Re: Motor controllers, RFI & stuff (esoteric question).

[Lee Hart <leeahart@earthlink.net>]

Lee Hart wrote:
>> The filter losses would be shifted into the filter components in the
>> controller. Most likely, the controller's switching frequency would be
>> increased to 50 KHz or so (roughly the state of the art in high power
>> switchers) to reduce the size and cost of the filter components.
>> Also, it is likely that some form of "soft switching" would be used
>> to reduce switching losses. Overall, the system efficiency would
>> probably rise a bit, but the cost would go up.

> Could you explain what 'soft switching' is?

Sure. Soft switching is a catch-all term for all the various techniques
that one can use so that a switch only opens/closes when the voltage and
current on it are (approximately) zero.

The usual method is to add a resonant L-C circuit to the switch. You
time things so the voltage across the switch just happens to be zero
when the switch closes, and the current in the switch just happens to be
zero when the switch opens. Thus switching losses are (approximately)
zero.

A simple example. Suppose you add a capacitor in parallel with the
MOSFET of a conventional PWM controller, and add an inductor in series
with the connection between MOSFET and motor+freewheel diode.

Assume the MOSFET is on. Therefore the voltage across it and the
capacitor are essentially zero. Now turn off the MOSFET; while it is
turning off (which takes a microsecond or less), its voltage stays at
zero because of the capacitor. Therefore, MOSFET current goes from max
to zero while its voltage stays near zero. Switching power loss in the
MOSFET is therefore essentially zero.

With the MOSFET off, the capacitor now charges from the inductor
current. The inductor and capacitor form a resonant circuit. The voltage
will rise sinusoidally to a peak (of about 2 times the supply voltage),
then resonantly ring back down to almost zero. At the instant the
capacitor voltage hits its minimum, you turn the MOSFET on again. During
its turnon time (a microsecond or so), it sees the curent go from zero
to max, while the voltage is nearly zero. Thus turn-on switching power
loss is essentially zero.
-- 
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
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