UCC27714: 2-SWITCH FLYBACK CONVERTER FIRING PROBLEM

Hi Eddie,

Welcome back to the forum! Thank you for using UCC27714 in your design and for the detailed description of your issue.

-At first glance, I noticed your boot cap = Vdd cap(Cbias). Generally, we advise Vdd cap(Cbias) to be 10X that of Cboot to allow Cboot to fully replenish.

Can you please re-upload your waveforms scope shots, the ones you have are blurry and cannot make out anything out of it.

While I look further into your design, I've attached couple technical documents for you to consider looking at to ensure best performance from UCC27714.

www.ti.com/.../slua887.pdf (Bootstrap Circuitry Selection Guide)
www.ti.com/.../sluub02a.pdf (UCC27714 user's Guide)

Expect to hear from me Monday/Tuesday.

Thanks for your patience.

Regards,

-Mamadou

Hello again Eddie,

I'm sorry to see that you're having so much difficulty getting this design working with UCC27714. Let's try to figure out what's going on.

While looking at your schematic, I noticed that you have timed the closing of Q1 and SH simultaneously, as opposed to the inverted scheme originally presented in the datasheet. As drawn, your schematic creates a shoot-through path for current through Q1 directly into SH. SH must be driven on (closed) only when Q1 is driven off (open). Unless this is an error in the drawing, I suspect this is the major cause of the observed failures.

Please double-check that the schematic as drawn is correct, as this is a critical issue if so. We can return to your other questions if it is not the case that SH and Q1 are being driven simultaneously.

Regards,