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UCC38050: UCC38050 PFC based SMPS - operation on 400Hz AC input

Janos Baroczi
Prodigy 20 points
Part Number: UCC38050

Dear Members! In a near-aircraft, fixed ground application i have to run a special, multi output SMPS which has a UCC38050 based PFC front end. So far this SMPS was run off regular 230V 50Hz mains, but due to the re-structuring of the whole application this supply will not be available anymore. The only power which will be available is 3x208V 400hz AC.

Before trying running the power supply on 1-phase of the 208V 400Hz supply I was browsing the datasheet of the UCC38050 and found no information on the maximum input frequency for the rectified half-sine waves that the controller is able to operate on. In case of 400Hz AC input the PFC circuit has to cope with a mains reference signal of 800Hz and not really shure if the multiplier block will cope with that frequency and cause instability or even oscillations in the control loop.

Other issue i considered running the SMPS on 400Hz is the  increased leakage currents in the EMI filter from line to neutral, but beeing a fixed installation this is now a minor issue.

As there is no requirement for the harmonics and/or power factor i was thinking to rectify+filter the 3x208V mains and power the SMPS with the filtered DC voltage of around ~280V. In case of this sort of DC operation (as i understood the transition operation of the UCC38050) the whole circuit will run as a current mode boost controller still operating on the border of DCM-CCM.

Do you have any experience of running PFC controllers designed for 50/60Hz mains on 400Hz? Do you see any risk of operating the unit on DC power? Thanks a lot!

  • 0
    TI__Expert 8210 points

    Janos

    Thank you for reaching out.  One of our experts will reply to you tomorrow.

    Best Regards,

    Eric

  • 0
    TI__Mastermind 33240 points

    Hello Janos,

     

    Thank you for your interest in the UCC38050 TM-PFC controller.

     

    To answer your last question first, there is no risk to operate the PFC on a DC input. It will simply act as a boost regulator and the input current waveshape should track the input voltage ripple on the DC level.

     

    Addressing the first issue: the multiplier block in the UCC38050 should have no trouble tracking the rectified 400Hz waveshape. There is plenty of bandwidth in it; no worry of oscillations. 

     

    Your middle issue is of the most concern: the effects from the EMI filter.
     Since this filter was designed for 50/60Hz, it will generate 8X higher capacitive current at 400Hz.  And the series inductance will have 8X higher impedance, shifting the voltage a little at the input to the diode bridge. 

    The voltage shift will cause the PFC to track that voltage and the resulting input current will be shifted from the raw input voltage. Furthermore, the displacement current will also add to this phase shift so PF will suffer.  It is not the PFC controller doing this, it is the EMI filter. 

     

    So if you simply take the 50/60Hz SPMPS without any modifications and run it on 400Hz, it should work but with higher phase-shift, more cross-over distortion and lower PF than before. PFC loop response (targeted for 60Hz) will be slow to 400Hz input deviations.  If the PFC stage and EMI filter can be redesigned for the higher input frequency, that should alleviate the PF/THD concerns. 


    As you mentioned, if there are no PF or harmonics specs to meet, then rectifiying 3x208V as an input to the SMPS might be the simplest solution, to balance the load across all three phases.  I don’t think that you need to add more filtering after the 3-Ph rectifier, but it does bear testing to verify this.

     

    Regard,
     Ulrich