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Registered Member #2591
Joined: Fri Jan 01 2010, 11:58AM
Location: netherlands
Posts: 76
on richie's website:
3. Double-ended Class E drive.
Two Class E amplifiers can be employed in a double ended arrangement to obtain more output power. In this arrangement the two amplifiers are fed with complementary drive signals (180 degrees out of phase) and the TC primary winding is connected between the drains of the two MOSFETs. This doubles the voltage applied across the TC primary winding and gives a considerable increase in output power without requiring larger switching devices. It also does not require such care in the matching of the two amplifiers that is so important in parallel operation.
How does this look like, does it mean that the 100nF blocking cap is removed and the other site of the primary coil is directly connected to the second class-e amp? (see attach file)
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Cool. Sounds like an awesome project.
I'll have to think about the 100nF (Richie is obviously the best person to ask about this), but you can greatly reduce the complexity of the drive circuity by simply replacing everything between the crystal oscillator and the gate of the MTW14N50 with a single gate driver IC - in this case, an IXYS IXDD414 or IXDD614 (new Claire Semiconductor version). These are commonly used in Class-E drivers up to at least 4MHz. I have used them successfully up to 8MHz, but mostly use them at 4MHz. They are about $5 bucks apiece, but you can eliminate about 17 components by using one of these and also don't have to futz around with those two drive transformers. It will certaintly make your life much easier.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
There are a number of ways you can achieve the double-ended drive. You can tie the TC-primary between the drains of the two MOSFETs and have a seperate RFC for each MOSFET drain choke as you have shown. However, if there is a DC path from the drain of one MOSFET to the drain of the other then you should only require a single drain choke to deliver the DC supply current to both MOSFETS. In which case you may fit only one drain choke provided there is no DC-blocking capacitor (or impedance matching capacitors) fitted in series with the TC-primary.
If the value of the RFC is quite low, then it may un-balance the drain waveforms of the two Class-E amplifiers. In which case it is better to use two matched chokes, or to deliver the DC supply B+ through a single RFC tapped to the mid-point of the TC-primary coil.
It's also worth noting that when you tie the TC-primary between the drains of two complementary Class-E amplifiers it will see twice the voltage swing compared to that of a single-ended Class-E amplifier. A normal resistive bridge-tied load like this will then draw twice the current when presented with double the voltage to give a four-fold increase in power processed! This doesn't happen with a TC as the load - Applying more voltage causes the arc to grow, the loading to increase, Q to fall, and the base-impedance of the resonator at resonance to rise. The result of this is that the load current drawn from the amplifier doesn't change much. The CW TC appears almost as a constant-current post breakout. The upshot of all of this is that you might have to rethink the matching circuitry when designing a double-ended Class-E RF power amp with bridge-tied load if you thought that the load impedance would stay the same as for the single-ended case. It will probably be closer to double the load impedance of the single-ended case. Life is never simple!
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double ended class-e hf tesla coil
