UCC28704: Valley switching not realized issue

Benjamin Zhou

Part Number: UCC28704

Hi Team,

My customer now is evaluating UCC28704 for Aux-power supply design, and they encounter an critical issue for the valley switching that valley-switching feature not realized and DRV turn on at very high voltage, could you kindly give comments on how to fix this issue?

The test waveform for VDS & Aux transformer voltage shown as figure 1, and the VDS & VS PIN voltage test waveform shown as figure 2, from the 2 test waveform, you can see the DRV turned on at higher voltage but not the valley.

Expect for your kindly reply, thanks.

Figure 1: VDS & Aux-Transformer Voltage

Ch1: Aux-Transformer Voltage; CH2: VDS

Figure 2 VDS & VS PIN voltage

CH1: VS Pin; CH2: VDS

Best Regards

Benjamin

BenjaBenBeBB

over 4 years ago

0 David Huang33 over 4 years ago

TI__Genius 9140 points

Hi Benjamin:

let me loop in an expert to support you in this case.

0 Benjamin Zhou over 4 years ago in reply to David Huang33

TI__Expert 4000 points

Hi David,

Expect for your kindly reply.

Best Regards

Benjamin

0 Jaden Ning over 4 years ago in reply to Benjamin Zhou

TI__Expert 7080 points

Hi Benjamin,

Could you please check if any capacitors on the VS pin of UCC28704 or if the VS pin divider resistor is far away from with the controller . long VS pin trace existing and ground plate under this trace.

If so , remove the caps , move VS pin resistors close to the controller to short VS pin trace.

Thanks

0 Benjamin Zhou over 4 years ago in reply to Jaden Ning

TI__Expert 4000 points

Hi Jaden,

Very appreciated for your kindly support.

Best Regards

Benjamin

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UCC28704: Valley switching not realized issue