UCC25660: Not starting under load

Jocelyn Harvey

Part Number: UCC25660

Hello, we have designed a 400W supply using the UCC256601. The power supply is not able to start under load, even with only a 50W load. It just keeps retrying at second interval. Any idea what could be the problem here?

Any help would be greatly appreciated. Thanks!

Jocelyn

4 months ago

0 JCHK 4 months ago

TI__Mastermind 44220 points

Hi Jocelyn,

Can you share scope shots of the BLK pin and the HO/LO pins during startup?

Thank you,

John

0 Jocelyn Harvey 4 months ago in reply to JCHK

Prodigy 20 points

0 Jocelyn Harvey 4 months ago in reply to JCHK

Prodigy 20 points

Hi John,

Ch1 is the BLK pin, Ch2 is the LO pin and Ch4 is the HO pin. The unit is started with a 50W load.

Thank you for your help!

Jocelyn

0 Mike O' 4 months ago in reply to Jocelyn Harvey

TI__Guru 70515 points

Hello,

Your Ho signal does not look correct. It should be a 180 degree phase shifted from Lo.

From your signal it looks like Ho may not be setup correctly.

Do you have a schematic that we could review?

Regards,

0 Jocelyn Harvey 4 months ago in reply to Mike O'

Prodigy 20 points

It was measured with a differential probe referenced to PGND. Do you want the HO signal referenced to HS?

0 Mike O' 4 months ago in reply to Jocelyn Harvey

TI__Guru 70515 points

Hello,

Yes.

Do you have a schematic for review as well.

Regards,

0 Jocelyn Harvey 4 months ago in reply to Mike O'

Prodigy 20 points

0 Mike O' 4 months ago in reply to Jocelyn Harvey

TI__Guru 70515 points

Hello,

Your inquiry has been received and is under review.

Regards,

0 Jocelyn Harvey 4 months ago in reply to Mike O'

Prodigy 20 points

Hi Mike,

I just replaced the capacitor in parallel with R_ISNS (C53 in my schematic) from 100pF to 220pF and now I can get it to start with a 350W load. What is the recommended value for this capacitor?

Thanks,

Jocelyn

0 Mike O' 4 months ago in reply to Jocelyn Harvey

TI__Guru 70515 points

Hello,

Your inquiry has been received and is under review.

Regards,

0 Mike O' 4 months ago in reply to Mike O'

TI__Guru 70515 points

Hello,

I would not recommend going larger than 220 pF. You might want to check to make sure your Isns resistor is the correct size.

When it comes to selecting C53 I would just ensure the capacitance reactance is at least 10 X the ISNS resistor to ground. In this way it will not affect the true current sense signal.

Regards,

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UCC25660: Not starting under load