UCC28780: Burst frequency at no load condition

HAMZA RHOUDDA

Part Number: UCC28780

Hi,

I'm working with the UCC28780 for an active clamp with a 20V output

I followed the debugging document provided by Texas Instruments and the output is regulated to 20V. But i have an issue with the burst frequency at no load condition, at the start Fbur = 3kHz and if i restart the converter immediately, Fbur raise to 24kHz as you can see in the pictures below :

When Fbur = 3kHz and i add a variable load,the converter works well and Ucc28780 seems to respect the ABM and AAM modes, but when the Fbur = 24kHz, the converter starts heating up and i found that the high-side mosfet is not switching cause the driver is not sending any pulses while the UCC28780 PWMH pin is switching.

Do you have any idea what can cause such a problem ?

Thank you,

over 3 years ago

0 Jaden Ning over 3 years ago

TI__Expert 7080 points

Hi HAMZA,

The last waveform shows UCC28780 has give out the PWMH but the high side driver didn't responded to the PWMH . Could you please check if the bootstrap circuit has correctly working to charge pump the bootstrap capacitor higher than UVLO. as well as VDD1 also be correctly biased.

Thanks.

0 HAMZA RHOUDDA over 3 years ago in reply to Jaden Ning

Prodigy 60 points

I checked the Vdd and the Vbst and they are on the correct level

0 Jaden Ning over 3 years ago in reply to HAMZA RHOUDDA

TI__Expert 7080 points

Hi HAMZA,

PWMH is high , VDD and Vbst are on the correct level . but high side FET is not turn on . could you please check the connection between PWMH to driver , driver to FET . and if the driver still survival .

Thanks.

0 HAMZA RHOUDDA over 3 years ago in reply to Jaden Ning

Prodigy 60 points

Hi Jaden,

I checked the connections and everything is connected. I realised something is that when i'm at 60V input (0.5*V brown-in), i have 4 pulses but if i add a differential prob on the bootstrap capacitor to measure the voltage, the UCC28780 starts sending 3 pulses and the first one is very small (about 130ns) as you can see in yellow at the below picture ?

0 Jaden Ning over 3 years ago in reply to HAMZA RHOUDDA

TI__Expert 7080 points

Hi Hamza,

130ns turn on time means it triggered lead edge blanking time . you may have to slow down the turn on slew rate of low side FET and increase the CS pin capacitor a little bit .

Thanks.

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UCC28780: Burst frequency at no load condition