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Sizing the common mode choke for output of a PWM inverter

Discussion in 'Electronic Design' started by [email protected], Aug 23, 2007.

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  1. Guest

    I'm working on a DC-AC inverter ckt using bipolar switching at
    switching frequency 39kHz. Could anyone kindly give me any advices on
    how the value of common mode choke is chosen? And also, what kind of
    magnetic material should be used for the core?

    Thanks for any replies.
     
  2. Eeyore

    Eeyore Guest

    Suck and see is the usual method. This kind of circuit behaviour is not readily
    modellable.

    Graham
     
  3. MooseFET

    MooseFET Guest


    You need to know what you are trying to block with the choke. The
    ideal common mode choke has a very high and resistive impedance at the
    troublesome frequencies.

    In general, you want the highest mu you can find. Saturation should
    not be an issue because the currents should match.

    If you can afford to do so, you may want to put a common mode choke on
    both the input and the output. The ground of the converter section
    should have only one route by which its clatter can get to ground.
    That route should be very low impedance.

    The common mode choke is creating a high impedance in one route.
    Unless there is a lower impedance route, the common mode choke is less
    effective.
     
  4. Paul Mathews

    Paul Mathews Guest

    Simple answer: One approach to this problem is to design your circuit
    so that your CM filter requirements are minimal, then choose CM filter
    components that aren't so big that their parasitics end up ruining
    their filter performance.

    Details:
    1) First, do everything you can do to minimize capacitance from high
    dv/dt nodes to anything relating to input or output wires, components,
    connectors.
    2) Provide designed minimum loop area paths for any remaining
    parasitic currents, e.g., snubbers and shunt capacitors. For example,
    for parasitic currents flowing through primary to secondary
    capacitances, electrostatically shield pri to sec and/or add shunt
    capacitance such that the inevitable return current associated with
    the parasitic current can flow in a capacitor placed very near to the
    transformer. In order to design these paths, you'll need to locate all
    high dv/dt elements and at least imagine where their parasitic
    currents will flow in complete circuit, returning to the source. I = C
    dv/dt, so you cannot 'block' the current, only reduce it by reducing C
    or dv/dt, or manage it by shunting it properly. Quite often, large
    currents flow to heatsinks of switching transistors and rectifiers,
    and the return paths for these currents should be considered and
    controlled. Designers sometimes use heatsink spacers to reduce
    parasitic C or even an intervening shield between the semiconductor
    and the heatsink.
    3) Bring differential EMI to acceptable levels using shunt capacitors
    and series inductance (if necessary). Some series L may be available
    from the leakage inductance of your CM filter (see below).
    4) All of the above measures will also reduce CM EMI. So, after the
    above steps, evaluate the need for CM filtering. Important
    considerations usually include cost, size, and weight. A common
    mistake is to choose the largest possible components, thinking that
    they will provide the best filtering. However, the parasitic
    capacitance of the CM filter components to other circuit components is
    often the determining factor in filter performance. Stray magnetic
    coupling is also sometimes a factor. In other words, your nice big CM
    transformer (aka CM choke) can have a lot of parasitic capacitance to
    that MOSFET on heatsink that might be just a few cm away, and you
    suddenly have 100s of microamps of parasitic current flowing directly
    into (one side of) your CM choke. For the same reason, the placement
    and orientation of the filter components are critical. Complete CM
    filters in metal enclosures or inside your own shielding components
    will generally have superior performance to collections of filter
    components unshielded from the switching elements.
    5) Quite small CM chokes an be very effective, particularly in
    combination with shunt capacitors. The main limitation tends to be
    wire size: you need to use a wire size large enough so that heat is
    not a problem. Obviously, a larger core makes it possible to achieve
    more inductance for a larger wire size, but beware the parasitic C.
    The CM choke will also have its own parasitic capacitance from turn to
    turn, which provides a shunt path for EMI. You also need to insure
    that any differential currents flowing in the choke are quite small,
    since saturation is otherwise a possibility.
    6) Many CM chokes are deliberately wound so that they develop a small
    amount of leakage inductance that can help with DM EMI filtering. This
    is done by putting the windings on opposite sides of a toroid or legs
    of an E-core.

    Paul Mathews
     
  5. Paul, all great stuff. The OP said DC/AC converter so the input DC return
    may possibly be tied to the chassis then a input CM choke is not needed.
    WSU?
    Harry
     
  6. Paul Mathews

    Paul Mathews Guest

    Depends greatly on how the power is brought into the enclosure and how
    the power connections connect to the outside world. If any appreciable
    amount of unshielded input power conductor is exposed to dv/dt, the
    resulting parasitic currents/voltages can exit the enclosure and
    radiate from cabling. However, low voltage switching, if that's what
    the OP is doing, does have the advantage of smaller dv in the first
    place.
    Paul Mathews
     
  7. Guest

    Thanks Paul. The reason why I'm asking this question is because I saw
    there is a CM choke used at the output stage of a 230VAC 300W inverter
    I bought. In the spirit of "reverse engineering" I thought I should
    steal the same idea and follow the same in my circuit design, although
    I haven't yet figured out clearly the cause and principal of common
    mode current ( I've read a bit and that it's somehow due to
    transformer leakage capacitance and the cap between heatsink and
    something ).

    My design follows the common architecture of most 12VDC-230VAC
    inverter: DC-DC (full bridge converter) then DC-AC (PIC driving H-
    bridge using IRF840). I've already made both parts of circuit working
    on breadboard (although haven't really loaded it yet since the
    breadboard can't handle any huge current on low voltage side) and is
    now entering the stage of PCB design. But before the circuit is
    finalized I wish to add the extra bit the CM choke in order to make my
    thesis supervisor happier. If the choice of inductance value happens
    to be a matter of trial and error as what Eeyore said, what
    conservative value would you guys suggest me to use, so that I can
    order from supplier straight away and start with the layout design?

    THX
     
  8. Guest

    Or.......In other words, how much the impedance CM choke appearing to
    switching frequency (39kHz) should I design
     
  9. Paul Mathews

    Paul Mathews Guest

    It depends on the nature of the hash on the output, some of which is
    invariably CM (for reasons described earlier, i.e., parasitics). For a
    DCDC converter, there is often energy in the 10s to 100s of MHz that
    results from rectifier turn-off transients. It's usually best to snub
    the rectifiers for this, which takes care of both CM and DM. However,
    any remaining CM requires a relatively small inductance due the high
    frequency. Obviously, the SRF of the inductor must be quite high, so
    single layer windings are the rule. For DCAC, there are no secondary
    rectifiers, and the transients on the secondary are a combination of
    primary-side transients seen through the turns-ratio of the
    transformer and transients arising from parasitic coupling from the
    primary side. Begin by minimizing all sources. Then, common mode
    couple a wide BW oscilloscope or spectrum analyzer to the output and
    see what's left. (One way to do this is to pass all output conductors
    through a wideband current probe.) If you don't have any means of
    probing to see what's there, guess that output CM EMI will have same
    spectrum as primary DM switching transients, which you should be able
    to observe. Suppose that this means you're trying to filter a broad
    spectrum around 10 MHz. You're going to want a few Kohms of impedance
    in CM, so 50 uH or so will get you there. Find a ferrite toroid that
    fits your housing and budget and see if a single-layer widely-spaced
    double winding of adequately sized wire will provide that much
    inductance. If not, go for a larger core.
    Paul Mathews
     
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