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I should start, possibly, by saying I am no expert on offline SMPS, but am having a go.

I have built up the schematic below: enter image description here

I am getting no proper voltage out of the outputs. The transformer outputs are switching to negative voltages.

The image below shows:

Yellow (bottom trace): FET gate (Q1 pin 1) Green: CTL (U1 pin 7) Blue: Protect (U1 pin 6) Purple: Vcc

enter image description here

The be absolutely clear, the bottom trace is a train of 17 pulses. The gate pulses are less than 1us, in the region of 700ns, which strikes me as being way too short.

The optimer pin on U1 doesn't seem to move. Vin_sense is bang in its correct range, I have an output load, but I'm not by any means stretching the transformer.

I have also looked at Isense, and below is the same type of scope traces, but with top trace as Isense, not Vcc.

enter image description here

I've read and re-read datasheets and apps notes, and although I did find three things with the original schematic that I have fixed to reflect what's in the above schematics I can see nothing else obviously wrong.

FWIW I am currently supplying the input voltage from a bench PSU at 142V, not yet directly from mains.

Do the very narrow pulses seem wrong? Can anyone see anything I am doing wrong? If you want some more scope traces or info, please ask.

The data sheet can be found here.

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  • \$\begingroup\$ Transformer pins 3 and 4 are reversed. \$\endgroup\$ – John Birckhead Sep 26 '19 at 18:13
  • \$\begingroup\$ Sadly, that's not the case .The dots on the transformer are wrong, the pinout is correct. I'll update the schematics \$\endgroup\$ – DiBosco Sep 26 '19 at 18:44
  • \$\begingroup\$ The pulses ought to be < =100kHz \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 26 '19 at 20:31
  • \$\begingroup\$ @SunnyskyguyEE75 The pulses are at about 65kHz, just very narrow. \$\endgroup\$ – DiBosco Sep 27 '19 at 7:32
  • \$\begingroup\$ Looks like you may be overloading at startup or might be failing VCC undervoltage. Things to try: Put some kind of load on your 5volt output. Remove or reduce output capacitors C12, C13, and C16. Check your input voltage and make sure your power supply is not being pulled down. You could also increase C2. \$\endgroup\$ – John Birckhead Sep 27 '19 at 13:26

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