Yo dawg, I heard you like assembly language, so the source code for my PIC-based fibre optic link for Tesla coil drivers is now on Github.
It was time to put it all together! (This actually happened in March- these are post hoc posts 🙂 )
First Odin’s control electronics had to be converted to run off 24V DC instead of the original 240V AC. (And mounted in a Eurorack while I was at it…) This wasn’t too difficult as they already used 20-something volts DC internally, derived from the mains with a traditional iron cored transformer and rectifier, and regulated to 15V.
I added DC input sockets to the driver and gate drive amplifier modules, and changed the fan for a 24V one too. The original 240V AC inputs are retained in case the PFC breaks down and I need to change back to the old power supply.
The PFC will be situated at the operator’s position with long cables for 750V and 24V running to the coil. This made everything simpler, as there was no need for remote control and the circuit breaker on the PFC could be the emergency shutoff for the whole system. But it did leave the 24V cable vulnerable to strikes and general pickup of the extreme levels of EMI around a Tesla coil. My solution was to make a DC input module using a surplus Traco 40W DC-DC converter to give galvanic isolation, and lots of EMI filtering on both input and output.
The red module is the receiver for my Teslink system that sends multiplexed control signals over a Toslink optical fibre. I finally got round to completing it (and making a Eurorack mounting transmitter too)
The idea is that the PFC accepts single or 3 phase power at anywhere between 208 and 415V, and supplies 24V DC to the Tesla coil electronics from its own control power supply. I didn’t want the hassle of having to change taps on control power transformers, or rather the carnage of connecting it to 415V with the taps set to 240. (I have done this before- it was messy)
The Tesla coil primary was set up using a water-filled steel pan as a dummy load.
It didn’t explode! 😀
The next step would have been to take the PFC and immersion heater bucket to a lab with 3 phase 415V power. Unfortunately this was made impossible by the COVID-19 lockdown. The debut was to have been the Nottingham Gaussfest, but this was also cancelled. Insert corona joke here 🙁
Before I could get on with building the PFC into an enclosure, I had one last design decision to make: What sort of EMI filtering to use. The size and shape of the EMI filters would affect the rest of the mechanical design. Ok, that’s management speak for “How am I going to get all this cr@p into the 3U rack enclosure I’ve already purchased?”
Now, I deal with EMC in my day job and am vaguely familiar with the standards and test procedures, but this is a one-off handmade power supply for a Tesla coil. It’s never going to get tested for emissions, and the emissions from the Tesla coil will dwarf the contribution from the power supply anyway.
So the main purpose of the EMI filters is to protect the PFC from malfunction or damage caused by the Tesla coil emissions. These tend to be common mode transients caused by ground strikes, containing frequencies up to the 10s of MHz. There isn’t a great deal of VHF or UHF energy due to the length of the spark channel. So they really aren’t super hard to filter out.
I prefer to connect the filters so the Y capacitors (jargon term for the capacitance between lines and ground) are at the end connected to the outside world. My reasoning is that I’d rather any incoming transients were dumped to chassis ground through the capacitors, than potentially flashing over a choke.
I started by trawling the RS, Farnell and Mouser catalogues for ready-made EMI filters. I ended up with a Delta 30TDVST2 for the input and a Schaffner FN2200-25-33 for the output. These both had the Y capacitors at the load end, so would have to be used backwards from the maker’s recommendation.
I soon discovered a serious problem with both filters: a very high Y capacitance. This isn’t a problem in the intended industrial application, but a bit of a show-stopper for mine. When the PFC is used on a single phase supply, the high capacitance causes enough earth leakage current to trip any RCD. Note that the Y capacitance of the DC output filter also contributes to the leakage, because the DC output is not isolated from the mains and has AC superimposed on it.
I couldn’t find any better filters, so I broke them open and set about reducing the Y capacitance.
I lifted the connection between Y capacitors and earth, and added a 68nF capacitor in series, with a 2.2M discharge resistor. This should give a total leakage current budget of around 10mA at 240V AC. (Odin has 2 68nF capacitors from DC bus to ground already, which contribute too)
The modified filters no longer tripped my house RCDs, so the job was done. In hindsight, I wouldn’t buy ready-made filters again. It would have been cheaper to buy the parts and make them.
