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UC3854: I use UC3854 for PFC bus, the Voltage test value are different between test samples

chen chen35
Intellectual 875 points
Part Number: UC3854
Other Parts Discussed in Thread: , UCC3817, UCC3817A

Dear All：

          I use UC3854 for PFC bus,when I test the U0,the value are different  between different  device .The schematic as follows：

The sample 1 voltage waveform as follow：

1、Device start

we can see the voltage on bus U0 is 378.5V-361.5V，∆U is 17V, the cycle is 3.4s.

2、Sample 2 voltage wave（ I just change another UC3854）.

we can see the voltage on bus U0 is 386.5V-378.5V，∆U is 8V, the cycle is 1.5s.

I want to kown why the test value are different between different chips?Is there something wrong in my design?Do you have Any good suggestions for improvement.Thanks a lot!

  • 0
    TI__Mastermind 34465 points

    Hello,

    You have a low frequency instability in your power supply. You should re-calculate the compensation network component values in the feed back circuit.
    The first sample shows an instability at 0.3Hz and the second is unstable at 0.6Hz. When your power supply is operating correctly , you will only see a low frequency  oscillation at twice the line frequency and a high frequency oscillation at the switching frequency. Somewhere in your circuit you have a very long time delay that is causing the controller to compensate for an error voltage that has been delayed for a very long time.
    Also the UC3854 is a very old part number. It has been superseded by the UC3854B,UCC3817 and the UCC3817A, and the UCC3817B.
    Regards,

    John Griffin