Building a DRSSTC Pt. 9 - The Schematic

Hi everyone, welcome back to my blog!

I lied when I said the next entry would be about the build. I left out something very important--the final schematic! I discussed the different parts, but never explained how they go together! This particular schematic is essentially one of Steve Ward's DRSSTC designs with a couple of tweaks for my coil. I cannot take full credit for the design.

Since my last post there have been many changes, the most notable of which being the addition of a flip-flop to allow soft-switching of the IGBTs (only turn them off during the zero-crossing point of the current), and of over-current protection. Let's start by taking a look at the full schematic. Please note that this circuit is subject to change--there are always things that will need to be adjusted throughout the design process.

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Pretty confusing, huh? I think so too. So let's break it down into its key pieces.

1) The Interrupter

I didn't actually include this in the schematic because there is any number of different ways to interrupt the coil. Generally, depending on your coil, you don't want the on-time to exceed 100uS, and generally you'll want even less. I believe I attached a calculator program written by Sigurthr (over at 4hv.org) in one of my previous posts. If you enter your coil's information, it will calculate the maximum burst length for you. Possible interrupter ideas range from simple single 555 timer circuits to complicated ones that require 3 555s, or even microcontroller-based ones. Regardless of the interrupter, however, there is one thing you should pay close attention to, and that's the optocoupler shown in the schematic:

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Many people replace this with an optic-fiber cable, which is certainly ideal and offers significantly more protection for the operator and potentially sensitive interrupter circuitry.

2) Soft-Switching Circuitry

As mentioned before, "soft-switching" means that we're only going to turn off our IGBTs at the zero-crossing point of the current. Trying to turn off IGBTs when there are hundreds of amps flowing through them puts a lot of stress on them and can cause them to fail rather quickly. Soft-switching only allows them to turn off when there is little or no current flowing through them. One of the most popular ways to do this is using a positive edge-triggered JK flip-flop such as the 74HC109.

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This particular circuit also incorporates the interrupter signal, the feedback signal, and the over-current detection signal so that all of them work together to turn off the IGBTs at just the right time.

3) Driver Chips

The driver chips are simply two paralleled UCC27425 MOSFET gate drivers. Each chip has one inverting and one non-inverting driver, which is required in order to generate a bipolar square wave to drive the gate drive transformer. Some designs use two separate chips such as the TC4429 and TC4420, one inverting and one non-inverting respectively. I chose the UCC27425 because it has both in one package, but failed to notice that it is only rated for 4 amps. I decided to use two of them in parallel to help share the load. Generally the higher the current rating, the better.

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You'll notice on the output I have a capacitor in parallel with a resistor. The capacitor is used to block DC which could cause the circuit to latch up, which could lead to excessive heating. However, when you have a capacitor in the same loop as an inductor (the primary of the GDT), you will get "ringing", or unwanted oscillations. In order to avoid this, we add the resistor to damp these oscillations which will ensure we only get a clean signal going to the GDT.

4) The GDT

The "Gate Drive Transformer", or "GDT" for short, is used to drive the gates of the IGBTs. It serves as an isolation transformer which prevents nasty kickback into our driver circuitry, and unwanted signals from going into the IGBTs. The GDT serves several purposes, but isolation is probably one of the most important. In this schematic I use two cores, but I will probably consolidate it onto a single core soon. It will depend on how your IGBTs are laid out whether you'll want one GDT or two.

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Notice that there are two pairs of secondaries. Each pair contains two secondaries that are in-phase with each other, but that the two pairs are out-of-phase with each other. If you recall the animation for the H-bridge that I made for one of the previous entries, two diagonally-opposite transistors need to be on and the other two should be off. We achieve this by using opposite-phased pairs of secondaries on the GDT. If you use a half-bridge you will only need two secondaries, each out-of-phase with the other.

