005.00 0.4705.01 1.0085.02 1.1355.10 1.1395.20 1.1425.30 1.1455.40 1.147Is that jump between 5V and 5.01V output voltage normal?

author: Psi
author_rank: Super Contributor
timestamp: January 30, 2014, 11:29:25 pm
content: Quote from: Sebastian on January 30, 2014, 04:56:24 pmYes, that's exactly what should happen. The opto LED is turning on/off at the threshold of 5V output.When the output voltage is correct (5V) the opto LED is on, so the switchmode controller IC knows the output is good and it can stop working.As soon as a load on the psu pulls the voltage down (under 5V) it causes the opto led to go out, telling the controller that it needs to do some work to pull the output back up to 5V (where the opto turns on and the controller stops working again).So there's nothing wrong with the output side of the feedback system.