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UC2825: PWM IC going to shutdown mode

Nesh Johny
Expert 1310 points
Part Number: UC2825

Dear SIr,

UC2825 is used as PWM controller for forward converter in my board with 100kHz and another driver IC is used for driving the MOSFET, (PWM signal only taken from UC2825). once the board reaches 65 degree the converter goes to shutdown mode even if there is no fault conditions, I have disabled all the protection circuits on my board. what could be the reason for the PWM controller to go to shutdown mode.I tried giving the bias supply from external power supply also (12V). Still the converter goes to CC mode (ie tries to draw huge current, since i have kept the external power supply on limit, converter goes to cc only).

Feedback is in voltage mode only. the shutdown happens only when the converter pcb temperature reaches around 55degree and  PWM IC temperature reaches 65degree (approx).

How can i stop this shutdown issue at full load / PCB temperature crosses 50degree then within 1hr IC will go to shutdown?

what all are the reasons for PWM controller shutdown.

Regards,

Nesh

  • 0
    TI__Genius 13660 points
    Hi Nesh,

    The transformer is most sensitive to the temperature in the circuit, May is there a issue of Magnetic saturation?
    Because Magnetic saturation will lead to low bias supply, and over over current. As you said, IC goes to shutdown when using bias supply, IC goes to CC mode when using external 12V.
    Besides, how did you measure IC junction tempeture TJ?
    an estimation of the chip-junction temperature TJ, can be obtained from the following equation:
    TJ = TA + ( RθJA × PD )...
    where: TA = ambient temperature for the package ( °C )
    RθJA = junction to ambient thermal resistance ( °C / W )
    PD = power dissipation in package (W)

    Regards,
    Teng
  • 0 in reply to Teng Feng
    Expert 1310 points
    Dear Sir,

    I have checked PWM IC case temperature only, which was around 65-70 degree when PCB temperature was 55 degree. As you suggested i will cross check the magnetic saturation issue.

    Regards,
    Nesh