5) H-Bridge (or "Full Bridge")

The H-bridge is what generates an alternating current from a steady DC bus. This AC signal is critical in order to get the proper oscillation from your primary tank circuit. The 3300uF capacitor is for smoothing, since the input to the bridge will be rectified mains (120VAC in the US). In order to find the DC voltage, simply multiply the RMS AC voltage (120) by the square root of 2 (1.414), and that means my DC bus will be around 170V. However, a large smoothing capacitor will be very important, otherwise we will get unwanted oscillation and poor overall performance from our coil.

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The tank cap (MMC) and primary coil will create a second-order LC circuit that will give us a resonant frequency determined by:

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where F is the resonant frequency in Hertz, L is the inductance of the primary coil in Henrys, and C is the capacitance of the MMC in Farads. The trick is to match this frequency with the resonant frequency of your secondary coil and topload. Once you do that, you have a "tuned" Tesla coil.

6) The Tesla Coil

The Tesla coil itself consists of the primary coil, the secondary coil, and the topload. This part is fairly straightforward. Something that should be noted, however, is that there are two current transformers (or "CTs") that are placed on the primary wire. In the schematic it shows two transformers labeled 1:1000, but really they're just toroids that the primary wire passes through. One toroid is for the feedback and one is for over-current detection.

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7) Feedback Circuitry

The feedback circuitry begins with one of the CTs that the primary wire passes through. This induces a current in the secondaries of the CT which we clean up using some diode clipping and a Schmitt-Trigger inverter to ensure we get a nice square wave. This signal is passed back into the soft-switching circuitry, and then fed back into the IGBT drivers. This creates a feedback loop which ensures a clean oscillation on the primary.

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One important thing to note in the feedback circuitry is the variable inductor in series with a 5 ohm resistor across the output of the transformer. This creates what is called "phase lead", which allows the IGBTs to handle the higher frequencies. Recall that we use the flip-flop circuitry to soft-switch the IGBTs. However, large brick-type IGBTs generally take a few hundred nanoseconds to switch. While this may not sound like much, the rise and fall times add up and cause the IGBTs to switch too late, which leads to significant heating especially at higher frequencies. The phase lead allows the IGBTs to switch slightly sooner than they would normally, to ensure that they actually turn off at the zero-crossing point. By using a variable inductor it is possible to adjust the phase lead to the best possible amount of time.

8) Over-Current Detection (or "OCD")

Over-current detection is used to make sure you don't stress your MMC and IGBTs past their operating limits. One of the best ways to do this is to use a current transformer on the primary coil, and a comparator to shut the coil off if the sensed current exceeds the specified level. I based mine on one of Steve Ward's designs which uses an LM311 comparator that feeds a signal into a one-shot 555 timer. The timer disables the driver for a millisecond or two to allow it to cool down before restarting again.

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If you're familiar with comparators and 555 timers, it will be easy for you to follow how this works. When the current exceeds the threshold, a falling-edge signal is sent to the 555 timer's trigger pin. This causes the output of the 555 to go high for a certain amount of time based on the values of C6 and R11. In our case, we use a 10k resistor and a 0.1uF capacitor. Based on the formula for a one-shot 555 ( T = 1.1 * R * C), these values give me an on-time of 1.1mS. While the 555 output is high, it pulls the input of the inverter low through a transistor, and the "clr" input of the flip-flop is held high. Since we're tapping off of Q/ ("NOT Q"), then the enable pins of the drivers which are connected to Q/ are held low for that 1.1mS, which means the coil stays off for that amount of time.


That's about it. Sure there are a lot of parts, but if you break it up into the individual sections it's not so bad, is it?

Anyway, now that we've looked at the final schematic, next time we can start following the actual build. Please remember that at this point in time there will likely be long delays between posts as I tweak things and make adjustments to the individual parts. I've already gone through several designs, so be warned that chances are nothing I have done is final!

I hope you've enjoyed this look at the overall schematic for a DRSSTC. Your first look at the schematic is always the most overwhelming, but it really helps to know how to break it down into the individual pieces.

As always, I welcome comments, questions, feedback, and suggestions. Feel free to post as a response to this entry, or to send me a PM on the forums.