# SSTC design guide

Published: July 15, 2019. Updated March 19, 2020.

This article is to point out some of the design decisions and calculations that is different from a DRSSTC, so this article will not contain a description of all parts used in a SSTC. The following topics are covered by the DRSSTC design guide as the guidance and best practices is the same.

As an example in this guide, I will use the dimensions and properties of my Kaizer SSTC 2 Tesla coil. It is a full-bridge IRFP460 running at 250 kHz, making up to 47 cm sparks, see all construction details on the link.

## Primary peak current calculation

To do an easy primary current calculation, to know if the MOSFETs can drive the load, it can be simplified by ignoring primary resistance and DC blocking capacitors reactance as they are very small factors.

First calculate the primary coil inductance for your helical or spiral primary coil. Use one of the two links for the online calculator.

Next step is to calculate the primary coil reactance. f is frequency in Hertz and L is inductance in Henry. Values are written in kHz and uH for ease of reading. The 8 turn helical coil with a diameter of 115 mm, with 1.78 mm wire and 2 mm spacing has a inductance of 10.16 uH.

$X_{L}=2 \cdot \pi \cdot f \cdot L$

$X_{L}=2 \cdot \pi \cdot 250 kHz \cdot 10.16 uH = 15.95 \Omega$

The peak current for the peak-to-peak square wave voltage envelope can now be calculated using Ohm’s law. I supplied my coil from full-wave rectified 230 VAC, which multiplied with square root (2) for the peak voltage is about 320 VDC.

$\text{Primary peak current} = \frac{Voltage}{Resistance X_{L}}$

$\text{Primary peak current} = \frac{320 VDC}{15.95 \Omega}=20 A_{peak}$

Conclusion on primary peak current. We now have a basic measure for how much current we are trying to push through our MOSFETs and primary coil. The MOSFETs should as a minimum be able to withstand this, with a safety margin added on top. Voltage rating should have around 33% head room, so if you are feeding the inverter 320VDC, a 600V MOSFET is to prefer.

## Primary Geometry and Coupling

There is generally four shapes of primary coils.

• Flat spiral coil (Used in SGTC and DRSSTC)
• Helical coil (Used in VTTC, SSTC and DRSSTC)
• Cone coil (Used in SGTC, DRSSTC)
• Half-circle coil (Used in QCWDRSSTC)

A SSTC will almost always use a helical coil with a high and tight coupling to the secondary coil. Due to the relatively low primary circuit current it is necessary to have a primary geometry that gives a high coupling to get a good energy transfer.

In my SSTC constructions I have often used regular machine tool wire wound directly around the base of the secondary coil, with nothing more than 2-10 mm of insulating material in-between and also to make it able to adjust coupling by moving it up or down. The insulation needs to extend further than the primary coil to avoid flash over damages, as I have experienced.

## DC Blocking Capacitor

The DC blocking capacitor is named after its purpose, to block any DC component of a signal to enter the transformer being driven by a half- or full-bridge. A DC offset current may cause an unbalance of the transformer that initiates a runaway process that ends up with the transformer saturated and the large current drawn in this mode will damage both transformer and MOSFETs. [1]

The DC blocking capacitor can either be in series with the primary coil for a full-bridge or in series with the primary coil for a half-bridge that connects to ground. For a half-bridge it can also be two capacitors forming a voltage splitter with a midpoint where the primary coil connects to, this voltage splitter can also be used in a voltage double configuration. The most important factor is that it is a very low ESR capacitor, in order to minimize the switching losses across it. Generally this means that the same type of MKP capacitors used in a MMC is suitable for DC blocking capacitors, given the capacitance is suitable. Below the DC blocking capacitor is the blue RIFA sitting in series with the black wires going out to the primary coil connectors.

There is two factors to calculate for the DC blocking capacitor. The capacitor reactance ratio to the inverter output impedance and the resonant frequency of the primary coil L and DC blocking capacitor C is much lower than the resonant frequency of the secondary coil circuit.

First we can calculate the DC blocking capacitors reactance. f is frequency in Hertz and C is capacitance in Farad. Values are written in kHz and uF for ease of reading. I used two 0.68uF X2 MKP capacitors in parallel.

$X_{L}=\frac{1}{2 \cdot \pi \cdot f \cdot C}$

$X_{L}=\frac{1}{2 \cdot \pi \cdot 250 kHz \cdot 1.36 uF}=0.468\Omega$

So now we can check the resonant frequency against the Tesla coil secondary circuit frequency. f is frequency in Hertz, L is inductance in H and C is capacitance in Farad. Values are written in kHz, uH and uF for ease of reading.

$Frequency=\frac{1}{2\cdot\pi\cdot\sqrt{L\cdot C}}$

$Frequency=\frac{1}{2\cdot\pi\cdot\sqrt{ 10.16 uH \cdot 1.36 uF}} = 42.8 kHz$

Being 5 times lower than the resonant frequency, there is no risk at the primary LC circuit resulting in a DRSSTC condition which would destroy the MOSFETs.

