UCC256303: Start-up failure

Kazuki Itoh

Genius 10710 points

Part Number: UCC256303
Other Parts Discussed in Thread: UCC256403

Hello team,

Could you please help me to address start-up issue of UCC256303?

Please find the internal link here for schematic and waveforms. (Please connect to VPN for download)

The switching sometimes stops when the output voltage (nom. 24V) reaches to approx. 19V.

Please note that this is not perfectly reproductive failure.

Also, I've posted another thread about UCC256303 before, but I think this one is another issue as BLK pin looks fine.

Regards,

Itoh

over 4 years ago

0 Yunsheng Qu over 4 years ago

TI__Genius 9790 points

Hi, Kazuki

I check the waveform, there is no fault protect related to BLK, BW, ISNS.

Could you test the HO-HS and LO signal? I suspect the missing slew rate is triggered, it means the slew rate of HS is less than 1V/ns.

another question, have you checked the first waveform, why BLK is pull low after output voltage decreased to 0V?

Yunsheng

0 Kazuki Itoh over 4 years ago in reply to Yunsheng Qu

TI__Genius 10710 points

Hi Yunsheng-san,

Please find the internal link here for HO-HS and LO. (Please connect to VPN for download)

The BLK is pulled low after output decreased to 0V because the external circuit forces VCC to turn off if 24V doesn't start up.
When the external circuit is triggered, the FET Q4 will turn off and then BLK goes to 0V.

Regards,

Itoh

0 Yunsheng Qu over 4 years ago in reply to Kazuki Itoh

TI__Genius 9790 points

Hi, Itoh

I checked the waveform you shared, it shows LO was stuck high.

It is need to add 100pF on ISNS pin or change to UCC256403 to avoid this issue.

The most solid way is changing to UCC256403.

Yunshneg

0 Kazuki Itoh over 4 years ago in reply to Yunsheng Qu

TI__Genius 10710 points

Hello Yunsheng-san,

Could you please clarify why the 100pF ISNS cap or switching to UCC256403 can resolve this issue?

Regards,

Itoh

0 Yunsheng Qu over 4 years ago in reply to Kazuki Itoh

TI__Genius 9790 points

Hi, Itoh

If there is high frequency noise on ISNS pin of UCC256303, and this noise make ISNS signal cross zero for many times in a short period, then there is risk to capture the waveform you provided.

And UCC256403 is upgrade version of UCC256303, with new design, the concern of ISNS cross zero for many times is avoid.

Yunsheng

0 Kazuki Itoh over 4 years ago in reply to Yunsheng Qu

TI__Genius 10710 points

Hello Yunsheng-san,
Looking at the waveform, I don't really think the number of the ISNS cross zero won't change by the 100pF cap, because it doesn't seem like it has large chattering at the cross zero point.

Do you mean the ISNS cross zero for many times should have some risk to stop the switching?

Could you please specifically elaborate on the condition when the switching stops due to the ISNS cross zero?
Does UCC256303 have switching stop trigger other than OCP1, OCP2, OCP3?

I'd appreciate if you show how the ISNS cross zero triggers the stop by timing chart or waveform for the better visibility.

Regards,

Itoh

0 Yunsheng Qu over 4 years ago in reply to Kazuki Itoh

TI__Genius 9790 points

Hi, Itoh

The timing diagram cannot share with customer without NDA, could you send me a email to yunsheng-qu@ti.com? I can share with you with email.

Yunsheng

0 Kazuki Itoh over 4 years ago in reply to Yunsheng Qu

TI__Genius 10710 points

Hi Yunsheng-san,

I sent email, please share.

Regards,

Itoh

0 Yunsheng Qu over 4 years ago in reply to Kazuki Itoh

TI__Genius 9790 points

Itoh,

Pls help to close this thread since we transfer it to email.

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UCC256303: Start-up failure