UCC256403: MOSFET shoot-through during the soft start

Rock Shao

Part Number: UCC256403

My customer used UCC256403 for a 54V/150W with 200kHz switch frequency, but the MOSFET will be shoot-through during the soft start,

The waveform as the below, ch1: gate, CH3: SS, CH4: ISNS

The schematic is the below, the RS5 used 470R now, L=510uH, turn raio=24:7, Cr=33n, Lr=70uH.

Will adjusting the capacitance of LL/SS help?

thanks and awaiting the feedback

over 4 years ago

0 Yunsheng Qu over 4 years ago

TI__Genius 9790 points

Hi, Rock

I cannot find the shoot through in your waveform, it is normal to me. Could you test both LO and Ho-Hs?

Yunsheng

0 Rock Shao over 4 years ago in reply to Yunsheng Qu

Expert 2995 points

Hi Yunsheng,

The current problem is that during the soft-start, the frequency of the first few cycles is normally 160KHz, but the latter will drop to 140Khz, causing OCP to start.

0 Yunsheng Qu over 4 years ago in reply to Rock Shao

TI__Genius 9790 points

Hi, Rock

then it is resonant over shoot issue, it can optimized by decrease pull down capacitor of VCR pin and increase pull down capacitor on LL/SS pin.

Yunsheng

0 Rock Shao over 4 years ago in reply to Yunsheng Qu

Expert 2995 points

Hi Yunsheng,

Thanks for your feedback, the turn-on issue is fixed. But there have other problems.

You can see the turn-on duty is OK for used CoolMOS, and you can see the below waveforms,

but if used the GaNMOS the turn-on duty is abnormal, and you can see the below waveforms,

In this case, to adjust the LL/SS pin resistance will helpful? thanks.

0 Yunsheng Qu over 4 years ago in reply to Rock Shao

TI__Genius 9790 points

Hi, Rock

Which GaN device you are using? Could you share the schematic with GaN device?

The waveform looks like VCR was disturbed, you may need to relayout the wire for pull down capacitor on VCR pin, make the loop of pull down capacitor is very short.

Yunsheng

0 Rock Shao over 4 years ago in reply to Yunsheng Qu

Expert 2995 points

Hi Yunsheng,

Please refer to the schematic and GaN MOSFET datasheet, thanks.

CoolGaN_1ch_140_PrelimDS_230320.pdf

0 Yunsheng Qu over 4 years ago in reply to Rock Shao

TI__Genius 9790 points

Rock,

QP3 should be low side GaN FET, could you also share the schematic of high side GaN. The VCC of high side is very critical to turn on high side GaN. If high side GaN cannot be turned on effectively, the voltage of resonant capacitor is not right which lead to wrong waveform of VCP pin, then the LO/HO signal is not right.

Yunsheng

0 Rock Shao over 4 years ago in reply to Yunsheng Qu

Expert 2995 points

Hi Yunsheng,

Please refer to the high side GaN MOS QP2, the Vcc from the UCC256403 charge pump, and the charge pump which is 470nF.

Is it OK to drive the MOS turn correctly? thanks.

0 Yunsheng Qu over 4 years ago in reply to Rock Shao

TI__Genius 9790 points

Hi, Rock

470nF as charger pump is too small, it is recommended to increase it to 1uF.

what is the value of CS6? Could you help to test waveform on VCR pin?

Yunsheng

0 Rock Shao over 4 years ago in reply to Yunsheng Qu

Expert 2995 points

Hi Yunsheng,

The CS6 is 100pF, and I will ask the customer to test the waveforms, thanks.

NW150W_SCH-LLC-UCC256404.pdf

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UCC256403: MOSFET shoot-through during the soft start