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Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608
Original thread here
Here is my first Class E Tesla Coil and also my first high frequency Project.
So far I am still testing the setup with only 50v. New board design helped a lot.
I had some issues with my secondary, I got a burned ring on my secondary and it runs hot. (probably cause the PVC is lossy and conducting..) I think It was cause by and RF burn that arced and melted the insulation all the way around. Wider dia. primary reduced heating but gave shorter arcs.
Versus
50v Setup specs:
45nf DC blocking Cap 960pf Shunt ( varies with supply voltage, every 220pf seems to increase or reduce Voltage by ~30v) 3 turn primary 2.89 MHz Res
Yellow:DC Blocking, Red: RF Choke, Blue: Ixdn630 runs COOL!! off of 12v supply,Green: Shunt caps
I used some scrap wire for the primary and will probably rewind the secondary. Just getting a feel for the E class...Feels good..
Schematic of course. Changes to it so far :
Used 12v 2amp regulator instead on 15v RFC= 26 turns of 18awg with two type 2 iron powder together.
Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608
Working on adding a GDT for the ixdn630. Cant believe I forgot to put one in there. Blew two of them and many fets. They are expensive. Returning shortly.
Update. Rewound secondary for 1Mhz. About 1in output at 50v Cant wait to see it at 170v
Registered Member #55219
Joined: Tue Jun 09 2015, 11:21PM
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Posts: 80
Looks good Zak, I like the board layout... About your resonator, I could be wrong, but maybe the twisted pair is adding excess resistance / capacitance and May be the source of some of the heat you mention. I get why you went that route, skin effect etc... Maybe build a single winding secondary and compare?
Also, Did you add / are you using an interrupter?
I was fortunate enough to have my friend come over and explain some of the things I didn't understand with the Class E TC diagrams (He's an electrical / electronics genius literally). He told me that I should consider (from standard E schematics) the DCB cap, L2, and Load resistor as the Secondary tank (internal stats). The RFC, Shunt Cap would then be the Primary tank. This was what confused me with comparing all the different builds.. and what I was trying to share in the other thread, but was just a little off...
Once again Looks good!. I think I am close with my 'No Scope' PLL Class E attempt @ 150khz, working on the soft start circuit then time to light it up :)
Registered Member #1316
Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365
Cool coil.
How much does the power FET heat up? SiC MOSFETs have a low gate charge (which is why your gate drive chip stays cool). However, they also have a high internal gate resistance, which I expect will limit the maximum frequency they can switch at when compared to a normal silicon MOSFET.
You should not need a GDT unless you are trying to galvanically isolate the driver from the power stage. Your schematic as drawn has a common ground so a GDT would just add leakage inductance and potentially slow your gate drive. A lack of a GDT would not lead to blown FETs.
Whats the reason for all the parallel inverters driving the IXDN630?
Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608
Weston wrote ...
Cool coil.
How much does the power FET heat up? SiC MOSFETs have a low gate charge (which is why your gate drive chip stays cool). However, they also have a high internal gate resistance, which I expect will limit the maximum frequency they can switch at when compared to a normal silicon MOSFET.
You should not need a GDT unless you are trying to galvanically isolate the driver from the power stage. Your schematic as drawn has a common ground so a GDT would just add leakage inductance and potentially slow your gate drive. A lack of a GDT would not lead to blown FETs.
Whats the reason for all the parallel inverters driving the IXDN630?
Im taking a little break on the project but I managed to get a little more progress.
I added the GDT cause every time I blew a FET it fried my gate drive chip. Those IXDN630's cost $7.77 a piece. Is there another way to prevent them from dying. I was not able to get good wave form from the GDT out of the ~14 cores I tested. I would prefer to go without it.
The T-220 or T-263 pkg type for the drive chip mounted to the heat sink eliminated all heating in the chip completely. However during testing the FET would get hot depending on the tuning after about 10sec. I have it mounted to a large base heat sink with a small fan blowing through the grooves keeping the heat down A LOT.
