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UC2854A: Multiplier optimization / behavior of the IAC pin

Part Number: UC2854A

Hi e2e,


I try to improve the THD of my system (400Hz, 115V). Therefore, the multiplier of the PFC controller UC2854A must be improved. The inputs Vsense and Vrms are understandable, but not the IAC input. After the rectifier arise 800Hz with positive half-waves. This should also be present at the entrance of the IAC. However, only every second half-wave is measured, which corresponds to a frequency of 400Hz. This behavior is very unusual and I would like to know more about the IAC input.

Thanks.

Yours sincerely,

A.Schwarz

  • Hello A.Schwarz,

    The multiplier output MOUT generates a reference signal for the line current and
    in order that the multiplier generate the correct output signal on MOUT it needs to monitor three input signals:

    (1) The output of the error amplifier

    (2) the RMS value of the line input voltage 

    (3) The instantaneous value of the line input .
    This IAC signal is a signal at twice the line frequenc
    Your system is operating at 400Hz so the signal on IAC should be at 800Hz.
    Please refer to Figure 1 in SLUA177A

    IAC is a high impedance low level current input and
    placing a scope probe on this pin may cause false operation of the controller because of the high switching noise present on the pfc.
    This signal you see on IAC may actually be just noise pickup on the scope or measurement noise coupling into IAC.
    In any case the signal on IAC is guaranteed to be at 800Hz if the IAC is only connected to the DC side of the rectifier and your line frequency is 400Hz.

    Regards

    John

  • Thanks for the reply.

    I was decoupling the Iac Pin with a low Capacitor with 100pF. But if this is just noise then the Measurement is useless.

    Is there a problem, when i am going to use a permanent current of 340µA at the Iac Pin? The Datasheet mentions a maximum Current of 250µA should be limited for best performance but nothing more.

  • As you say the data sheet recommends the maximum current at high line should be limited to 250uA for best performance.
    If you wish to use 340uA here then you can expect to extra distortion of the line current due to the non linearity of the multiplier.

  • Since it has been over 2 weeks without further information I am going to close out this post since I think it has been resolved.

    Please open a new post if you have further questions.

    Regards

    John