INN2904K FORWARD pin grounded.

Posted by: treez on 03/04/2018

Hello,
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? (not connect it to the switching node of the transformer on the secondary side)

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:
https://ac-dc.power.com/sites/defaul..._datasheet.pdf

Comments

Hi Treez,

Thanks for your inquiry.
If you will use a rectifier diode as replacement for the SR Mosfet, just ground the gate drive. The FWD pin should still be connected. You need to reduce the leakage and\or change your reflected voltage so that the sensed voltage will not exceed 150V.

Regards,
PI-Picard

Thanks for your reply.
This is very inconvenient for us. We will have to change the transformer turns ratio so that NS/NP is smaller. Are you sure we have to connect the FORWARD pin? I mean, the INN2904K is a constant frequency part, it does not do Boundary conduction mode where it needs to sense the secondary side switching node. Why do we have to connect it? Can we connect it via a potential divider. We really dont want such a low turns ratio, as it will reflect more back to the primary. There surely can be no point in the connection of the FORWARD pin with INN2904K.?.. it is a constant frequency part.

Also, surely a gate drive output should never be grounded? Gate drive outputs are low impedance, and grounding them would surely result in short circuit damage? Surely a gate drive output should be just left unconnected if not used?

They likely mean't leave the gate drive pin open and ground the ground pin...

Hey, treez.

I do agree with all of your points and need the same kind of information... But PI often doesn't provide answers beyond the datasheet...

So, how did you solve your questions? Where did you connect the driver and forward pins?

Regards,
Christos