I removed most of the paralleled inverters as they added delays and ringing to the drive frequency.
I was really just trying to get a foot hold on what does what when I adjust something in the circuit. I set up my primary with lossy 18awg wire with taps to test the affects on the wave form. Also just kept adding or reducing shut caps to change the max V seen by the FET.
The height of the pri. played a big part in spark output. To high up the secondary and it blew my FET.
Pictures are old, Secondary is now made of single 32awg on acrylic ~2in diameter. Res. 1.18MHz
I want to experiment with antenna or CT feedback for tuning but wouldn't I require something to start the oscillation?
I will also be updating my schematic here soon as well
Registered Member #55219
Joined: Tue Jun 09 2015, 11:21PM
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Posts: 80
With the GDT from personal experience, Start with the standard 15 turn 1:1:1 ratio, measure, then try 8 turns, and 20.. I found some like more turns, others like less.. Or for $5us you can buy a small pre-wound gdt from mouser P0584NL (product number) buddy sent me one, but have not used yet... With the antenna feedback, if using the 74ac14 ic, unless you use a pull-up resistor on the input( to set output low ) you should be able to get it working without much more, as the ic will send a High output when circuit is turned on, and will/should start oscillations ..
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
Ahhh, I think I know why your 'FETs are dying, I used those exact same SiC MOSFETs on my inverter. It would run fine for 10 or 20 minutes and then, seemingly without reason two (they were in a full bridge) of my 'FETs would die. Using a 'scope I found that the gate voltage at the leg of the MOSFET was ringing to almost 20v. They aren't designed to withstand that! I added a 6.8ohm gate resistor, this damped the oscillations to acceptable levels and I didn't have any more problems.
Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
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Posts: 608
hen918 wrote ...
Ahhh, I think I know why your 'FETs are dying, I used those exact same SiC MOSFETs on my inverter. It would run fine for 10 or 20 minutes and then, seemingly without reason two (they were in a full bridge) of my 'FETs would die. Using a 'scope I found that the gate voltage at the leg of the MOSFET was ringing to almost 20v. They aren't designed to withstand that! I added a 6.8ohm gate resistor, this damped the oscillations to acceptable levels and I didn't have any more problems.
BTW I didn't use a GDT
I dont believe that is an issue, During operation the drive voltage is at ~16v-- 18v max. Im using 3 different types although one of the FET has such a large Gate capacitance it really doesn't work. However the WolfSpeed is fantastic!! It seems whats killing them is the fact that I do not have it tuned properly or it is detuned and cause the Voltage and Current to exist simultaneously causing massive heating. Drain and source pin on one FET got RED hot and lit on on fire. The FET itself had a glowing circle between the two pins. That intern either sends an EMF spike into the IXDN or shorts the chip so that it breaks the drive chip along with it .
Would a high voltage high current diode across the Drain and Source pin of my FET protect the drive chip during a failure?
With that being said, while I am tuning it I was trying to be careful and notice patterns to avoid the coil running with the voltage and current overlapping. It happens. Just costs a lot.
Ill break it out again in a couple days and mess with it some more.
Registered Member #1316
Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365
You also want to avoid a GDT as the SiC FETs have an asymmetrical gate drive voltage rating. Its easy to overvoltage them with negative voltages.
If you are trying to reduce the number of gate drive chips killed, consider adding external diodes to ground / VCC, a small SMD fuse, or an external gate drive resistor. The SiC FETs have a high internal gate resistance (26 ohms in the one I linked), so anything less than that will not adversely effect your gate drive waveforms while still protecting the gate driver chip from a short.
Additionally, consider using a cheaper gate drive chip, such as the IXDD609CI ($3 on digikey). The 30 amp gate drive chips are designed for driving huge transistors. The internal gate resistance of most FETs you would consider in this application is high enough such that you will get nowhere near 30 amps of gate current. For the SiC fets you are using it's going to peak at less than an amp.
The smaller IXYS gate drive chips are just as fast, if not faster. I have used them up to 6.78MHz in a class E coil and they worked fine.
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My Class E
