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UC28025: short circuit protection does not work

Yue TANG
Expert 8705 points
Part Number: UC28025

Hi,

My customer use below design,but found the short-circuit protection function of UC28025 does not work. please kindly suggest what may be the reason?

thanks.

  • 0
    TI__Mastermind 31580 points
    Hi Yue,

    The UC28025 has an internal comparator that shuts down the PWM when the signal on the CS pin exceeds 1V. Under short circuit conditions the primary side peak current will be limited by this function but the average current in the secondary side can be very high and this may cause the power components to become over stressed if they have not be designed to meet this higher power level, or if the cooling provided is not sufficient to keep the devices withing the recommended ratings.

    Please ask the customer to increase the load past full load and monitor the voltage signal on the CS pin. Once the peak voltage on the CS pin they should see the PWM duty cycle reduce.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    TI__Expert 8705 points
    Peter,
    Thanks for the reply. What they need is to short down the PWM once the secondary side meet the short circuit conditions. Obviously, you clearly explain why their current design can't do this. would you please suggest if can add some circuit to meet this short circuit protection requirement? thanks.
  • 0 in reply to Yue TANG
    TI__Mastermind 31580 points
    Hi Yue,

    Normally the IC has a start-up circuit like the one in the customers circuit but once the device is running there is a feed from an aux winding on the transformer which provides the voltage rail to the IC. If the circuit goes into short circuit the aux voltage drops and the IC turns off. The resistors in the start-up circuit are chosen so they can charge the vdd cap to the UVLO turn-on level but they are too high to allow the IC to run continuously from the start-up circuit. This allows the power stage to go into a hiccup mode of operation which reduces the power dissipation.

    One way to try is to measure the voltage on the EAOUT pin and if it is above 4.7V shutdown the supply rail to the IC and let the chip go into hiccup mode and restart. The EAOUT should only be above 4.7V during a short circuit condition but the customer will need to check operation at low input voltage and full load to avoid shutting down by mistake.

    Regards

    Peter