The reactance of the two capacitors was 0.468 Ω and the inverter output impedance should be higher than this.

$\text{Inverter output impedance}=\frac{\text{Voltage output}}{\text{Current output}}$

$\text{Inverter output impedance}=\frac{320 VDC}{20 A}=16\Omega$

It is worth noting that the inverter output impedance should be almost identical to the primary coil reactance as the pure Ohm resistance of the primary coil is very small.

The DC blocking capacitor reactance is 32 times smaller than the inverter output impedance and the design requirement is fulfilled. A lower capacitance would result in even smaller losses in the DC blocking capacitor.

Conclusion on DC blocking capacitors is that they should behave like a dead short at the resonant frequency of the Tesla coil or inverter. So anything below 2 Ω reactance should be considered to work.

## Gate Drive Current

Many different driving methods are used for SSTC’s and their half- or full-bridge of MOSFETs / IGBTs. Unlike the DRSSTC universal drivers, there is a wide variety of gate drive ICs, transistors or other implemented in different SSTC driver circuits.

It is important that there is enough current and fast enough rise time when driving the gates of the MOSFET/IGBT that switching losses are minimized as much as possible.

Use the MOSFET / IGBT Gate Drive Calculator to estimate the required peak current needed and RMS power consumption for the power supply design.

## References

[1] Alexander Gertsman, Sam Ben-Yaakov, “Zeroing Transformer’s DC Current in Resonant Converters with No Series Capacitors”, Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, 2010.

### How to make a DRSSTC?

If you have never build a DRSSTC before, the best practice is to copy someone else’s proven design. There are many pits to fall in doing design, construction and testing that can be avoided by building a replicate of a working machine.

The experience gained from building a DRSSTC from a proven design can be used to build another later, where you can make changes to the design, use different parts, add other features etc.

The experience you now have after building a couple of DRSSTCs is enough to start designing your own from scratch, you now know enough of the founding ideas, what works and what does not, how to build optimized parts of the system and what it takes to push electronics beyond their rated limits.

### Which skills do you need to make a DRSSTC?

Basic electronic knowledge is enough and the more advanced subjects can be learned along the way of reading, designing and building. Forums like http://www.4hv.org is a great place to ask for help. If you show interest, have done your research and ask for help on specific problems, there is all the help you could ever dream of.

You can use your hands and tools. Soldering is necessary and knowledge about using a oscilloscope. It is again skills that can be learned.

Be realistic about the time, money, space and power supply available to power a DRSSTC.

### Which tools do you need to make a DRSSTC?

Regular tools: Screw drivers, pliers, cutter, hammer, saw etc.

Electronics tools: Soldering iron, multimeter and oscilloscope.

Nice to have: differential probe for isolated measurements with the oscilloscope, isolation transformer to be able to ground the negative rail of the power circuit when testing, industrial wideband current monitor to monitor primary current wave form.

### How to tune a DRSSTC?

The best way to tune a DRSSTC is setting up exactly at the turn number that can be calculated with the wonderful calculator JavaTC.

To find the sweet spot, where sparks are the longest at full input voltage, detuning of the primary is used. When a spark of a certain length is flying output from the topload, this presents a load with a capacitance, this lowers the resonant frequency of the secondary coil. By tuning the primary circuit for a lower frequency than then secondary, the purpose is to find a lower resonant frequency of the primary circuit that corresponds to the loaded resonant frequency of the secondary circuit.

This topic is described in much more detail in the DRSSTC design guide chapter on tuning.

### How big is a DRSSTC?

One thing that can come as a surprise is how large a DRSSTC system is when it is put together. It can all seem very handy when you are working on the different parts, but once it is assembled it can seem much bigger. Another practical issue with size and weight is if you can handle to put it together with just one person, maybe you need to be two to rise the secondary and mount topload and perhaps it is even needed to use lifting equipment for heavy weights.

 Small Medium Large XLarge Power 400 W 2500 W 10 kW 25 kW Peak current 280 A 800 A 2000 A 5000 A Frequency 327 kHz 70 kHz 38 kHz 35 kHz Weight 2 kg 30 kg 120 kg – Height 0.45 meter 1.2 meter 2.8 meter –

### How much does it cost to make a DRSSTC?

It all depends on how long you want to wait to find a specific part a good price. The listed prices in these tables are mostly second hand used items for the electronics and new for the mechanical parts. A good example is how I got a good deal on the CM600 IGBTs and not as good a deal on the CM300, they actually cost me more per brick.

Most of the listed parts below are found over a long period of time, so obtain parts at the best price possible.

