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UCC38503: UCC38503 : Abnormal AC input current when no load

Ginger Lin
Prodigy 100 points
Part Number: UCC38503

Hi Ti ,

I use UCC38503 as a PFC + forward power module design which rated output power is 300w. When output no load , the AC input current will become abnormal. The current waveform like inrush current when power on. The input AC current become normal when I connect very light load on the forward side, for example 3W load. Is that because the poor compensation of PFC or or current shunt? 

Attached files is my circuit of PFC and abnormal waveform. 

thank you very much

  • 0
    TI__Mastermind 34465 points

    Hello Ginger Lin,

    What is happening is the controller is going into an asynchronous type of burst type of mode at very light loads.

    At light loads the error amplifier output is less than 0.33V and the zero power amplifier turns the pwm output off.
    You can see this in the block schematic on the datasheet for the UCC38503.
    Since the pwm is off the output voltage drops and  the pwm is enabled once the error voltage rises above 0.33V
    So that is why you see an irregular burst on the AC input current.
    In this situation the system is only supplying the overall no load losses.

    In your case this is 3W out of a 300W system.

    Regards

    John

  • 0 in reply to John Griffin1
    Prodigy 100 points

    Hi John

    Could I disable burst mode function? The inrush current made some noise of inductance and EMI noise when no load.

    thank 

  • 0 in reply to Ginger Lin
    TI__Mastermind 34465 points

    Hi Ginger,

    You schematic does not show a bypass diode from the VAC+ to the cathode of D3.
    If you fit a slow speed diode Vout will pre=charge before the pwm starts.
    This will greatly help with startup noise.

    One other thing you can do is increase the value of the resistors R6,R13,R32 from VAC+ to the IAC pin.
    If you do this the EA output is forced to increase and so the zero power amplifier will not activate for this power level.
    Test at high line and minimum load

    If you reduce the power further then the zero power amplifier of course will trip at this lower level.
    It may be possible to find a power level that is outside your range of operation and if you are lucky the no load power level will produce an EA output greater than 0.33V . In that case you will achieve continuous pwm over the no load to full load .

    Regards

    John