Bob Masta wrote...
Actually, it was the reverse conduction that I was talking
about. I have been thinking about how to use MOSFETS for
high-power AC control, and this is the problem. It's easy
to find high-voltage, high-current devices, but they don't
block the opposite polarity at all.
Correct, all vertical MOSFETs (often called DMOS, VMOS, etc)
have an intrinsic diode in parallel with the FET.
I was hoping to take advantage of low MOSFET losses and easy
load-sharing, but it seems you need a rectifier in series
with the device... so there goes all your power savings into
the diode drop!
If you wish to use FETs, the solution to this is to use two
N-channel parts back-to-back (connect their sources together
and their gates together). FETs happily conduct current in
both directions, so when you apply gate voltage, the FET that
has reverse current will be on, shunting its intrinsic diode
with a low Ron, so you won't suffer any diode drop.
..
.. --------+-|<|-+-+-+-|>|-+------- +/-400V and 2A rms max
.. | | | | | (w/o any heatsinks)
.. IRF740A ', ,-,' | ',-, ,'
.. (2) | V |s | s| V | the diodes shown across each
.. ----- | ----- FET's drain-source leads are
.. PVI5033R ---, | ,--- intrinsic to all vertical FETs
.. ------, | | |
.. gate |-----+--|--'
.. drive |--------' isolated gate-drive voltage
.. ------'
..
Excuse the poor ASCII drawing. One thing that's evident from
the drawing is that for AC-line switching, the gate drive will
have to be floating. That's the problem faced in a solid-state
relay, or SSR, that uses FETs, and this is solved by using a
stack of photodiodes driven by an IR LED. It's not fast (200us,
etc) but it works well and gives 2500V of optical isolation.
If you want to make an optocoupler SSR with your own choice of
FETs, you can use IR's PVI5033R isolated 10V 5uA gate-driver,
which includes a special fast shut-off circuit (see figure 4).
http://www.irf.com/product-info/datasheets/data/pvi5033r.pdf
DigiKey (and Newark) has these in stock at $5.75 ($5.45) each.
For example, using a PVI5033R and two IRF740A MOSFETs you can
make a +/- 400V, 1-ohm switch that will turn on within 12 ms
and off in about 0.5 ms, using 5mA drive to the PVI5033's LED.
Thanks,
- Win
(email: use hill_at_rowland-dot-org for now)