 Enclosure Scrap plexi Cost: Free Wood and wheels Cost: 45 Euro Wood and wheels Cost: 120 Euro – Rectifier 1x 25A bridge Cost: 2 Euro 2x 25A bridge Cost: 4 Euro 3x 200A double modules Cost: 10 Euro – Filter capacitor Scrap capacitors Cost: Free 2x BHC 3300uF / 450V Cost: 4 Euro 4x Siemens 6000uF / 350V Cost: 10 Euro Heat sink Scrap heat sink Cost: Free Small heat sink Cost: 5 Euro Large heat sink Cost: 15 Euro – Switches 2x IXGN60N60 Cost: 28 Euro 2x CM300HA-24H Cost: 100 Euro 2x CM600FA-24H Cost: 62 Euro 8x MBI800-120 Cost: Busbar 2,5 mm2 solid copper wire Cost: 1 Euro 40 mm2 from 1,5 mm copper sheet Cost: 12 Euro 300 mm2 from 10×30 mm copper Cost: 25 Euro – Wiring PCB and wire Cost: 1 Euro 6 mm2 wire Cost: 10 Euro 3×35 mm2 wire Cost: 30 Euro – Secondary wire 0.127 mm, 0.1 kg spool Cost: 10 Euro 0.25 mm, 2.5 kg spool Cost: 30 Euro 0.7 mm, 10 kg spool Cost: 200 Euro – Secondary form 50×500 mm grey PVC Cost: 3 Euro 160×1000 mm orange PVC Cost: 31 Euro 315×3000 mm orange PVC Cost: 200 Euro – Varnish Spray varnish Cost: 10 Euro 0.5L varnish and brush Cost: 33 Euro 0.75L varnish Cost: 25 Euro – Topload Styrofoam and aluminium tape Cost: 10 Euro Wood, 127 mm alu ducting and alu tape Cost: 40 Euro 6x laser cut alu holders, 7x rings of 25 mm coax cable Cost: 340 Euro – Primary coil 2.5 mm2 solid copper wire Cost: 2 Euro 10 meter of 10 mm copper tubing Cost: 24 Euro 14 meter of 10 mm copper tubing Cost: 34 Euro – MMC 2x 942C Cost: 5 Euro 12x 942C Cost: 32 Euro 5x 4uF/3kV GTO snubber capacitor Cost: 266 Euro – Driver Universal driver 1.3 Cost: 15 Euro Universal driver 1.3 Cost: 15 Euro Universal driver 2.1 Cost: 25 Euro – GDT 1x ring core Cost: 5 Euro 1x ring core Cost: 5 Euro 2x ring core Cost: 20 Euro – CT 2x ring core Cost: 10 Euro 2x ring core Cost: 10 Euro 2x ring core Cost: 10 Euro – Total cost 103 Euro 400 Euro 1404 Euro –

### Where to find parts to build a DRSSTC?

Ebay is the best source of exotic parts like large IGBT bricks, large pulse power capacitors, electrolytic filter capacitors and snubber capacitors.

Look in your area for metal scrap yards, here you can often buy new materials that have been thrown out for reuse, at scrap metal price. This is a good place to source copper clad, busbar, copper tubing and other metal parts for construction.

You should also look for local HAM markets, those are flea markets by and for radio amateurs. Here you can usually find cheap power supplies, oscilloscopes and small parts. With a bit of luck you can also find power electronics and filter capacitors.

Varnished copper wire for the secondary coil can be found locally at motor or transformer winding companies. It can also be bought from electronic stores and ebay. Prices are about the same in all places due to it mostly being copper and is governed by the metal prices.

Secondary tubes, pipes and forms can be found in home depot shops ranging from 40 mm diameter to 160 mm diameter. Some times up to 200 mm can be found in the same places. 315 mm diameter pipe and large is usually only to be found through construction companies, wholesale or ordering online.

### What does the different DRSSTC abbreviations mean?

DRSSTC – Dual Resonant Solid State Tesla Coil

MMC – Multi Mini Capacitor

BPS – Bangs Per Second

GDT – Gate Drive Transformer

CT – Current Transformer

FRES – Resonant frequency

### Introduction to the guide

This is a guide that aims to explain a few in depth details and best practices that one should have in mind when designing and building a Dual Resonant Solid State Tesla Coil (DRSSTC).

It is not a complete instruction in how to make a entire DRSSTC system work, but food for thought and explanations on why some parts are chosen over others. The lessons learned over the years by other people who built Tesla coils have resulted in some parts being the only one used, just due to the fact that they were used by the original designer and proven to work good.

This guide should hopefully give you an insight to chose among a wider selection of parts for your DRSSTC and that could help on lowering the construction price and time spent looking for parts. The math and calculations used are simplified for practical use.

### Topics of the DRSSTC design guide

1. Rectifiers
2. Busbar and primary circuit
3. IGBTs
4. DC bus capacitor
5. PFC (20% done)
6. Snubber capacitor
7. MMC / tank capacitors
8. GDT / driver (20% done)
9. Secondary coil