Voltage limit on FWD pin of innoswitch INN3947cq

Posted by: aBh1 on 01/14/2024

Hi, the fwd pin on the inn3947cq has a limit of 150V which demands high primary to secondary turns ratio in the flyback transformer. Is there a way to circumvent this limit? I am thinking to keep the resistor connecting fwd pin to the transformer secondary very large(~1meg) to create enough voltage difference while keeping low current. But there is no mention in the datasheet of what current should go in the pin to still work.

Also if I am not using the SR-FET, is it still necessary to use this pin?

Hi,

Thank you for using Pi forums.

you cant use a large resistor on the fwd pin. A 47 Ω, 5% resistor is recommended to ensure sufficient IC supply current and works for wide range of output voltages. This pin is also used to charge the BPS (SECONDARY BYPASS pin) capacitor when output voltage is lower than the BPS voltage.

if you are not using the SR pin. connect this pin to GND.

regards,

Sigris

Hi Sigris,

Thanks for the reply. So just to confirm, If I am using a diode instead of SR fet, I can short the fwd pin and SR pin to gnd. In this case the secondary side controller will be powered through Vo pin and we can expect normal operation.

Please confirm if I understood it right. And will there be any other consequences or any functionality that I would miss if I do this?

Thanks and regards

Abhishek

Hi,

you can short the SR pin. but not the FWD pin. as suggested a A 47 Ω, 5% resistor is recommended. since you are using a diode instead of a srfet. the FWD pin will sense the voltage from the diode instead of the drain voltage from the SRFET.

regards,

Sigris

Thanks for clarifying Sigris.

Best,

Abhishek

Voltage limit on FWD pin of innoswitch INN3947cq

评论