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INN3977CQ

Posted by: SignoraMin on

I use INN3977CQ for  DC-DC converter to design 12V 25W output, 120V-800V input, and reference design is 30 W Isolated Flyback Power Supply with StackFET Using InnoSwitchTM3-AQ INN3977CQ.

And I find when the input voltage is 120V with the load 0,5W, the Vds for StackFET and Vsr (Voltage for SR pin)is normal but when the input voltage is up to 300V, the Vds is delay than Vsr. it seems Vsr is off before Vds, as the oscilloscope shown.

 

Could you help me how to solve this problem?

 

Comments

Submitted by PI-BERJ on 11/22/2022

Hi SignoraMin,

At 0.5W load and higher input voltages, the currents on the SR FET could be very low that INN3977CQ does not see the need to turn on the FET, i.e. internal current sense thresholds needed to turn on the sync. FET are not triggered. No gate signal from the SR pin does not actually turn off the synchronous FET. The current just flows through the body diode or a parallel Schottky if any.

Regardless, I don't think this should be a concern.

Regards,