UCC2897A: UCC2897A power on issue

Hi

The Vdd is discharging during the start-up and it looks like the system is not starting up on the first attempt.

Try increasing the VCC capacitor PC63 to 10uF.

Please take some new waveforms and include Vin and Vout with VDD and SR Vds stress. You can also add a TVS across te mosfet to clamp the Vds voltage. I suspect the Vds spike occurs during the shutdown when the current in the SR mosfet flows in the reverse direction.

Regards

Peter

Hi

For some reason the voltage on the VCC capacitor is falling during start-up and the device is turning off and restarting. During the turn-off I think the current is reversing through the SR mosfets and when they turn-off with reverse current the Vds is increasing and causing the high voltage stress.

I suggest you also test the operation during shutdown.

Regards

Peter

Hi,

Thank you for your reply.

We still have some problems.

Could you kindly help to describe or draw the path of reverse current.

Cage Chen63 Cage Chen21

Hi Cage,

Its quite simple the current flows in the opposite direction during the turn off event and when the fet turns off the vds spikes up because the SR is breaking a current flowing through an inductor. If the current is in the normal direction when the SR turns off the current continues to flow in the body diode until it drops to zero and there is no voltage spike.

If you still have a voltage spike you can add a TVS across the SR or you can add a clamping snubber circuit.

Regards

Peter

Hi Cage,

We have moved support to email so I am closing this post.

Regards

Peter