The PFC engine is working, but there are a few other things needed to make it usable. (Operationalise it? Or Heaven forbid, weaponise it? 🙂 )
Auxiliary power supply: The PFC needs a small amount of power to run its own control electronics. I decided to use a Meanwell WDR-120-24 switching power supply to provide 24V DC. This is an industrial grade unit that will accept any input voltage from 200 to 500V AC.
The WDR-120-24 is a bit more expensive than the usual 85-265V input range units, but vital for my goal of being able to run the PFC off either 230V single phase or 400V 3 phase power, without any kind of voltage selector switch that could cause carnage if set wrongly.
Precharge: The bus capacitance of the DRSSTC is very substantial. Odin has 4700uF after a recent upgrade. The PFC itself will also need another 1000uF to allow it to work without the DRSSTC connected. All of this has to be charged to the peak value of the mains voltage before the PFC can even start, in an orderly manner without tripping any breakers.
I chose a capacitive ballast for this job, consisting of 22uF motor run capacitors with 10 ohm resistors in series. The capacitors do most of the current limiting while the resistors protect the capacitors and main contactor from the surge when the capacitors are shorted out. The resistors are attached to the main heatsink and protected by the overtemperature cutout.
The precharge controller is based around a time delay and voltage sensing relay. (Schematic in a future post) The voltage between D1 and D2 must get over 200V, and the voltage between D2 and U2 below about 20V, before the sensing relay will pull in. This energises the main contactor, connecting the PFC input rectifier directly to the mains, and powering up the PFC controller through its auxiliary contact. The PFC then goes through its own soft start procedure, charging the DC bus capacitance to full voltage.
Dump load: The large DC bus capacitance also needs discharged when the system is powered down. My previous coils all relied on bleed resistors and took over a minute to discharge. For this build I decided to try some PTC thermistors from Epcos. (Details in a future post.)
The main advantage of PTCs is that, unlike normal resistors, they limit their own temperature and won’t catch fire or explode if the switch controlling them accidentally turns on while the DC bus is powered. This allows me to switch them with a SCR which was already present in the bypass diode module.
EMI filtering: This is as much to protect the PFC from damage by the huge transients generated by the Tesla coil, as to protect the mains from the hash thrown out by the PFC. My search for suitable off-the-shelf EMI filters is documented in another post.
The low power test was a success, so a larger dummy load was obtained- 3x 240V, 3kW water heaters connected in series.
The PFC was placed on a special tea tray test fixture and temporarily wired into the electric cooker supply in the kitchen.
According to my smart meter the result was 8.1kW input, though the other house appliances were using a few hundred watts at this point. The line current waveform looked nice. I used an EMI filter this time so the garbage at the switching frequency is reduced.
The moment of truth could be put off no longer 🙂 The PFC was tested at reduced input voltage using a 110V, 100 watt heater as a load.
In the previous post I mentioned that I couldn’t drive the MOSFETs directly from the controller chip because they need a negative voltage to turn off. The easiest way to provide this was with an isolated gate driver using the excellent Silicon Labs Si8271 IC.
Due to the tiny size of this IC and the requirement for a very tight layout to minimise stray inductance, I designed a PCB for it. This was also an excuse to finally start learning Kicad after years of using Eagle.
Power is supplied by a Murata MGJ6 DC-DC converter connected to J2. +15V to pin 1 and -5V to pin 2. The same DC-DC converter powers the opamp on the voltage feedback isolator board.
R7 is a small value like 22 ohms. I installed a 10k resistor in place of C6 to pull down the input if it became disconnected. R2 was not fitted and R1 was a 0 ohm link, the opposite of the schematic (I got the enable pin logic the wrong way round)
I uploaded the Gerber files to JLC– a Chinese manufacturer based in Shenzhen who ship worldwide- and had the boards the following week for a cost of about £5. JLC are 1/10 the cost of any UK PCB house and have really lowered the barrier to entry for hobbyists.
All this power is nothing without control.
Luckily by using silicon carbide power devices, we were able to handle all the power we needed with a simple boost converter switching at 100kHz. No need for multi-phase or fancy energy recovery snubbers- it’s just a domestic appliance-sized PFC front end scaled up.
