palmer@tallis.enet.dec.com (Colonel Mode) (08/22/90)
I've had a HVHF resonant air core transformer project on the back burner for several years now and would like to revive it. If you're familiar with Tesla coils, this is not one, but it's output is similar. A high current MOSFET RF oscillator drives and is tuned to/by an LC tank composed of 3-10 turns of 1/2 inch copper tubing and a .01-.1uF capacitor. The secondary is several hundred turns of 20-24 gauge wire whose electrical length is 1/4 wavelength of the LC tank frequency. The difficulty I've been having is in the design of the RF oscillator. How can I prevent HV kickback from the secondary from burning out the MOSFETs and/or the rest of the oscillator? Kickback can occur whenever the load on the secondary changes suddenly, or if the secondary is not tuned to the LC tank. I don't want to use vacuum tubes. Someone suggested using a PWM chopper instead of a sinusoidal oscillator, but I fear that that would only make the kickback worse. Any suggestions? ***** Chris Palmer palmer@tallis.enet.dec.com phone (508)486-6667 dtn 226-6667
grege@gold.GVG.TEK.COM (Greg Ebert) (08/23/90)
Using a cascode connection of a MOSFET and a bipolar transistor will give you the fast switching time from (from the MOSFET), and high-voltage protection (from the bipolar). I seem to recall a fairly high-voltage bipolar used for the horizontal-output section on TV's is (used to be ?) available from Radio Snack. Since your circuit is resonant, all you really need to do is give it a 'kick-in-the-pants' with an impulse, and it will ring at the resonant frequency. The pulse rate of the impulse should be an 'integer fraction' [sorry for such a silly term, but you know what I mean] of the tank frequency. You can use lower pulse rates for a higher Q. As far as the 'kickback' is concerned, it's just something you will have to live with. You can compute it if you know the peak secondary voltage, which will vary wildly depending upon your load; just use the turns-ratio of your transformer. Be aware (read: BEWARE!) that under worst-case, the voltage across the cascode circuit is the kickback voltage PLUS the DC supply voltage. Egad ! MOSFETs are wonderful devices, and I've made them switch several amps on/off in less than 100nsec with blatant disrespect for circuit layout and wimpy gate-drivers.