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Re: transistors instead of diodes?

Original poster: "Jolyon Cox" <jolyon@xxxxxxxxxxxxxxxxxxxxxxxx>

you say you use the 4046 PLL -what other PLLs are suitable for tesla coil
use -I mean would it be possible to use the 567 tone decoder/PLL chip for
instance -or even the 555 timer with additional circuitry?


----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Wednesday, March 23, 2005 7:08 PM
Subject: RE: transistors instead of diodes?

> Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx> > > >I am wondering if in place of fast-recovery diode it would be possible to > >use a second transistor in anti-parallel with each transistor of the > >H-bridge > > Hi Jolyon, > > If you use MOSFETs then this happens automatically since MOSFETs conduct > equally well in either direction when they're on. The trick is used in > industry (it's called a synchronous rectifier) > > However, there is no real advantage to it since the fast recovery diodes > already make a good job of returning as much spike energy as possible to the > DC supply. It's only useful in low voltage high current circuits where the > MOSFET has lower conduction losses than the best schottky diodes. > > IGBTs only conduct in one direction so you would need two in anti-parallel. > But it's totally pointless in this case since an IGBT actually has a P-N > junction, so it always has a higher volt drop than a diode of the same > current and voltage rating. (so does a 600v MOSFET for that matter) > > As for your other questions on suppressing unwanted harmonic modes, I use a > 4046 PLL chip in my controller and I can set it to reject any modes outside > of a narrow frequency range. Steve Ward uses a somewhat different logic > circuit but the result is similar- it won't allow the driver to drive modes > above a certain frequency. > > I don't think there's any point in adding extra coils and capacitors to the > primary circuit, it just increases IR losses. > > Steve Conner > > >