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UCC28720 / R4 on the figure 30

Other Parts Discussed in Thread: UCC28720


I received the following question from my customer.

There is the R4 on the figure 30 of the datasheet.

Is this resistor for VDD filter?

Could you show me how to calculate this resistor?

Best Regards,


  • Kuramochi-san,

    Yes this resistor R4 is used as a filter for VDD. It limits the peak charging of VDD due to the leakage inductance energy.

    It's not easy to explicitly calculate the value of R4, usually the user needs to experiment with the value needed for each design. Mostly it depends on the aux winding leakage inductance. For a high leakage, a bigger resistor value is needed.

    If R4 is too small, VDD will increase to a very high level at full load. If R4 is too big, VDD will fall too low at light load or no-load.

    Usually, R4 value can be from 1 ohm to maybe 22 ohm, depending on the power level and the transformer leakage inductance.

    For the UCC28720 datasheet example, at 5-W output, a 22-ohm resistor is used. At higher power levels, usually a smaller resistor value is used. At 65 W, I have used 4R7 in the past, but it depends on the transformer leakage inductance.

    Usually, it's better to keep the R4 value smaller, and only increase it if the VDD level is getting too high in the application.

    I hope this helps.