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Class E PLL SSTC

Class E PLL SSTC

A small Tesla coil will have a high resonant frequency, making conventional driving techniques difficult. What were once low capacitances suddenly seem huge, and insignificant switching times become a considerable portion of the switching period. A whole array of new problems arise as frequency increases. However small coils only need small amounts of power to make a decent spark-show, allowing for simpler topologies. Originally I had intended to run this coil off-line with a half-bridge but I was unable to achieve decent gate drive through the GDT. So instead I switched to class E, which only requires one switching device and no GDT.

Notice: Although this circuit works, for a class E coil you should use a fixed frequency oscillator. This is because the output stage must be carefully tuned to your drive frequency, and any small deviation either frequency or loading will increase losses. See my other class E coil for a fixed frequency design.

schematic
Click on the schematic for a larger image.

Circuit Function: The phase locking is preformed by the 4046 chip. First some basics. The 4046 has an internal VCO, or voltage controlled oscillator. It's frequency is controlled by the voltage at pin 9. The 4046 also has internal phase comparators, in this circuit only phase comparator 1 is used, which is just a XOR logic gate. First one sets the frequency range of the internal VCO with the 100pF capacitor, and resistors on pins 11 and 12. The pin 11 resistor sets the upper frequency, while pin 12 the lower frequency. By adjusting the voltage at pin 9 the VCO frequency can be moved up or down between the set frequency points. Pin 4 is the VCO output, which will oscillate regardless of input. This allows it to start the SSTC, which in turn provides feedback through the secondary base current transformer. The base current signal and the output from the VCO itself are compared by a XOR gate. (The VCO output is actually sampled at the IRF630 drain, due to delays in the driver transistors and IRF630 itself.) The output from the XOR gate is a PWMed signal, which represents the phase difference between the VCO output and base current itself. Since the VCO is controlled linearly by the voltage at pin 9, pulsed DC would simply send it to max frequency and back down again. What is needed is a steady DC voltage proportional to the PWMed signal, which is created by the 120k resistor and 1nF cap. The VCO can be biased by changing the constant voltage at pin 9 with the potentiometer. This effectively allows one to adjust the phase angle. Music can be modulated into the output signal by inserting it into pin 9. Unfortunately the corona hisses and distorts the music.

Class E is almost as simple as it looks, basically one switches in resonance with the series resonant circuit formed by the primary inductance and matching capacitor. The whole point is to tune the primary and resonant capacitor until the circuit is critically damped (doesn't ring below zero), and turn on the mosfet just as the voltage reaches zero. This allows the mosfet to turn on with no voltage across it, ZVS, which eases switching and decreases switching losses. Damping of the resonant circuit is done by adjusting the primary coupling. Tighter coupling causes energy to be drawn from the primary faster, which causes more damping. Unlike conventional SSTCs coupling should be fairly loose, almost like a SGTC. Since this has been elaborated much better before, I'll point you to some other pages which describe class E switching much better. Richie's page and Steve Ward's page are great resources. Steve Ward's page has simulated waveforms which greatly help the tuning process. See if you can recognize these:

overdamped waveform perfect clas E
Bottom trace is gate voltage, top is drain. The gate signal sure looks nasty when out of tune.

The phasing of the CT and primary are very important. While experimenting with an antenna I found that I could only achieve breakout with the primary phased oppositely as the secondary. The CT phasing was also critical for a phase lock, but I was unable to determine which direction is required. The symptoms of improper phasing are no breakout, breakout only after "coaxing" one out with an arc or sudden loss of phase lock while tuning.

My coil draws about 80W from 50V, and runs at 1.38MHz. The secondary former is 5 diameter * 8 cm tall. The entire coil with control electronics fit in the palm of my hand, hence the nickname palm-top SSTC.

all boxed up1 all boxed up 2 Completed coil in my hand
CW streamer from coil Interrupted streamer Lego minifig with streamers from helmet

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Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.


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