Re: SISG Power Supply Theory (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 28 Jun 2007 13:15:55 +0200
From: Finn Hammer <f-h@xxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: SISG Power Supply Theory (fwd)

Mark, all.
Comments are interleaved below:


> A number of TCML members have requested that I write an explanation of how
> the power supply charges the tank cap and triggers the SISG circuit in an
> SISG Tesla Coil Application.  For the experts this will likely be trivial
> and boring.

Not at all.
> Finn Hammer has built a
> triggered version which really is a new concept in and of itself.  You will
> see why later.
> 

New Concept? hardly. Terry`s SISG circuit is a work of genius. The 
trigger feature is more like an add-on, which could not work without the 
safety of the Sidac overvoltage clamp.
>     
> 
> The SISG coil uses a pulsed DC power supply to charge the tank cap.  When
> the tank cap reaches a �threshold� voltage (set by the # of SISG circuits)
> the SISG fires and acts like a switch that allows the coil to resonate just
> like a spark gap coil.  In fact view the SISG as replacing an RSG � 

Sidac`s are voltage triggered devices, so the SISG is rather like a 
static gap in my view.
The RSG analogy is justified only, when the trigger circuitry is added.

with the
> additional power supply modifications.  Terry�s basic SISG circuit fires at
> 900 volts, so the choice of threshold voltage is a multiple of 900 volts if
> you use his circuit.  I built some boards (identified as SISG4), which a
> number of you have, that have (4) SISG circuits on each board.  So each
> board gets 3600 volts threshold.  (2) boards 7200 volts.  (4) boards 14,400
> volts.
> 
>  
> 
> The way the SISG circuit works, it can only be triggered with a DC voltage
> of correct polarity.  If you study Terry�s circuit, you will see that the
> gate will never fire if the polarity is reversed.  Also, once the SISG
> fires, it cannot turn off until the DC voltage is removed.

This is not correct.
The Sidac`s stop to conduct as soon as the IGBT takes over, although 
with some delay. I don`t recall how long the delay is, but the fact that 
you can achieve breakrates in the 1200BPS range with a 120Hz pulsing Dc 
supply is testimony to the fact that they turn off well.
   Thus the need
> for a pulsed DC power supply.  The rising DC turns the SISG on and when the
> DC is removed the SISG turns off. 

The DC across the Sidacs is brought below their conducting threshold by 
the clamping action of the conducting parallel IGBT.

  Now for discussion purposes we can talk
> about the BPS(breaks per second) for the SISG at steady state in a similar
> way we would discuss BPS for an RSG.
> 
>  
> 
> Just like a spark gap, when the SISG turns on the coil �rings up� and �rings
> down�.  The issue that causes some headaches, is that you do not get to
> independently control the BPS like in an RSG. 

Because it is analogous to a static gap, being voltage triggered.

  The BPS is tied to the
> charging rate.  So if the tank cap is charged very quickly then the SISG BPS
> will be very high.  For example, when I first converted my DC coil and had
> (4) MOT�s charging a 37 nF cap, my SISG was operating at 1200 BPS.  This was
> a terribly inefficient condition.  As I reduced the impedance of the
> transformers and increased the value of the tank cap the BPS came down.  I
> eventually, wound up with (2) MOT�s ( a voltage doubler) and a tank cap
> value of  ~118nF to get the break rate down to 120 BPS.  Terry has used a
> 165 nF cap with his single MOT design.

So again, the right analogy of the SISG is that of a static gap.
> 
>  
> 
> Now you see why Finn�s development is interesting.  By independently
> controlling the IGBT firing rate we could work with any MOT and cap
> combination (as long as the cap wasn�t too large to charge before we hit the
> peak) and simply fire the gate at the appropriate time. 


The triggered SISG is designed to be able to fire at the initial voltage 
of the raw DC supply, but hold off the resulting voltage of the primary 
cap, after the first recharging cycle, which is twice the voltage due to 
the effect of the charging choke between the supply and the primary cap.

Since the trigger element is a SCR, which does not shut off when the 
voltage across it drops, but rather does so, when the current goes to 
zero, there is always a single section of the top Sidac in series with 
the SCR. This means that the trigger can operate btwn. 125V and 1000 
Volts. I use 4 pcs. 250 volts Sidacs because I like the round numbers.
> 
>  

Snip
> 
>  
> 
> Some have asked about using pure DC for this application.  There are two
> problems with this.  The first was previously mentioned � the SISG will fire
> once and then never turn off if fed pure DC.

The Sidac`s stop conducting as soon as, or shortly after, the voltage 
across them is clamped low by the parallel conducting IGBT.


   The second is that to get the
> pure DC you need filter caps and the filter caps will add to the tank cap
> during resonance.  There are ways around the 2nd issue, but not the first.

There is a charging choke in between the filter and the tank cap. At 
ringing frequency the impedance of the charging choke is so large, that 
it effectively isolates the 2 caps from each other.

> 
>  
> 
> Well that�s it.  Hope you can follow my ramblings.  
> 
> I did recently receive a shipment of SISG IGBT�s that I ordered last fall so
> if anyone needs any contact me off list.  Took about 38 weeks.  I think they
> were swam here from Singapore.
> 
> Everyone should try SISG.  You�ll never go back.  It lasts forever and never
> wears out.  Been running over a year now and never blown an IGBT.  Some
> think SISG is expensive, but I spent a lot more screwing around with RSG
> variations for a couple of years than what SISG costs.


> 
>  
> 
> SISG schematics and other SISG info are here:
> http://www.teslaboys.com/SISG/index.html
> 
>  
> 
> Mark Dunn

Hope this helps,
Cheers, Finn Hammer
> 
> 
> 
> 
>