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Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I'm working on a 4MHz class-E coil and got it working somewhat (need to adjust the coupling as it's somewhat under-damped right now), but there seem to be two modes of operation, one correct and one incorrect.
The power stage topology is simply a FET with primary between the drain and positive rail. When everything is tuned right, I get an expected class-E waveform (VDS on top, resonant gate drive on bottom):
But, if I adjust the frequency by as little as 50-100khz away from 4MHz, it changes mode into the following (voltage scaling is same as the previous waveform):
So far, no more than 50V rail has been put on it since I fear that if it enters this undesirable mode of operation as voltage goes up, it'll blow the FET from VDS breakdown.
According to SPICE simulation, this actually is expected behavior. I'm having a hard time getting a good intuition for what is causing it, though. It seems to be an alternation between charging up the secondary tank and then having it dump back to the primary. Is there a way to avoid this mode, or make it require more frequency deviation before entering it?
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
you are going off tune and the output transfer is going at half freq. This is one of the problems with high order classes of amplifier drive. I have had to rework glass G Rf amplifiers where the drive level as well as the resonance is important to maintain output at the desired level. A 1dB change in input made a 5 dB and mode change in output level.
A reduction in Q may help ie more L less C. In general tuned circuits the Q is proportional to the C value ie lower l higher c = higher Q
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Class-E Coil Modes of Operation
