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UCC28782： REF pin does not start(LOW TEMP)

TETSU
Prodigy 250 points
Part Number: UCC28782

Hello TI-Team

UCC28782 does not start at low temperatures (-10 ° C).

Please check the attached file.

REF don't stand up(Low Temp ).xlsx

As the temperature is lowered, VDD rises, but REF does not rise.

P13 goes up, but after reaching VDD(on), it doesn't go down to 13V.

It is a power supply with a fixe output voltage and does not use the Vsub booster circuit. (Not a USB-PD application)

Vsw and BIN are connected to GND.

The IC version is UCC28782ADRTWR.
I've tried a few, but some start.

Are there any possible factors?
Is it a malfunction of the IC?

TETSU

  • 0
    TI__Expert 7080 points

    Hello TETSU,

    UCC28782 can support -40℃ temperature operation, so it should not be malfunction problem . but some external components parameters and internal threshold will changed as the temperature reduce . Could you please try below actions

    1. Remove the Zener or TVS on P13 pin , remove the TVS between HV start depletion FET Vgs , since the clamp voltage maybe will decrease at low temperature which is very close to VDD_ON threshold.

    2. Re-solder the IC to ensure the solder is OK

    3. Check the capacitors on REF/VDD pin and rectifier diode on AUX winding path.

    3. Swap the "bad" IC and good IC to see if the failure follow with IC or follow with the board.

    Thanks.

  • 0 in reply to Jaden Ning
    Prodigy 250 points

    Hi, Jaden

    Thank you for your quick response.

    I changed the Zener of P13 from 18V to 20V and it worked.
    The cause was that the Zener voltage and UVLOon were reversed.
    I connected the EDZ V18 (18V Zener).
    Including the variation, it seems that the 18V Zener is calculated to be reversed.
    The data sheet said that I would recommend a 20V Zener.
    Should I connect a 20V Zener?
    Is the 20V Zener to protect the SWS pin when the P13 is open?
    Please tell me how to determine the Zener voltage of pin P13.


    Thanks.

  • 0 in reply to TETSU
    TI__Expert 7080 points

    Hi TETSU,

    20V Zener on P13 pin is recommended , since the Zener voltage has variation range , take 18V Zener for example . the Zener voltage maybe will from 17.1V~18.9V , 17.1V already touch the VDD_ON range that may cause control start failure .

    The 20V Zener is to protect P13 avoid damage when surge energy coupling to this pin.

    Thanks.

  • 0 in reply to Jaden Ning
    Prodigy 250 points

    Hi, Jaden

    What is the absolute maximum rating of the P13 pin? (20V?)
    If the absolute maximum rating is 20V, the 20V Zener cannot keep the absolute maximum rating of P13 due to variations.
    What should I think about the surge exceeding 20V?

    Thanks.

  • 0 in reply to TETSU
    TI__Expert 7080 points

    Hi TETSU,

    20V is the absolute maximum rating for P13 which written in the datasheet

    Except for the Zener on P13 we also has a 1uF capacitor on this pin , so the Zener is to clamp the high spike voltage while capacitor also can filter some energy of coupling noise . the part number of MM3Z20VST1G has a narrow range Zener voltage is proposed by EVM.

    Thanks.

  • 0 in reply to Jaden Ning
    Prodigy 250 points

    Hi, Jaden

    I checked the data sheet of MM3Z20VST1G.
    However, it seems that it exceeds 20V due to variations.
    This Zener cannot keep the absolute maximum rating of the IC.
    Is it okay for the P13 terminal to exceed 20V for a short time such as a surge?
    The data sheet stated that it would be used to protect P13 when it was opened.
    Isn't it connected for the purpose of protecting the SWS pin when P13 is not connected?

    Thanks.

  • +1 in reply to TETSU
    TI__Expert 7080 points

    Hi TETSU,

    Actually, SWS can be protected by the bidirectional TVS which connected on Vgs of depletion Mosfet . and the maximum allowed voltage for SWS is 38V.  firstly we should guarantee the clamp voltage on P13 never less than 17.5V typical VDD_ON voltage , then we can consideration the effectiveness of over voltage stress . I believe it can afford a shot overvoltage stress in a very short time , even it didn't indicated in the datasheet.

    Thanks.