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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
GeordieBoy wrote ...
The resonant gate-drive is, of course, a narrow-band solution. I'm not sure if Chris wants to operate his switching amplifier across several bands?
I apologize for my last post for one other thing...
After doing some math and thinking again I see additional trouble with it - For 600pF gate capacitance the characteristic impedance will be only about 20ohms, which may require up to an amp of drive current which is unrealistic for direct output of the HC240.
So I would say a transformer is pretty necessary in this case, to transform the output impedance into something very low and maybe omitting the resistor completely (The 74HC240 has lots of output channel resistance alone).
Still the transformer could be made easily from a tiny ferrite bead if needed and would yeld very efficient drive (far more than non-resonant drive).
... not Russel! Registered Member #1
Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052
Quick followup -- after spending several hours toying with various models, I can see why this isn't as quick a question as I thought. Resonant drive seems to do very well, happily, but without adding a transformer, some bias, and a clamping zener, I wasn't able to get the fast rise times that one would like to see, and projected efficiency plummeted. At least I'm no longer groping around in the dark! Thanks for all the input.
Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
This might seem like a bit of a dumb suggestion/question, I don't know too much about triggering mosfets, but would it be possible to use a pull up resistor so that the gate voltage sits just below threshold? I've used these before with the 74HCxxs to work with 74LSxxs, so when the 74HC240 output goes high, the gate voltage is more than enough to trigger, without forcing the chip to run at 7v? This is out of curiosity since you seem to have gotten somewhere with it.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
You can certainly bias a MOSFET just below the Vgs(threshold) voltage to reduce the AC voltage swing required - Therefore easing gate drive requirements. However you have to be careful in power applications for the following reasons...
Power MOSFETs optimised for switching applications have a negative temperature coefficient to Vgs(threshold) and a somewhat high gain. This means that a bias voltage of 3.5V may be in the pinch-off region when the device is cold, but well into the linear conduction region when the device has reached steady-state operating temperature.
This problem is particularly apparent with high voltage MOSFETs or when multiple devices are paralleled. The shift in quiescent operating point isn't always noticed in RF amplifiers until the RF drive is turned off, at which point the sustained DC conduction through the device causes excessive dissipation, further decrease in Vgs(th) and thermal runaway.
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Quick MOSFET Question