This means we can also use a controller intended for said domestic appliance PFC. I used the UCC28180 PFC controller chip from TI, because it only has 8 pins and is marketed as “easy to use”, therefore it can’t possibly go wrong! 😀
(OK, I bought the evaluation board and tested it with some Tesla coil-like loads before committing)
This is the circuit I ended up using. It’s quite different to the original TI application circuit, so I’ll describe the differences.
Compensation: TI provide 2 full pages of formulae for calculating the compensation components, and ain’t nobody got time for that! So I took the values from the evaluation circuit, and doubled the capacitors and halved the resistor because it seemed like a good idea.
Gate drive: Silicon carbide MOSFETs need a negative gate voltage to turn off, and the UCC28180 doesn’t provide this, so I had to make a separate gate driver, which I’ll describe in the next post. The Silabs isolated gate driver chip needed a 5V supply and a 5V signal, so I had to add a level shifter and a 5V regulator.
Inductor current feedback: The UCC28180 is designed to take feedback from a resistor in the DC bus negative. This is a very common design choice in small PFCs. I added the 4x 1N5819 diodes and associated circuitry to adapt the signal from the CTs to look like it came from the original resistor. The 13V zener is used to clamp the spikes when the CT cores reset. A value of 13V guarantees reset even with the maximum duty cycle allowed by the controller chip.
I tested the CT feedback on the UCC28180 eval board before committing to building the full sized version, in case the chip turned out to be unhappy with it for some unforeseen reason, but it performed identically to the original resistor feedback, as far as I could tell.
Voltage feedback: Since the CTs provide isolation of the current signal, and I ended up using an isolated gate driver, I decided to isolate the voltage feedback too so the control circuit would be completely isolated from the high voltage side. I used a simple circuit based on the HCNR201 linear optocoupler and a TL071 opamp.
The voltage feedback input of the UCC28180 has a high enough impedance that it can accept the current from the photodiode (4.5uA per 100V DC bus voltage) directly with no amplification or buffering. This simplifies the circuit considerably and provides some nice synergy. The UCC28180 has circuitry to detect an open feedback network, a dangerous condition that can cause the output voltage to run away, blowing up expensive pieces of power electronics. By configuring the voltage feedback in this way, the protection still works for an open circuit anywhere in the signal path, including forgetting to plug the isolator into the main board, and loss of power to the isolator’s opamp.
Both circuits were easily constructed on protoboard with through hole components (and an adaptor for the UCC28180)
When I started designing I already knew that the heart of the thing would be silicon carbide MOSFETs and diodes. I heard so much from Anders Mikkelsen of Advantics raving about how awesome they are. 🙂
On recommendation from Anders I chose the C3M0065100K MOSFET and C4D10120D diode. These are available from Farnell, Mouser etc. for about £10 each. I figured that 2 in parallel would allow me to run an inductor current of 32A RMS, a reasonable match to the mains supplies available in the UK and Europe. Anders also kindly donated an inductor capable of handling 32A RMS. 🙂
The remaining design decisions were set by the dumpster find of a large heatsink and set of 3 Semikron dual diode bricks. Having such a beefy oversized rectifier made short circuit protection easy: I just needed to come up with one more heavy duty diode to bypass the delicate SiC diodes, and then a short on the output would simply pop the circuit breakers on the mains input.
CT1 and CT2 are small ferrite cored 1:200 current transformers sensing the MOSFET and diode currents. Their outputs are combined in the controller to give the inductor current.
SiC MOSFETs can switch incredibly fast, so a good layout with minimal stray inductance is important. I decided to make a multi-layer structure out of pieces of single-sided copper-clad PCB and copper sheet.
I didn’t manage to fit as many capacitors as I wanted: only 5 on the input and 4 on the output. The output ones in particular will have a hard time with serious amounts of HF ripple current. They are only there to filter out the HF ripple. Large electrolytic capacitors will be needed to handle the power frequency ripple, they will be mounted somewhere else and connected by wires.
Clamps for the MOSFETs and diodes made from pieces of Ikea cabinet legs. 🙂