What causes UC3842 to repeatedly restart?

Technical Discussion Power Electronics
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1

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At first, with normal power-up, the output was only 18 volts. Then, while checking the circuit, I blew the 0.2-ohm resistor. After resoldering it, there was no output. The image shows the current VCC waveform on the oscilloscope. Why does the UC3842 keep restarting? What’s wrong?

2

Be clearer: what power topology is this?
Also provide the schematic, otherwise there’s no way to know where that 0.2-ohm resistor you mentioned is…

3

It’s just the 72W flyback you open-sourced, blogger :melting_face:

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This R8 fried, and then the output voltage was gone :persevering_face:

5

After the 0.2R resistor blew once, did the VCC waveform become like in the image above?

Check whether the primary switching transistor is damaged.

After winding the transformer, did you measure the parameters with an LCR bridge? What are the primary and secondary inductance values and the leakage inductance, and was the core tightly wrapped with tape?