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TPS23750 Flyback-Converter over heating?

eric kim
Expert 1400 points
Other Parts Discussed in Thread: TPS23750, PMP9175

Hi,

I have question about TPS23750  Flyback-Converter heating issue,

in case of Zener D7(BZT52C18, ) and T2(C1174-AL: Coilcraft),

These two components are over heating upto 100 'C at room temperature..

If changed D7 to DFLZ18-7 or PTZTE2518B, It's almost same temperature around 100 'C.

Could you help me how to decrease the temperature?

Best regards,

eric

  • 0
    TI__Mastermind 40395 points
    Hi Eric,

    Are you testing the orderable EVM? Or your own layout board using the EVM circuit design? What output voltage version are you using?

    What are your operating conditions (input voltage, output power)?
  • 0 in reply to Darwin Fernandez
    Expert 1400 points

    Hi darwin,
    Thanks for your guideline..

    The test board is not EVM but real Board
    that size is small 30x30(mm) and using the EVM circuit design.

    Topology is flyack converter,
    Input Voltage is 48Vdc,
    Output voltage is 5W(5V/1A(normal) ~ 5V/1.4A(Max.)).

    Thank you,
    eric

  • 0 in reply to eric kim
    TI__Mastermind 40395 points
    Hi Eric,

    I recommended double checking first the board that there are no shorts, misconnections, and there are correct component values (as there are BOM changes for a 5V solution compared to the schematic shown in the user's guide). As this is a basic flyback design 500mW for the zeners are typical to many similar designs with no issues. I would also recommend ordering the EVM so as to have a working solution to compare to (with BOM change).
  • 0 in reply to Darwin Fernandez
    Expert 1400 points

    Hi darwin,
    Thank you for the recommendation,
    So I have ordered the EVM for debugging.
    after getting the EVM, I will check what is differential against real board.
    real Board design is almost same using the EVM circuit design...

    Thanks...

  • 0 in reply to eric kim
    Expert 1400 points

    Hi Darwin,

    I've checked TPS23750 flyback converter EVM after changing 5.0V output according to the Load Spec of the EVM's BOM  as below: 

    (Because of EVM's default output is 3.0V)

    R12 : 1k

    R13 : 1.5k

    R18 : 86.6k

    R19 : 13.3k

    R20 : 000

    T2 : C1174-AL

    and captured a thermal picture by attached.

    It's appeared to 102 'C degree at D7(BZT52C18, 18V Zener),

    Could you please let me how to decrease the heating Zener diode?

    Best regards,

    Eric

  • 0 in reply to eric kim
    TI__Mastermind 40395 points
    Hi Eric,

    These zeners may get hot as zeners are relatively slow so they don't clamp well. Even with TVS which are faster, they can get hot. The best you can do with current solution is get high rated zener and more copper to dissipate the heat. But this can only help slightly.

    Best solution is to use new clamping circuit we use on our new self driven reference designs like shown in PMP9175.
    www.ti.com/.../tidr643.pdf

    This is a more thermally efficient solution and cost of circuit is same as two zeners.
  • 0 in reply to Darwin Fernandez
    Expert 1400 points

    Hi Darwin,

    Thank you for the solution,

    Can I replace direct p2p to two Diode(MMSD4148T1G) instead of D7(BZT52C18) without any additional componets? for example TR, Cap, Resistor?

    Thanks,