# [TCML] Re: Solid state efficiency, was: mini Tesla coil specs

```Hi Dex,

```
The turn-off switching losses are zero in the twin design, since the IGBT current is already zero at turn-off [commutated to diode.] The carriers in the IGBT substrate get swept long before the next haversine, so the turn-off tail isn't an issue.
```
```
I really don't think that 30uF is a primary resonant capacitor; it seems more like the filter bank that gets stuck in front of a DRSSTC bridge. If that 30uF was the Cpri, consider the Ipeak at 1kV:
```
Ipeak = 1kV / 0.081 = 12,345A.

-Greg

```
```Hi Greg,
Thanks for the explanation of R=0.6 ohm for the rotary gap
of your coil (it makes sense).
But what do you mean by saying there is no IGBT switching loss?
Switching losses of IGBTs at typical tesla coil frequencies
are not so insignificant (althought generaly much lower than
in a spark of SGTC of same power).
The price paid for reduced conducting state voltage drop of IGBT,
compared with say that of MOSFET are increased switching times.
```
As it should be known IGBT turn-off transition exhibits phenomenon known as current tailing during which increased power
```loss occur.Typical turn-off times of IGBTs are in the range
0.3...3 microsec.Assume ,for example,a typical coil frequency of
100 khz and coupling between primary & secondary k=0.2.
Assume that this is a high power coil and one decides to use a
typical commercialy available HV IGBT module rated 4500 V.
It has Toff~1.25 microsec.
Turning-off last for about 25% time of 1 semicycle of the coil's
operating frequency.

That's normal coil only with IGBT instead of a spark gap.
Characteristic surge impedance of the primary can be calculated
from 2 parameters given in the coil's specification:

Cp = 30 uF
F  = 66 kHz (w = 414 000 s^-1)

And so:
Zch=1/(Cp*w)=0.081 ohm
```
```

_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla
```