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4.096MHz Class E SSTC
4.096MHz Class E SSTC

Wanting to take the step up from my Palmtop SSTC I decided to do a proper, class E, 4MHz SSTC. For those who have never seen these before, the basic idea is to use a switching topology commonly used in RF amplifiers, instead of the usual half or fullbridge. The reason for this is too greatly reduce switching losses in the power mosfets. Richie Burnett was the first to use this topology in a Tesla coil, and since then many have built similar coils.

schematic

The driver used for the 4MHz class E SSTC is a string of several amplifiers, which buff up the signal from a 4.096MHz crystal. It's pretty straightforward until the first mosfet stage, where the high frequency fun begins. The IRF630 gate is driven through a small transformer, which is biased at around 3V. The bias reduces the amount of voltage swing required to reach for gate potential, and the transformer helps match the 2N390X stage to the gate impedance. Leakage inductance in this transformer also play a part, by creating a resonant circuit along with the gate capacitace. If tuned properly a nice sine wave can be created on the IRF630 gate, allowing for proper gate drive. Which is all well and good, but there are still two stages left to tune! The next stage is actually a class E stage which is used just to drive the IRFP450 gate. Again the gate transformer must be carefully tuned for the best waveform, but in addition you want to tune for a class E waveform on the IRF630 drain. (See below for how to tune class E) To be honest here, I had already tuned the IRF630 when I discovered moving the windings on the gate transformer had a profound effect on the waveform amplitude. At this point I simply moved the windings until the IRFP450 received a perfect sine-wave gate waveform, and simply let the IRF630 drain waveform be. If overall system losses are a major concern, you'll need to put more effort into tuning the IRF630 than I did! One fun thing to note, is that at this stage we're already generating roughly 5W of RF power just in gate drive. Without a heatsink your IRFP450 could overheat just from gatedrive alone, given time.

IRFP450 gate waveform
IFRP450 gate, 5V div, 40ns

Once the IRFP450 gate waveform is perfected, you're only about halfway. The final class E stage is the one that counts, and tuning it can be tricky. At this frequency minor changes of the secondary or even primary can destroy the tuning. I experienced this myself, as I had originally tuned everything to working order, and then foolishly decided to cut off any remaining PVC from the formers to get a more deliberate look to the project. Little did I know that the 1cm of PVC removed from the secondary and primary formers were adding vital amounts of capacitance to the setup! I was forced to make a new secondary, so make sure you don't repeat my mistake! In essence you'll need to use a more or less set value of drain capacitance, and tune the primary and secondary until you get both breakout, and a good class E waveform. This process takes time, so be prepared to do some experimenting.

prototype Prototype 2

Tuning for Class E

As mentioned earlier the premise for class E is to reduce switching losses. This is achieved by turning on the switch with zero current and zero voltage across it.

Despite having tuned my coil, I still don't have a good procedure for this. When you tune or adjust one component, everything else that was previously optimized needs readjusting. For this reason you'll need to tune and retune many times before arriving at an optimal tuning. For starters wind a coil with a resonant frequency somewhat below your target frequency. This will allow you to remove turns later. Then estimate the values (or use the ones in the schematic) of the class E components, and power it up while watching the drain waveform. If it appears as a half-sine wave that is cut off before reaching zero, you need to remove some turns from the secondary. If it appears as a sharp spike, at less than 50% duty cycle, you need to add some turns to the secondary. Once correct, you should have breakout when run from 50V.

Now you'll need to tune the primary side. You'll need to experiment with both coupling and inductance, and possibly the drain-source capacitance and RF choke. Decent values of the RF choke and drain source cap can be found even if your setup isn't tuned perfectly, so start here. Once you've settled on values for these components, you need to experiment with the primary winding. Some of the factors that come into play are:

Length of wire
Number of turns
Spacing between turns
Distance above base

Class E coil with wavefor  


Video of Class E coil playing a MIDI song (Zelda)

Similar projects and references:
Richie Burnett's HFSSTC
4MHz amplitude modulated audio sstc
SSTC-4: HFSSTC 4MHz
HFSSTC classE
Steve Ward's Class E SSTC




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This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.

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.