Re: Toroid cores for GDTs

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>


> Silly question!!  Why not just buy a gate drive transformer?  It is 
> a fairly standard part.

Solid-state Tesla coils are not a standard application though. 
Commercial GDT parts are aimed at switched mode power designs that 
need to meet electrical safety standards. The GDT is part of the 
isolation barrier, so to meet code it needs lots of insulating 
plastic between the primary and secondary windings, which means high 
leakage inductance.

We need to drive several large MOSFETs at high frequency, so we need 
absolute minimum leakage inductance. We usually get that by 
forgetting we ever knew about "code" and making the transformer with 
a multi-filar winding of ordinary PVC insulated wire. If you're a 
radio ham you might know this as a "Transmission line transformer". 
You are limited to integer ratios (1:1, 3:2, 4:3, 2:1 etc.) but that 
is hardly a problem. Finally, using a large toroid core of high 
permeability ferrite lets you reduce the number of turns, reducing 
the leakage even further, while keeping the magnetizing current acceptable.

The result is a transformer that doesn't meet any electrical code, 
but can drive a H-bridge of IRFP460s with 10 ohm gate resistors at 
250kHz all day. And can be made in 5 minutes with $2 worth of parts. 
The isolation barrier is only two thicknesses of PVC which is not 
enough to be "Safe" as far as electrical codes are concerned. But we 
hardly care about that since the average Tesla coil is unlikely to 
pass code anyway, on account of having an exposed live terminal that 
shoots out enormous voltages and colossal discharges.

Steve Conner
http://www.scopeboy.com/