RE: Terry's DRSSTC -"different" H-drive functions...

["Tesla list" <tesla@xxxxxxxxxx>]

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

>If all the IGBTs are
>not conducting, then the primary Q is very low and the energy stored in the
>primary is very quickly being dissipated in the free-wheeling diodes in the
>IGBTs.

This is not quite right. If you turn all the IGBTs off, then the free-wheel
diodes act like a bridge rectifier and transfer the primary energy back into
the DC bus. It's not wasted, just recycled. It seems that about half the
energy in the primary at the time of gate drive stopping goes back to the DC
bus and the other half carries on to the secondary.

Of course it would be nice to make sure all this energy goes forwards
instead of back. I remember Jimmy Hynes originally planned to use the idea
of shorting two IGBTs on to make sure the energy all goes into the
secondary. But it's not possible if you use transformer drive to the gates.
Jason Judd tried the method with success. He uses optoisolated gate drive
ICs from Agilent, controlled by a PIC or suchlike, so he can have any
combination of IGBTs on at any time.

I decided not to do it because I'm determined to use transformer coupled
drive (the reason being that it makes arm shoot-through incidents almost
impossible- a transformer can't turn two devices on at the same time) and
I'll just live with the sub-optimal use of energy.

I reckon it's not such a big deal. If you use the "turn all IGBTs off"
method they have to deliver more energy over and above your desired bang
energy, to make up for the energy that rectifies back. So they will
dissipate more heat. If you short two on, then you don't need to deliver
this extra energy, but the two "on" IGBTs will still dissipate more heat
because of the extra current flowing in them. The freewheel diodes work
harder in the former case (they don't do anything at all in the latter apart
from catching random spikes) but I still think they will hardly heat
compared to the IGBTs.

The Antonio/Hynes style tunings can arrange it so there is practically no
energy in the primary when the gate drive ends, which avoids the problem
altogether. But probably at the cost of needing a bigger resonator for a
given spark length.

Steve C.