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Registered Member #952
Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388
Arcstarter wrote ...
Place a transistor with the emitter on the negative supply and the collector on the negative side of the circuit. Connect the audio source's negative side to the emitter and positive to the base on the transistor.
Sounds good. BTW, do I have to use a BJT or could a MOSFET work too?
Registered Member #289
Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154
It is not necessarily as simple as just turning the negative rail ON/OFF. If you modulate a current feed inverter with the above mentioned method you will ring the series inductor on the positive rail. This will cause a large voltage to build up across it along with very high startup currents. Self oscillating current feed circuits (basically anything with an inductor in series with its input) general rely on both the switching devices to be on at startup. This means at the instant of start up the current across the series inductor will rise at a given rate (Vs/L assuming the input V is constant and your inductor does not saturate) until the circuit starts at which point the inductor load current will change significantly, causing the circuit to ring down at a low frequency. In normal operation this is harmless and allows the system to startup. However if you modulate the rail too quickly (like with audio or PWM) you will be switching on and off during this period of ring down. This creates all kinds of switching transits and will eventually kill something.
So bottom line is it will work, but if you are going to try and modulate that circuit with a low side FET make sure to pay attention to the current and voltage across it. The L of your inductor will, to some extent, control the max frequency at which you can “safely†modulate the circuit.
Registered Member #952
Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388
Let's compare WaveRider's Royer driver and the Mazzilli ZVS flyback driver. Both can be used to drive a SSTC.
The main difference is that the Mazzilli circuit has a capacitor in parallel with the primary, and the operating frequency is determined by the LC circuit. WaveRider's Royer circuit works from a center-tapped feedback coil and it tunes itself.
However, some people report just making a single center-tap primary for a SSTC and plugging it in an old Mazzilli flyback driver and voilà , it works! Has he just been lucky, or does the Mazzilli still tune itself somehow? I know it tunes itself for Zero Voltage Switching, but is the frequency always constant?
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
The Mazzilli driver's frequency is set by the primary inductance and primary capacitor so it's as constant as the LC component's values. In order to run a TC secondary you need to tune the driver to the secondary fres by changing the primary capacitor/turns.
Registered Member #952
Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388
That's exactly what I had understood.
It'd be pretty hard to tune a SSTC with that kind of a driver, especially if you only had one fixed value capacitor. Then it should be tuned with the primary, and that would go pretty complex because of the center-tap (or well, at least it would take a lot of time)
So I'll focus on WaveRiders Royer driver as it tunes itself and you don't have to worry about tuning it yourself.
Registered Member #952
Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388
Yes, I thought I could also give that circuit a try. One thing about the class E tuning capacitor: Which type do I need? Would some small-value ceramic caps do it?
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Royer SSTC questions
