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Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Nice work
Any thoughts on making the gate drive resonant, that might help with the driver heating (then again it might not).
I copied your biasing circuit thing, but I then decided to add an extra transistor. The current taken from the extra winding on the gate drive transformer is used to provide a tiny regulated (zener) supply, and a pot is used to bias an npn transistor which provides the bias for the mosfet from the regulated +5v rail. This way it's still failsafe if the driver blows up.
I'll have to get back into this, my stuff has been temporarily abandoned
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Avalanche wrote ...
Any thoughts on making the gate drive resonant, that might help with the driver heating (then again it might not).
I can afford to add a bit more inductance, because the IRF730 gate capacitance is on the order of ~700pF, not ~2600pF. I'll make the secondary two turns, and change the primary from two to four or five.. that should reduce the magnetizing current, too.
wrote ... I copied your biasing circuit thing, but I then decided to add an extra transistor. The current taken from the extra winding on the gate drive transformer is used to provide a tiny regulated (zener) supply, and a pot is used to bias an npn transistor which provides the bias for the mosfet from the regulated +5v rail. This way it's still failsafe if the driver blows up.
Pffft!! Well bahhh, fine. Complicate my circuit. See if I care.
Mine seemed pretty failsafe, but there's really no guarantee that the gate will discharge properly, except that under DC any charge will short through the inductors.
Steve will remember though my describing my 6.5MHz class-E coil which would run with no gatedrive. The resonant circuit was so finely tuned and of sufficient Q that when gate drive was removed, there was enough leakage/capacitance from the channel that the drain current managed to force the FET on and off... dangerous!!
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
wrote ... Pffft!! Well bahhh, fine. Complicate my circuit. See if I care.
lol, sorry, I'm just used to complicating other peoples circuits It's what kept me in employment for a year by making me look like I knew what I was doing, before I got the boot
I don't think it's any more failsafe than your version, I think I just did it so I could adjust the bias when messing around with my driver. Sourcing the bias current from a regulated rail might help with the impedence thing you mentioned, I'm not sure.
No gate drive is pretty impressive I wonder if I'm having a similar 'problem' with my setup. Somehow I'm seeing an unwanted 24Mhz when I put a load on, which probably has something to do with the extra capacitance in series with the gate
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
I revised my transformer to use a four-turn primary, two-turn secondary, and single-turn bias winding... and when I connected it back up to the IXDD414 and IRF730, it still runs hot... but I didn't need (or at least, didn't want) to add any extra inductance -- resonance was ~15MHz, and the Q is so low that 13.56MHz is just fine.
The efficiency is a little disappointing, because it draws 980mA at 22V.. but I could justify this saying that, as opposed to driving the IRFP450, I'm getting almost 18V p/p across the gate, I'm driving it "significantly" faster (up to 14MHz not 11.5MHz), or that the IXDD gets less efficient as its load increases, blah blah. It works, and with a 40mm fan, it stays cool enough (hot.), for now.
After mounting the IRF730 to a free heatsink, I wound a little resonator, plonked it on a groundplane, and used my signal generator to find that, tweaking the height of the primary, it works at ~12.7 - 14MHz.
The picture below shows what the combined gatedriver/bias/decoupling/FET looks like.
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HF gate drive experiments
It's what kept me in employment for a year by making me look like I knew what I was doing, before I got the boot 
