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Registered Member #538
Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181
I'm making a class-E SSTC driver based on Richie Burnett's driver and I have a small question about winding the GDTs.
Because the GDTs have different amount of secondary and primary turns, how do I wind the transformer so I get the lowest possible leakage inductance? Should I just wind the secondary and primary bifilarly "like usual" and just take the other end of the secondary (secondary has less turns than the primary because the gate drive is resonant I presume) from the middle/between the ends of the whole bifilar winding? Or should I wind a *filar winding with more than 2 wires and connect several of the windings in series as the primary?
Or do I really have to care about the leakage inductance that much because theres added series inductance anyway to achieve resonance with the FET gate capacitance and the leakage/series inductance?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
You've really answered your own question in the last line!
At 4MHz I had to add an external series inductor because the leakage inductance of the GDT alone was insufficient to series resonate with the transformed gate capacitance at a low enough frequency (4MHz).
At 8MHz, the gate could me made to series resonate with the leakage inductance just by moving the secondary around the toroid closer or further away from the primary. This fine tunes the leakage inductance.
When you get up to tens of MHz then it becomes important to do everything you can to minimise leakage inductance. When trying to operate large die MOSFETs in the tens of MHz, the series resonance of the gate with even the lowest achievable leakage inductance is usually still below the wanted frequency! Then it is possible to raise the resonant frequency by inserting a small capacitor in series with the GDT primary to cancel some of the excess leakage inductance. (This can be contrasted with the need for a series inductor to suplement the inductive reactance at lower frequencies)
There is some info about GDT's in this folder on my site:
In particular see "ferrite.txt" and "ferrite2.txt". Although these don't cover sinusoidal drive for Class E specifically, most of the comments still apply.
A neat trick to minimise leakage inductance and maximise coupling when the turns ratio is far from 1:1 is as follows. For example if the turns ratio is 4:1 then you wind a single 8 turn primary, and four 2 turn secondary windings. The 8 turn primary is wound to take up the entire circumference of the toroid. And the four 2 turn secondarys are spaced equally around the circumference, also in an attempt to take up as much as possible of the core. Another popular method is to fully enclose one winding within another winding made of copper foil or tube.
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How to achieve lowest leakage inductance?
