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

Discussion in 'Electronic Components' 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 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 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
    to form zener clamp. Is it appropriate? How about using RC Clamp ( as
    shown in )?

    5. When winding transformer, should the direction be "back and fro"
    as in, or always follow
    one direction

    Thankyou for your kind helping ; )
  2. Salmon Egg

    Salmon Egg Guest

    Parts of speech called articles were invented to aid written and spoken
    communication. USE THEM!

    -- Fermez le Bush--about two years to go.
  3. Guest

    I'm not native speaker of English and sorry for my poor expression
  4. Salmon Egg

    Salmon Egg Guest

    I apologize. I am sure your English is better than my capability in your
    language whatever it is.

    -- Fermez le Bush--about two years to go.
  5. legg

    legg Guest

    In figure 3 the deadtime is illustrated as being dependant on the
    timing capacitor size, modified slightly by the optional resistor in
    series with pin7.

    Softstart: using the 50uA present on pin8. the rise in softstart
    capacitor voltage will be linear - i = C dv/dt. How this voltage rise
    will affect your circuit, you'll have to determine yourself.

    SG3525 is originally a Silicon General device - now Microsemi

    Application notes for jellybean two-phase pwm drivers may all be
    applied to a degree. There's not much special about the IC itself.
    In your push-pull topology, the snubber should act somehow to limit
    voltage peaks and reduce radiation when the mosfets turn off. A
    current snubber will limit dv/dt. At the low voltage you're operating
    at (12V), a voltage clamp might not be needed to protect parts.

    You do not mention a power level.
    How you clamp is your choice. Ask yourself what's going to get hot.

    If a lot of energy is involved, you might consider trying to recover
    it, rather than burning it off.

    Energy stored at any time is ( Li^2)/2
    Leakage energy is stored in the leakage inductance. See references

    Check out the old unitrode seminar at TI

    2001 magnetics design handbook chapters are downloadable

  6. Guest

    Hello legg, thankyou for your reply which has been useful. But there
    are still some bits which I'm not quite sure:

    1. You said deadtime is illustrated in figure 3 on datasheet, so is
    deadtime = oscillator discharge time? And which application note on
    jellybean two-phase pwm driver are you referring to, since I couldn't
    locate on microsemi website?

    2. I ran a simulation on push pull stage of the circuit, and found that as soon as M1
    and M2 has just turned off, the energy stored by MAGNETIZING flux from
    Lm (not leakage inductance) will get discharged by lower section of
    primary winding. Demagnetizing current will flow upwards back to 12V
    source through body diode of M3 and M4, as shown in this picture . I've heard that body
    diode of MOSFET is a by product during fabrication, and is of slow
    recovery time. So should I place a fast recovery diode or schottky
    across each MOSFETs to facilitate the flow of this current?

    P.S. I'm working on a 300W DC-DC converter which steps up 12V to
    360VDC. The transformer core is ETD49, Np=6T+6T, Ns=198T. Calculated
    magnetizing inductance =200uH. Total primary current at rated load =

    thanks again for helping ^_^
  7. legg

    legg Guest

    Look at SG3524, UC3525

    Intersil an6915
    Nat Semi AN-292
    Philips AN126

    The links are straight Google hits.

    You will be looking for power stage info, not info on the driver or
    it's interface.
    Your reference is not to a conventional push-pull converter, it is to
    a DC-DC transformer, with no energy storage element in the output
    rectifier filter. This is basically capactor-to-capacitor energy
    transfer, with no regulation ability.

    The control chip is completely wasted in this application.

    As to your model, I have no comments, as there are models and then
    there are models.
    For example, current will not appear 'across' the mosfet switches - it
    will flow 'through' them. If your GUI makes this kind of an error, it
    can hardly be expected to keep track of it's own bootlaces.

    Remember to apply a load to your model, as unloaded operation will
    hide the relative importance of the currents involved.

    With both switches off, the magnetization current will have the
    opportunity to ramp to zero (in a perfectly coupled transformer),
    through which-ever path presnts itself. As your model image does not
    illustrate drain voltages, you can have no idea where the magnetizing
    current is flowing, or why.

    Once you apply a load and view the resulting waveforms, you may
    understand why capacitive-capacitive energy transfer has it's
    problems, without the presence of effective intermediate inductive
    storage. Perhaps leakage terms will suffice. Don't forget them in your

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