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

["Dex Dexter" <dexterlabs@xxxxxxxxxxx>]

Steve,I've just noticed the link  claims change in a top load
(toroid) size too.Apparently 81" long spark was achieved with
48" toroid.This would mean that if 66 khz is resonant frequency
with smaller toroid,with bigger one the resonant 
frequency would be lower.Perhaps about 55 khz.
But still ,at that frequency Zchar of primary is still below 
0.1 ohm (assuming claimed Cp=30 uF).
Quite low and quite suspicious.

As concerns statement that commutation loss of circuits 
involving any kind of a semiconductor device is 0,that
certainly can't be true.All semiconductor devices are
charge controlled.To switch them between "on" and "off"
states,the controlling charge must be inserted or removed
from their interior's (equivalent) capacities.
That process takes time , consumes energy, and depends
on the construction of a semiconductor device.
Various soft-switching principles are used to mitigate
switching losses involved.But they cannot be totally
avoided.
Interestingly ,quite sometimes reducing a switching
loss of a device results in the increasage of
a conducting loss or/and shifting part of the loss
to other dissipating semiconductors and parts of 
a circuit.Anyhow,the switching frequencies of 
PWM converters with advanced topologies are limited
to 2-20 khz for the sake of switching dissipation.
There's no free lunch.

To reduce further IGBT power loss of the switching action
it is desirable ,as concerns tesla coil design,to strive
for lower resonant frequencies and lower switching
frequencies (lower BPS) I think.

Dex
    

      

--- steve.ward@xxxxxxxxx wrote:

From: Steve Ward <steve.ward@xxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Subject: Re: [TCML] Re: Solid state efficiency, was: mini Tesla coil specs
Date: Thu, 19 Nov 2009 09:27:30 -0600

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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