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Problems of transformer, snubber and freewhee design of push pull DC converter using SG3525

Discussion in 'Electronic Design' started by [email protected], Apr 19, 2007.

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  1. Guest

    Hi everyone, as part of inverter project I need to build a step up
    converter that converts 12VDC to 360VDC using push pull topology.
    Switching frequency is at 55kHz and using SG3525 as PWM chip. I have
    a few question:

    1. Datasheet of SG3525 from manufacture is too brief. It does not
    explain clearly how exactly to size Rd for dead time nor how to
    calculate value of slow start capacitor. Does anyone know where I
    might find a reference design or app note of at least a buck converter
    using this chip, as I could find none from ST website?

    2. What is the exact use of snubber circuit with MOSFET? Is it to
    absorb voltage spike due to leakage or magnetising inductance of
    primary winding when MOSFET is turning off? Or to damp ringing at
    rising edge due to to fast dV/dt? Where should it be placed? As in one
    example http://sound.westhost.com/project89.htm the snubber is placed
    across primary winding, but I also saw some circuits where it is put
    across D and S of MOSFET.

    3. MOSFET use gate resistor to slow down turn on transition which
    could otherwise cause ringing. Isn't that overlap the purpose of
    snubber? Sorry I've read too much app notes I messed up the concept.

    4. When switching high frequency transformer, the ideal transformer
    output waveform should look like:

    ------- --------
    | | |
    | | |
    -- --- ---
    | |
    | |
    -------
    it's necessary to discharge energy stored by leakage inductance at
    crossover. Does snubber do this as well? How about the energy stored
    by magnetizing inductance and magnetizing current? does it also need
    to be discharged? And if so, should I use a freewheeling diode (fast
    recovery type) across primary winding? My first thought was to connect
    two diode this way http://www.geocities.com/w2kwong/Untitled.gif which
    proved failed in simulation as D1 will short the upper winding when
    voltage is induced by lower winding as lower MOSFET is on. So instead
    I added two zener diodes http://www.geocities.com/w2kwong/Untitled2.gif
    to form zener clamp. Is it appropriate? How about using RC Clamp ( as
    shown in http://www.geocities.com/w2kwong/Untitled3.gif )?

    5. When winding transformer, should the direction be "back and fro"
    as in http://www.geocities.com/w2kwong/Untitled4.gif, or always follow
    one direction http://www.geocities.com/w2kwong/Untitled5.gif?

    Thankyou for your kind helping ; )
     
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