.
>
> Correct, it's the dv/dt that causes RMI. No matter if it's
> positiv or negative. Also no matter of it's an traic, mosfet, IGBT
> or whatever.

That would be true if there is no snubber cap across the Fet or IGBT. That
cap will allow a low dV/dT without causing losses in the fet. Like this the
switch-off losses are zero, and as you stitch on at zero volts, the turn -
on losses are also zero. Fets can switch much faster than triacs or SCRs. I
will see if I can find my old circuit.


> > The practical difference
> > being that the triac "turns itself off" at the next z-c point, which
> > keeps switching and s/w simpler
>
> Yes, you just have to "count" from the last z-c point ant "trig"
> the triac, possibly with just a short puls. The shutoff is handled
> by the triac itself at the next z-c. Note that for inductive loads
> this gets a little more complicated...
>
> > From what I can gather, series inductors, shielding and good layout
> > are fairly effective at minimising RFI
>
> I also thinks that you can get pretty low RMI by using suitable
> shokes on the line (mains) terminals. The large part of RMI is
> transitted from the mains cords attached to the switching device.
>
> > To keep it simple, it seems that
> > something other than a triac is needed for PWM if you wish to use
> > the leading z-c and turn the juice off mid-wave
>
> Yes, that's why people are looking at MOSFETs, IGBT's and other
> devices. But note, the reason these gives less RMI are that they are
> switching off slower (at mid cycle) then the triac turns on
> (also at mid cycle). Slower switching also gives higher loss (heat)
> in the swithing device. If you are switching the MOSFET hard (as
> you normaly want to do) up to the same speed as the Triac, you stll
> get the same level of RMI. There's no free lunch, right ? :-)
>
> Jan-Erik.
>
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