Solid State Tesla Coil

From HvWiki

Abbreviated SSTC, See also VTTC

Contents 1 Introduction 2 Topologies of SSTCs 2.1 Conventional SSTC 2.2 Common switching circuits 2.3 Interrupted SSTC 2.4 Dual Resonant SSTC 2.5 Off Line TC 2.6 SSTC Magnifier 3 Characteristics of a typical SSTC 3.1 Toroid 3.2 Secondary 3.3 Primary 3.4 Feedback Characteristics 3.5 RF ground

Introduction

Any Tesla coil that uses a solid state driver, replacing the spark gap with semiconductors. SSTCs usually use, but are not limited to MOSFETs and IGBTs. Streamers feature a firey appearance due to the fact that many of them run "CW" or continuous wave. Basically that means that unlike the SGTC and the DRSSTC, they are fed continuously and do not give the circuitry a rest. By adding an interrupter the coil can be pulsed on and off however, much like SGTC and DRSSTCs. This changes streamer appearance and gives the driver circuitry a break. SSTCs can even feature the sword-like streamers characteristic of a VTTC if operating at the right frequencies. Currently the record holder for the highest running frequency is 14Mhz, by our very own BlackPlasma.

Topologies of SSTCs

There are a few different types of SSTCs.

Conventional SSTC

This is the most common type of SSTC. Commonly, it consits of an oscillator, or feedback mechanism, driving a set of MOSFET's, most likely two or four, in a half bridge or full bridge configuration.

These types of coils are generally limited to around 20" of spark length, though greater legnths have been acheived, it is uncommon.

Common switching circuits

Conventional solid state tesla coils, and dual resonant tesla coils usually use a half-bridge or a full-bridge to switch power to the primary.

Interrupted SSTC

(ISSTC) An SSTC with an Interrupter, sometimes as in VTTCs this is referred to stacatto. This often allows for greater streamer length. Because it runs the switching devices (IGBTS,MOSFETs,etc) at a duty cycle, one can put much more current through the primary coil and inverter (for a shorter period of time). This is the fundamental principal of the DRSSTC.

Dual Resonant SSTC

See DRSSTC. Similar to a conventional Solid State driver but utilising a capacitor in the tank circuit along with the primary coil, so as to form a resonant circuit like in SGTC's. This allows voltage and current gain in the primary circuit before the secondary resonance multiplies it further, giving greater output voltage. This is where the dual resonant part comes in - 'normal' SSTC's only have a single resonant circuit, which is the tesla resonator (secondary coil and topload pair). The primary capacitor's reactance also cancels the inductive reactance, as at resonance they are equal and opposite. This lowers the impedance of the load presented to the half bridge or full bridge (for example) and allows much higher current to flow, resulting is massive increase in power throughput. As a consequence of increased current, the 'duty cycle', or pulse duration is lowered (otherwise the switching devices would be stressed to overheating and exploding).

Off Line TC

An SSTC similar to a spark gap coil, yet different in that it uses IGBT's to switch the power into the primary rather than a spark gap.

SSTC Magnifier

To be continued...

Characteristics of a typical SSTC

(Note: These do not especially apply to the DRSSTC topology)

Toroid

Toroids are generally smaller, sometimes nonexistent. Very few do not feature a breakout point.

Secondary

Squatty compared to it's DRSSTC and SGTC cousins. Because sparklength is often limited, and directed upward by the breakout point, height is no longer a problem. Strike rails are not necessary either.

Primary

Because high voltage is not present in the primary (generally 9V up to 300V). Primary is very close to the secondary, is vertical, and inter-turn spacing is minimal. The primary value is generally determinied by trial and error. Fewer turns allows more current to flow (by lowering the impedance) and will allow a higher voltage output on the secondary. However, this is limited by the driver. Therefore, current is limited (thus saving the MOSFETs) by adding more turns to the primary.

Feedback Characteristics

Feedback to the circuit is obtained with an antenna or feedback transformer. The antenna must be far enough away to prevent frying your circuitry either by induced voltage or direct strike. Therefore, current transformer has become the most used method of supplying feedback. The primary of the feedback transformer is the RF ground wire of the Secondary resonator coil. Generally, 1-2 turns is used on the primary. The secondary is generally between 50-200 turns. Varies widely depending on desired performance. The phase difference of various voltage and current waveforms in a SSTC are predicable and used to sense resonance in solid state drivers. When in resonance all reactances are practically cancelled out, so the primary voltage and secondary current will be in phase for example.

RF ground

...is directed just like any other tesla coil. Directly to mother earth or through an EMI filter to the power line's ground.