TOP261Y Regulation and power-up problem

I forgot mention, the input is nominal 120VAC/60Hz for this design.

I see the schematic on PI expert format so I assume your schematic is the same as shown in the PI expert file.

The schematic does not show anything wrong. So in order for you to trouble shooting I suggest the follow.

check with an oscilloscope the phase of the transformer. Connect CH1 of the scope to DRAIN node. Then connect CH2 probe to anode of D5 and compare the phase of both nodes when switching voltage is present. Repeat the measurements DRAIN to anode of D4, DRAIN to anode of D3, DRAIN to anode of D2.

If they are in phase, then disconnect D5, D4 and D2 from the circuit. Connect an external 12 VDC to the collector of U2B and to primary return. With the external 12 on, turn on the input voltage and measure your voltage at 12 Volts. if you have regulation reconnect the diodes one by one. every time you connect a diode the power supply should stay in regulation. If not, then that particular output has something wrong.

Next; disconnect U3 and C12 from the circuit. Connect a 11 V zener from Cathode od U2A to secondary return. If the supplies regulates, you have a problem around the TL431 circuit.
Let us know if this help you to fix the problem.

Hi Wolf,
Yes the schematic is the same with the exception of the transformer pinouts. We had some made by Premier Magnetics based on this same design (I did send them the same design file as I attached with my earlier post).

I did the checks for the phasing. All anodes look just about the same as the picture (Drain_and_AnodeOfBias.jpg) with the exception of the amplitude.
Then I disconnected all Diodes but D3 for the 12V output. Attached an external 12V source between collector of U2 and primary return.
There was no regulation, the 12V output went to a bit over 28V.

Next step was to remove U3 and C12 and replaced them with a Zener. I only had a 10V Zener at hand, but the result is more or less the same, - no regulation. The 12V output went to 30V.

I am lost at this point. Especially since my prototype is just working fine.

As I mentioned if my previous post, if I remove the opto (U2) completely, the TOP261Y is starting up and switching as there would be a heavy load on it. The control pin never discharges as I would expect and no Auto restart is happening. Am I correct in expecting Auto restart in this situation?

Attached are some scope plots

About Your external bias picture.

Looks the power supply is regulating at 28 V am I right? Is the 12 volts output (measuring 28 V) steady or is it pulsing?

I am attaching a picture of the OUTPUT voltage (TOP), Control Pin Voltage (middle) and DRAIN current (bottom). The trace form the opto coupler to the control pin is disconnected. As you can see every eight time the control pin electrolytic capacitor is charged the topswitch starts to run. The wavefors you show when you removed the opoto does not have any sense. Please check for your top switch pins not to be opened.

If possible try to send me a picture as the one I sent to you where shows DRAIN current, VOUT and control pin electrolytic capacitor voltage. Make sure your electrolytic capacitor is connected to control pin
If possible try to send me a picture as the one I sent to you where shows DRAIN current, VOUT and control pin electrolytic capacitor voltage. Make sure your electrolytic capacitor is connected to control pin

I know, the control voltage does not make any sense.

Here is what I did to take the attached scope plot.
Physically cut the trace coming from the Emitter of the opto couple, replaced the TOP261 with a new TOP260, checked again to make sure that the control pin has connection to C5.
(I actually hooked the scope physically to the pins of the capacitor C5 when I took the scope plot). Measured C5, R6 and C4 with a meter to verify a correct value, they are all good.
As you can see on the scope plot, After power-up, the control voltage slowly rises to about 5V and stays there. NO FEEDBACK IS PROVIDED. At one point I would expect the control voltage starting to fall again due to the TOP260 discharging it.

I just noticed one thing and it might be a clue, the Control voltage rises to 5.6V and stays there, measured with a Fluke meter.
What could prevent the capacitor from being charged to 5.8V?
I have a 6.8Ohm resistor, R6, in line from the control pin to the capacitor C5, per PI Expert.
Could this be my problem?
How tight are the tolerances on the control voltages?

talking about the picture you sent to me;
what happen to the output after the start up? does it stay in regulation?

If you are referring to the picture (Control_DrainCurrent.jpg) from my last post, no there is no regulation because the feed back loop is open.

The control voltage across C5 rises to 5.6V and stays there. The FET is constantly switching.

OK, I have resolved the problem.
It was indeed caused by the control voltage not reaching 5.8V, therefor the FET kept on switching.
My problem is layout related. The ground from C5 together with the Bias return was connected to the primary return at the input capacitor C2.

I just simply cut the trace from Bias return to the ground of C5 and place a jumper from the transformer bias return pin to the primary return at input capacitor C2.

The control pin now behaves as expected and output is in regulation and works just as my prototype.

I still don't know why the control pin behaved that way before.

Thank you for your support

Pete