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Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Just curious to see what everyone has been doing as far as Class-E tesla coils go and how far people have been pushing them.
I'm presently looking at designing a 1kW Class-E 4MHz audio modulated coil, using an offline PFC supply to generate the 300V Drain Voltage, and using IXYSRF Gate Drivers / RF MOSFETs for the actual switching element.
One of the major requirements is ultra high quality audio, so I may even be using a 2nd Post Regulator (Current Mode Buck) to provide a very clean Drain Voltage to clean up quiescent arc noise as well.
Registered Member #2390
Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381
EastVoltResearch wrote ...
Just curious to see what everyone has been doing as far as Class-E tesla coils go and how far people have been pushing them.
I'm presently looking at designing a 1kW Class-E 4MHz audio modulated coil, using an offline PFC supply to generate the 300V Drain Voltage, and using IXYSRF Gate Drivers / RF MOSFETs for the actual switching element.
One of the major requirements is ultra high quality audio, so I may even be using a 2nd Post Regulator (Current Mode Buck) to provide a very clean Drain Voltage to clean up quiescent arc noise as well. Do let me know when those are available
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
ST were the first with a 1500V silicon MOSFET, DigiKey stocks STW4N150 suitable for off-line work. Keep them clean and dry otherwise they track between the legs!
I blinked and ST now make a 9A die too: STW9N150 (PDF). I've played with the former, and 4MHz was OK, but driven with a square wave to give more freedom to choose shunt C! You're sure to tear less hair out if you go with a metal-gate MOSFET and a DEIC420 or so.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
I've dabbled quite a bit with using various Class E amplifiers to drive TC resonators.
First prototype was 4MHz but only low power. Around 500 watts or so with a IRFP450A or MTW14N50E switch-mode MOSFET. Corona was purple in colour and almost completely silent unless disturbed by movement of air.
Second design was 8MHz, initially low power (approx 650W) and then driven by two single-ended Class-E amplifiers with a bridge-tied load. Power level was approx 1500W using two IRFP460A devices. PA drive was IRF530. Total current draw was about 11A at 160V. Corona was completely silent and more flame like than at 4MHz.
Third design ran at 18MHz. Power output was only about 400W and it required about 15W of RF drive! Switching device was IRFP450LC with a few volts of gate bias to reduce drive requirements. Corona was like a big silent candle flame, but VERY hot!
I also investigated three audio modulation techniques:
1. FM. It is easy to implement Frequency modulation at low-level but Class-E amplifiers bandwidth is very narrow if you want good efficiency. You have to tune the amp so the quiescent operating point (when there is no audio present) is near maximum efficiency otherwise it quickly overheats. Then, when it's tuned like that the linearity is poor for positive audio modulation so it doesn't sound so great.
2. High-level Class-H modulator. This is a linear mode AM modulator like a Class-A or -AB series pass modulator, but with a neat modification. It uses two linear pass transistors and runs from a split rail supply consisting of +V and +V/2. Excellent efficiency is achived around the quiescent point because the +V/2 rail is fed through an almost fully saturated transistor to the RF amplifier. Negative modulation is achieved by bringing this device progressively out of saturation, and positive modulation is achieved by directing current from the +V rail through the other device. It is much more efficient than a simple linear Class A series pass modulator without the complexity of a switched-mode modulator. You can find more details by searching for "Class-H modulator"
3. High-level "Class-D" Pulse Duration Modulation. This is basically a series pass modulator consisting of a buck converter and a reconstruction filter to remove the switching noise. (I actually used two phase-shifted buck converters that were 180 degrees out of phase to increase the effective switching rate, spread heat dissipation and ease the filtering requirements.) The same as the polyphase PWM modulators used in some high-power AM broadcast transmitters. Efficiency is very high because there are none of the dissipation issues of the linear-mode Class-A modulator. In theory this switching modulator should be very linear but there was an unexpected problem...
In an AM radio transmitter the load (antenna) is fixed at say 50 ohms, so the final RF amplifier sees an essentially fixed load resistance regardless of modulation. This in turn means that the RF amplifier itself looks like a fairly constant DC load to the modulator that preceeds it. This enables the modulator's reconstruction filter to be designed with the right frequency response and a correctly damped rolloff.
Unfortunately the breakout from a TC resonator doesn't behave as nicely as a broadcast antenna does!!! The corona voltage is more or less clamped above breakout, so the quarter-wave effect of the resonator makes the RF current out of the amplifier be approximately constant above breakout regardless of modulation. This implies a varying load resistance to the RF amplifier throughout the modulation cycle. This in turn means that the DC load presented to the modulator by the RF amp varies throughout the modulation cycle too. This leads to peaking of the reconstruction filter when the corona load is small (modulation minima,) and excessive HF droop when the corona load is large (modulation maxima). I think I got that right, it was a long time ago! The end result is lots of intermod distortion at the high-frequency end.
One final thing I experimented with was carrier-level modulation. This is a technique borrowed from SW broadcasting where the resting "quiescent" carrier level is altered depending on the instantaneous modulation depth. Basically the instantaneous carrier level is only that which is required to support the audio modulation depth at that particular time. In AM broadcasting this saves a lot of electricity costs because the carrier level can be decreased when the audio level is quiet, and then only increased to full when required to support deeper modulation. For an audio modulated SSTC I found that it reduces corona hiss when it is otherwise most noticeable - during quiet parts in music where the hiss is not masked.
So there you have it! That's my experience with Class E amps driving TCs!
-Richie,
PS. There is a lot of info about Class-E amps and techniques to achieve AM (for radio transmitters) here:
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Class-E Tesla Coils - What have you done?
