UCC28950: Channel D Output Unexpected Long On-Time

Antonio Fadhel

Part Number: UCC28950
Other Parts Discussed in Thread: UCC27714, UCC28951

Hi Team,

My customer is having a UCC28950 issue with long on-times on the D FET causing high current and eventual FET failure. They tried increasing the slope comp and it did not help, so they believe it might be due to noise in the current sense cabling. It only happens at very light loads when the controller is right on the edge of burst mode.

They hypothesize that the controller is about to change states from continuous to burst mode when noise causes it to lock the “D” output for 2 clock cycles. It can only be replicated with their customer's load which is fairly noise. Have you seen this issue? Any initial thoughts on what could be happening and how to mitigate it?

Please see waveform below:

-CH1: transformer current

-CH2: Vds of B FET after going through a UCC27714

-CH3: D FET drive out of 28950

-CH4: Vgs of D FET after going through a UCC27714

Thanks,

Antonio

over 4 years ago

0 Yunsheng Qu over 4 years ago

TI__Genius 9790 points

Hi, Antonio

Is the waveform happened at load transient condition?

During transient conditions the controller must change the duty cycle applied to the Power Stage and this requires a change in the phase between the OUTA/OUTB and OUTC/OUTD pairs. The duty cycle of the OUTA/ OUTB pair never changes so the only way to change the phase is to temporarily change the duty cycle of OUTC and OUTD. This change is temporary and the OUTC/OUTD pair return to their nominal 50% duty cycle once the new phase has been reached. The duty cycle of OUTC and OUTD may be greater or less than 50% - depending on the direction of the phase change being made.

Yunsheng

0 Antonio Fadhel over 4 years ago in reply to Yunsheng Qu

TI__Genius 9410 points

Hi Yunsheng,

It happens during light load conditions and does not seems like a simple increase in duty cycle for a transient response. The on-time seems unexpectedly long (about 2 complete clock cycles) - enough to saturate the transformer and cause FET failure.

Regards,

Antonio

0 Yunsheng Qu over 4 years ago in reply to Antonio Fadhel

TI__Genius 9790 points

Hi, Antonio

it may have such risk with UCC28950 if layout is not good. Is that possible to switch UCC28950 to UCC28951?

Yunsheng

0 Antonio Fadhel over 4 years ago in reply to Yunsheng Qu

TI__Genius 9410 points

Hi Yunsheng,

What would be the reason behind it? Does the UCC28951 have better noise immunity than the 28950?

Thanks,

Antonio

0 Antonio Fadhel over 4 years ago in reply to Antonio Fadhel

TI__Genius 9410 points

Hi Yunsheng,

I will share the schematic offline for review.

Regards,

Antonio

0 Yunsheng Qu over 4 years ago in reply to Antonio Fadhel

TI__Genius 9790 points

Antonio,

Let's move the support to email, Blake's team will support during chinese new year holiday.\

Yunsheng

Power management

Power management forum

UCC28950: Channel D Output Unexpected Long On-Time