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.
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
