I have a design 193V output voltage and 700mA current, but when the current drawn from output, the auxilary winding voltage fall down, and IC shut down PWM gate signal as you can see in the attachment and the output voltage go to zero. And I could not understand, why auxilary voltage is like sawtooth wave, I think the IC try to open PWM but can not start. Can any body help me about that issue? enter image description here enter image description here enter image description here

  • \$\begingroup\$ Decreasing R9 and increasing C9 would tend to provide VCCP for a longer time. This may help "ride through" until the next pulse comes. \$\endgroup\$ – rdtsc Feb 13 at 17:33
  • \$\begingroup\$ Actually I tried 18R and 0R for R9 but result is same as you can see in ossiloscope screen. \$\endgroup\$ – ali mutlu Feb 13 at 18:22
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    \$\begingroup\$ Which channel is which signal? \$\endgroup\$ – winny Feb 13 at 21:31

The FAN6754 will shut down when the supply drops below 9 volts and not restart until that voltage rises above 16.5 volts. Given that it can work at a peak supply voltage that is closer to 30 volts, it seems that more turns on your auxiliary winding might be the route to go.

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  • \$\begingroup\$ Actually as you can in waveform it comes nearly 16.5V, but it can not start, the voltage is drop again. It can be related to R8,R9 or V1, because if the current is enough to turn to zener, voltage will be 22V but also voltage is dropping. \$\endgroup\$ – ali mutlu Feb 13 at 19:59
  • \$\begingroup\$ The o scope shots don’t give me enough detail to recognise that. What switching frequency are you using? \$\endgroup\$ – Andy aka Feb 13 at 20:15
  • \$\begingroup\$ 65Khz. The scope show c9 and c10 voltage. \$\endgroup\$ – ali mutlu Feb 13 at 20:31
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    \$\begingroup\$ Unloaded, what do those voltages look like and please remember I don’t know where your 0 volt references on the scope are nor, which colour is which signal. Try and show max and min rather than max and mean. \$\endgroup\$ – Andy aka Feb 13 at 20:35
  • \$\begingroup\$ Yellow is C9 voltage and Green is C10. But it must settle in 22V due to zener, but it fall down. \$\endgroup\$ – ali mutlu Feb 14 at 6:37

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