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

[Steve Ward <steve.ward@xxxxxxxxx>]

Just to clarify, Greg, the coil in the link Dex sent is (i suspect) almost
an identical topology to your twin setup, where the IGBT acts like the spark
gap.  Its not a DRSSTC system.  Just to be sure, this was the coil in
question:

http://scopeboy.com/tesla/t4spec.html

Of course the big drawback is the 1kV max firing voltage, so Mr Conner never
got really juicy bangs out of it ;-).  Also note that he only ever used
15uF, not the 30uF that it was mechanically expandable to.  I think he
eventually saw the flexibility and power in the DRSSTC and shifted his
attention there.

For reference, the DRSSTC filter bank often has to be 10-20X the intended
bang energy, if not to provide a low ripple voltage, but to have capacitors
that will deal with the large RMS current (for us little guys it ends up
being electrolytic-based).

Steve



On Thu, Nov 19, 2009 at 1:33 AM, Greg Leyh <lod@xxxxxxxxxxx> wrote:

> 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.
>>
>> For the coil posted in the link before,that's not DRSSTC.
>> 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
>>
>
>
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