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Offline Flyback 24V....using INN2904K controller


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Thread starter #1
We are using the INN2904K controller to make a 15W, 25V Offline 240VAC Flyback.
We are not using the synchronous rectifier facility in the secondary side. As such, will it be allright for us to simply ground the FORWARD pin?

The Forward pin is only rated up to 150V and with our turns ratio, the reflected voltage (+leakage spike ring) would result in the FORWARD pin getting overvoltaged if we connect it up to the switching node of the secondary side.

INN2904K datasheet:


Well-Known Member
The first line from this paragraph on page 14 of your link implies that the FORWARD pin is part of the power supply of the controller. Grounding it might leave it kind of hungry.

"FORWARD Pin Resistor A 47 W, 5% resistor is recommended to ensure sufficient IC supply current. A higher or lower resistor value should not be used as it can affect device operation such as the synchronous rectifier drive timing."


Well-Known Member
Thread starter #3
Thanks, PI just got back to me....FWD pin is indeed used to power the secondary side of the controller at startup....this means on mains 200-285vac the transformer tunrs ratio ns/np shoudl be 1/5 or less....not what we wanted.
I dont think we can even add some auxiliary circuitry so that we can have say ns/np = 1/2 and still work with it.
The circuitry woudl be too extensive.


Well-Known Member
You could add another secondary winding just to power this part. That might be simpler than adding auxiliary circuitry. Don't know what the cost impact would be though.


Well-Known Member
Thread starter #5
Thanks, you're right, i wonder what the dropout voltage of the (seconday side) internal regulator (inside the IN2904K) is though? As you know, this info is needed so that its known what to set the turns ratio to of the "auxiliary" coil.

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