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Original question:

UCC28064A: About using in single mode

UCC28064A: About single mode startup

kiri yasuda
Prodigy 40 points
Part Number: UCC28064A


Tool/software:

Hello.

The UCC28064A is continuously operated in single-phase mode to operate the PFC circuit.
Look at the waveforms [ch1: FET_Vds, ch2: IL, ch3: FET_Vgs, ch4: VCin].
This shows the waveform at startup, where the falling edge of the coil current waveform is not straight.(waveform1) In addition, the triangular waveform is extremely small.(waveform2) When these occur, the voltage waveform of Cin behind the diode bridge is disturbed. Only at startup is such an unusual waveform seen. The current capacitance of the X capacitor and the Cin is 2μF. This is the limit of the capacitance size because the board layout makes it impossible to increase the size. Also, the coil is not saturated.

Are there any other countermeasure suggestions besides changing the constants of the capacitor and coil?

Does the waveform disturbance at startup cause any abnormality in normal operation?

Thanks.

Waveform 1

Waveform 2

  • 0
    TI__Mastermind 40580 points

    Hello Yasuda-san, 

    Start-up of the PFC is a special transient condition, where Vout is very close to the peak of Vin.  
    In this case, the inductor demagnetizes with a small voltage Vout - Vin, so the current down-slope takes a long time. 
    The inductor down current is not straight because the input votlage (Vin) is the voltage across Cin and this is varying as you can see in the zoomed green trace. 
    So Vout -Vin has a varying value and di/dt of the inductor varies with it. 
    This is normal behavior.  It goes away as Vout increases. 

    In waveform 2, most of the current is limited by the Peak Limit threshold.  
    There is one small pulse that has an early turn-off, compared to the others.  I think this can be from noise at the CS input.  

    Sine these effects are of short duration, they do not affect the steady-state THDi.
    They do not cause any abnormality in normal operation.

    Regards,
    Ulrich