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SMPS history Q

Discussion in 'Electronic Design' started by Bob LaFrance, Jul 20, 2004.

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  1. Bob LaFrance

    Bob LaFrance Guest

    Flyback power supplies are most always drawn with a capacitor from
    input to output. Capacitor is Y rated and goes from either the plus
    or minus bus to circuit common on the secondary side.

    I know the cap will help keep dv/dt down across the transformer. This
    would tend to help calm things down if there is an opto feedback
    scheme. I suppose it may also help with emi considerations.

    I once used one of those servo type optocouplers - TIL300/HCNR200 for
    analog current feedback from negative bus to circuit common.
    (Application was 480v induction drive) With long motor leads, I
    needed to increase cap to 0.1uf to get circuit to work properly.

    Are there any other reasons for cap? Does anyone have any history
    with part. SMPS lore? Have you seen problems that required the part,
    or more or less capacitance?

    Reason is my superiors would like to decrease the value of part in
    order to meet some CE leakage requirements. I told tech to go run
    some drives without the cap and report back to me.

  2. legg

    legg Guest

    Some other methods of reducing effects of common mode return current
    from the secondary are:

    - screening the transformer properly to a 'quiet' primary node.

    - using a screened optocoupler ($ and safety implications)

    - improving transformer winding layout to balance internal currents so
    that the secondary looks like a null point.

    -Reducing secondary-side generated common mode current by reducing
    voltages appearing on grounded heatsink rectifier isolation barriers
    and by using the same nulling techniques as for primary-generated

    - pumping antiphase current from a dedicated winding into the coupled
    ground reference, to cancel strays at the frequencies of interest
    (using small controlled coupling caps and larger winding voltages).

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