TOP257 PS output OVP causing issue

With a 1.8Mohm resistance connected to the V-pin, the line UV and OV thresholds are 45.5V and 203.84V respectively. What is the line voltage when you make your measurements?

Can you confirm that the line UV/OV and output OV functions work fine independently?

Try making your measurements with a DC source fixed at 170VDC -or- 115VAC (in case the DC source is not available) .

Can you make sure that you have used the right types of components for the output OV functionality? 

Attached is the actual schematic for the circuit.

With the “output OVP” disconnected from the circuit, I’m able to increase the line voltage up to 156VAC, at which point the TopSwitch shuts off.  The DC voltage at the input bulk cap is 216VDC.  The bulk cap is a 250v capacitor.  I’m able to decrease the line voltage down to 45VAC, at which point the TopSwitch shuts off.  The OV values are what I was shooting for, not too concerned about the UV.

If I then connect the output OVP circuit, I can turn the line voltage up past those levels.  I easily get to the bulk caps 250V.  Measuring at the “V” pin is a little touchy.  It seems easily effected by a meter.  The parts of the circuit are the same.

I do not have a DC source that goes to that level.  Not sure what you mean by “making measurements at 170vdc or 115vac”?

Thank you

"If I then connect the output OVP circuit, I can turn the line voltage up past those levels."

Does this mean that with the OVP now connected, the line UV/OV fuctionality works fine now?  

" Measuring at the “V” pin is a little touchy.  It seems easily effected by a meter."

This indicates a PCB layout issue; did you refer to the layout recommendations by PIExpert? 

Umm, I probably wasn't very clear.

My intention with the circuit was to have 2 forms OVP.  One on the input side & one on the output side.

On the input side I adjusted down the values of the sense resistors to get protection from accidental hi line voltage.

I added the "output OVP" (zener, resistor, diode & cap on the bias winding) to the output loop to have OVP on the output side.

With the sense resistors @ 910K each (and without the output OVP), the circuit is functioning as desired - if 156VAC line voltage is reached the power supply shuts down.  This can only be achieved without the "output OVP" circuit (zener, resistor, diode & cap on the bias winding) connected.   If I connect those components, the supply will no longer shut down at 156VAC.  It acts like the output OVP components are keeping the current at the "V" pin low.  So the answer to your first question is "no".  Any suggestions?

Yes the recommendations were considered when doing the layout.

Is it possible for you to capture the V-pin voltage+Load Voltage+Drain Current on the oscilloscope? 

I can capture the V-pin + Load Voltage, but not the drain current (no current probe).  Do you want to see the noise on the output?  Otherwise, the output voltage is a steady DC level, not much to see.

I can capture the V-pin + Load Voltage, but not the drain current (no current probe).

Please see the attached screen shot of the V-pin & 5V load voltage. Picture is a little blurry (2v/div & 5.0us/div).

Thanks

Remove D5 from the Output OVP circuit and check to see if the problem persists.

That seems to have worked for now.  The OVP shut down when the DCV got to about 215V. Thanks for getting me over that hump.  I'll look a little deeper into it tomorrow.  I had previously tried a 1n4148 in its place, but still had the same problem.  Thank you for the help.

There is excessive noise on the V-pin of the device. I have attached a copy of the V-pin voltage observed by me on a TopSwitch board af full load and low-line. Kindly compare the two for reference. Since the V-pin is a high impedance trace, you might want to route it as far away from the high-current switching traces as possible to avoid stray noise pickup.