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UCC2818 Line to Line PFC

Adria Martinez
Prodigy 30 points
Other Parts Discussed in Thread: UCC3818, UCC3817

Hi,

I am desinging a 2KW PFC controller with the UC2818 controller. One of the requeriments of this design is that the

input voltage could be compressed between 380VAC to 440VAC, 50Hz. I have already validate my PFC when the voltage

is less, with 220V to 250V, and the results were satisfactory. But when I change the input voltage, and all the resistances and

capacitors as the datasheed suggest, the results are bad, the Power Factor only is 0.9 at low power, and then decreases.

Please help me if you know something.

Thanks!

  • 0
    TI__Guru 73585 points

    Why do you have to change all of the Rs and Cs for the high line condition?  They should be the same as low line.

    At light loads PF will go down do to the phase shift between input current and input voltage.  This is do to the shift between input current and input voltage caused by the input filter.  At these power levels the specification that you need to meet is input current harmonic content EN61000-3-2.  You can easily hit this with a PF of 0.9.

    Regards,

     

  • 0 in reply to Mike O'
    Prodigy 30 points

    Hi Mike,

    I think my explanation was not clear enough. We started the design considering the PFC connected at monophasic mains. That means 230V nominal voltage, i.e., low line voltage 210Vac and high line voltage 265Vac. The output voltage was set at 600Vdc. Under these conditions, everything was Ok.

    Afterwards, our customer required the equipment to work on triphasic connections. So low and high line voltages changed to 380Vac and 440Vac respectively. So in order to keep Iac below de 500uA limit, we have to change the associated resistors. We also change the output voltage to 700Vdc. Under these conditions, we get a power factor close to 0.9 at relatively low loads. But when we increase the load current, the PF starts to decrease instead of aproximating 1.

    Looks like there is something we didn't take into account when we calculate the new values.

    Many thanks for your help.

  • 0 in reply to Adria Martinez
    TI__Guru 73585 points

    I have not personally done a 3 phase design.  If you are using one UCC3818 for a three phase design this might be the issue.  If you are taking IAC from the output of a three phase rectifier the UCC3817 is trying to get the current to track that waveform which will have a large DC component.  That being said I have heard of some customers doing this and thier designs meet their harmonic content requirements even though the PF is not that great.

    I have also heard that some customers if they are trying to get high power factor on these designs have used three isolated PFC pre-regulators on each phase.  They would need to be isolated from each other with there own down stream converter that need to be load shared.  However, to save money I believe a lot of customers are doing what you are doing and meeting their harmonic content requirements.

     If there is someone else on the forum that has specialised in these designs maybe they would like to contribute some more information.

    Regards,

     

  • 0 in reply to Mike O'
    Prodigy 30 points

    That's also what we're doing, we have 3 isolated PFC's, each one connected to a different phase pair.

    We're now at the stage of testing one single PFC but we're facing the problems commented before.

    Some more information, may be it helps. The input current waveform is senoidal at low output power, but when the power rises, the PF starts to fall and the input  current is having some distorsion.