UCC27714: UCC27714

John Coughlan

Part Number: UCC27714

https://www.ti.com/lit/ds/symlink/ucc27714.pdf

I want to use UCC27714 in a 2-switch flyback. Datasheet section 7.4.3 has fig 49 which shows an implementation for circuit to achieve this.

Section 7.4.3 mentions that 2 additional diodes and FETs required, while fig 49 only shows one additional diode.

Can you advise where the second additional diode is meant to go.

When I simulate this circuit I need to make Rboot>100ohm to stop VDD from effectively shorting or hitting UVLO. Power in the resistor is 12^2/100R=1.44W (average approx. 0.8W)

I'm assuming/hoping that if I put in the missing diode this wouldn't be as bad.

Can you advise where the other diode

over 4 years ago

0 Richard Herring over 4 years ago

TI__Mastermind 23015 points

Hello John,

Thank you for the interest in the UCC27714. There is a drawback in sharing the boot resistor as the pullup for the auxillary FET as you mention. The best solution is to provide a separate pull up to the top aux FET back to VDD.

Confirm if you can see the edited diagram above.

Regards,

0 John Coughlan over 4 years ago in reply to Richard Herring

Prodigy 10 points

Thanks for the fast reply. What type of range would you expect for the resistor >1K; >10k.

Simulation seems to work with 10k but that doesn't mean much.

0 Richard Herring over 4 years ago in reply to John Coughlan

TI__Mastermind 23015 points

I think it depends on the switch you use for the top aux FET, and the gate charge of that part. But I dont think some delay of turning on that FET will be a concern. I would suggest the 1K as a starting point.

Regards,

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UCC27714: UCC27714