RE: transistors instead of diodes?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>

>I am wondering if in place of fast-recovery diode it would be possible to
>use a second transistor in anti-parallel with each transistor of the
>H-bridge

Hi Jolyon,

If you use MOSFETs then this happens automatically since MOSFETs conduct
equally well in either direction when they're on. The trick is used in
industry (it's called a synchronous rectifier)

However, there is no real advantage to it since the fast recovery diodes
already make a good job of returning as much spike energy as possible to the
DC supply. It's only useful in low voltage high current circuits where the
MOSFET has lower conduction losses than the best schottky diodes.

IGBTs only conduct in one direction so you would need two in anti-parallel.
But it's totally pointless in this case since an IGBT actually has a P-N
junction, so it always has a higher volt drop than a diode of the same
current and voltage rating. (so does a 600v MOSFET for that matter)

As for your other questions on suppressing unwanted harmonic modes, I use a
4046 PLL chip in my controller and I can set it to reject any modes outside
of a narrow frequency range. Steve Ward uses a somewhat different logic
circuit but the result is similar- it won't allow the driver to drive modes
above a certain frequency.

I don't think there's any point in adding extra coils and capacitors to the
primary circuit, it just increases IR losses.

Steve Conner