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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

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  • \$\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 '20 at 17:33
  • \$\begingroup\$ Actually I tried 18R and 0R for R9 but result is same as you can see in ossiloscope screen. \$\endgroup\$
    – ivanmateo
    Feb 13 '20 at 18:22
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    \$\begingroup\$ Which channel is which signal? \$\endgroup\$
    – winny
    Feb 13 '20 at 21:31
  • \$\begingroup\$ @ivanmateo are we done with this question and answer now? If not then please leave a comment for further clarification else formally accept my answer. If you are unaware how to do this see this. \$\endgroup\$
    – Andy aka
    Mar 25 '21 at 11:23
  • \$\begingroup\$ @ivanmateo are we done with this question and answer now? See also the comment immediately above and take note that if you wish to receive the full attention and help of folk on this site, you should remember that upvoting useful answers and accepting the best answer is good practice and a sign of respect to those giving their free time to help you. \$\endgroup\$
    – Andy aka
    Jan 3 at 15:30
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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\$
    – ivanmateo
    Feb 13 '20 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 '20 at 20:15
  • \$\begingroup\$ 65Khz. The scope show c9 and c10 voltage. \$\endgroup\$
    – ivanmateo
    Feb 13 '20 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 '20 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\$
    – ivanmateo
    Feb 14 '20 at 6:37

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