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Universal AC Input 5V/10A/50W PSR Flyback Power Supply Reference design PMP10974 schemtic question

Steven Weyrich
Prodigy 100 points
Other Parts Discussed in Thread: PMP10974

Hi,

When I look at the schematic of the reference design PMP10974 I'm wondering about the circutry between the bridge rectifier and the Flyback transformer with the Mosfet Q900.

What I've understand is that the Q900 FET is switched on with the low side FET Q903 (thats clear otherwise no current flow in the transformer).

In the off state ground and Vin are disconnected from the transformer.
So when I got it right in the off state current flows out of the transformer in the source via D900 and the flows inside it via D904 from ground

--> Snubber like?

Thanks for a clearification.

Steven

  • 0
    TI__Expert 4015 points

    Hi Steven, you're analysis is correct. The leakage current in the transformer primary causes node 1 of the transformer to ring down until it is clamped at Gnd-Vd by D904, and node 3 of the transformer to ring up until it is clamped at Vbulk + Vd by D900. 

    The leakage current therefore flows into the bulk cap and is not dissipated in a snubber, hence the circuit can achieve higher efficiency.

    Hope this helps.

    Thanks

    Billy