UCC28780: Converter startup problem

Raju Baddipadige

Part Number: UCC28780

Hi,

We are designing a 12V/65W power supply using UCC28780(QFN package). During testing, we observed that there are PWML pulses. Please see the below image CH1:Vdd CH2:Voltage on HVG pin CH3: Voltage on SWS pin, CH4: Reference voltage on REF pin. Please also find attached the sheet for BOM. D_SWS=18V, D_HVG=27V. We tested three cards, it was the same. It looks startup sequence is okay, but there isare PWML pulses. Please let me know the reason. Did I make any mistakes?

8863.BOM.xlsx

over 2 years ago

0 Raju Baddipadige over 2 years ago

Prodigy 200 points

and also it took more than 50us to discharge the HVG pin voltage to 11V.

0 Raju Baddipadige over 2 years ago in reply to Raju Baddipadige

Prodigy 200 points

Hi,

Sorry, CH4 is not exactly on the reference voltage. It is on the BUR pin. There is a resistor divider between the BUR pin and the reference pin. RUBR1=120kE, RBUR2=51.1kOhm. If we compute the back voltage on the Reference pin is 5V(1.5V*(RBUR1+RBUR2)/(RBUR2).

0 Ulrich Goerke over 2 years ago in reply to Raju Baddipadige

TI__Mastermind 40100 points

Hello Raju,

Thank you for your interest in the UCC28780 ACF controller.

Unfortunately I am unable to determine the problem that you are experiencing from the information provided. Providing the full Calculator Tool would be more helpful because it lists your design targets, and a schematic diagram of your design would be most useful.

In your first posting, you mention that "...there are PWML pulses". Normally this is completely expected and is a good thing.
Am I correct in assuming the you meant "...there are NO PWML pulses"?

Please follow this debug guide for start-up issues, which can help you solve them: https://www.ti.com/lit/pdf/slua982

Regards,
Ulrich

0 Raju Baddipadige over 2 years ago in reply to Ulrich Goerke

Prodigy 200 points

Hi Ulrich,

Thanks for your quick reply. Sorry, there was a typo, there are NO PWML pulses. Please find attached the design sheet.

1588.UCC28780 Excel Design Calculator (Rev. C) SLUC664C.xlsx

Thanks and Regards,

Raju

0 Ulrich Goerke over 2 years ago in reply to Raju Baddipadige

TI__Mastermind 40100 points

Hello Raju,

Thank you for providing your full Calculator file. It will help a lot to understand your system.
But I do have some questions about your choice of certain component values. Several of them are far from the recommended values.

But first, have you had a chance to examine your board for problems related to HVG, RDM and/or RTZ?
In the debug guide, Table 3 (page 9) lists those 3 pins as having possible opens or shorts to GND which can prevent all PWML pulses.

Please check those possibilities.

Regards,
Ulrich

0 Ulrich Goerke over 2 years ago in reply to Ulrich Goerke

TI__Mastermind 40100 points

Hello Raju,

I haven't heard back from you in almost 2 weeks. Have you been able to resolve your start-up issues?
If so, please let me know and I will close this thread.

Regards,

Ulrich

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UCC28780: Converter startup problem