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UC3875: Input filter capacitor

ghasem aghdam
Intellectual 455 points
Part Number: UC3875

Hi there,

I'm trying to find some guidelines for choosing an input capacitor for regular phase-shifted full bridge (PSFB) topology. Do you have something (any documents/ANs) to share with me? Any help you could provide would be much appreciated.  

Looking forward to hearing from you.

Thanks & BR,

Ghasem

  • 0
    TI__Mastermind 34465 points

    Hello Ghasem,

    The capacitor needs to be able to supply the input current switching wave shape to the converter whether it is a PSFB or any other kind of converter.
    So you can calculate the RMS input current and ensure that the input capacitor is rated for this value.
    That is the minimum input capacitor requirement. The actual value used will normally exceed this number.
    In practice because the power rating is proportional to the input capacitance times the input voltage squared then the amount of input capacitance is proportional 
    to W/(V^2). So for example the UCC28950EVM has an input voltage of 390V  with a power rating of 600W and it uses 330uF input capacitance.
    This is sort of a rule of thumb used for the initial prototype design.

    Your next step would perhaps be to simulate your design with Power Stage Designer tool and this will accurately calculate the capacitor ripple current and voltage.
    You will then be able to check the effects of increasing or decreasing the capacitor value.
    Your final design will be limited by the volume of space available for Cin and EMI considerations since differential mode noise is minimized by increasing Cin.

    There also may be a requirement for a holdup time for the converter and the value of input capacitance may be determined solely by the operating range of input voltage and the hold up time.

    Hope this helps .

    Regards

    John