Can't get new class e to work!!

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Can't get new class e to work!!

Postby wa2onk » Sat Nov 17, 2007 4:45 pm

Greetings,

Well, got a problem trying to get 6 fet p/p 40m rig going. Sine wave input, to a driver fet, out of the driver I can get about 20v p-p on each three fet leg. Problem is I can't get out put legs to draw any current.

I pulled a couple fets out an did resistance checks comparing them to a new fet. They all read about the same, resistance wise. I don't know if this actually is a valid check. The only thing I can figure is that the fets are bad. Could I have gotten a bad bunch :?: , although thats seems hard to believe. I'd hate to change them all out, they're a real pain to get to.

I checked the input wave forms to the p/p legs to make sure they were 180deg different. All voltages ar getting to where they should. Output goes to a 13 turn coil 1.75" dia of #12 wire. Tuning cap is 50-500pf and loading cap is 50-750pf.

Any ideas :?:

Thanks, Chuck...
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Postby frank carcia » Sat Nov 17, 2007 7:34 pm

Sounds like the source leads are not connected to DC ground.
The source lead has 2 functions Low side of gate drive and DC return for the Drain current. Sounds like yours are floating. The best thing to do is put a screw in the heat sink at each source lead with a lug to terminate it to the heat sink. This would be in parallel with the source bus gfz
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Postby kf1z » Sat Nov 17, 2007 8:03 pm

Absolutely NO current....or, just not much?



What do you have between the driver FET and each of the 3 FET modules?

I assume a transformer? how's it wired?

with voltage applied to the drains of the 2 modules, do you have 20 + volts at the gates of those 2 modules?

If the FETS are bad (possible 1 , but not likely all 6) they would likely have shorted drain to source...you'd be popping fuses...


Are you using a published design, or can you post a schematic?


It does sound like your source leads are floating (not tied directly to ground)....


Your driver is working, so don't get offended... but, if you're using FQA11N90 FETS, the pinout is Gate, Drain, Source (left to right facing front of the device)
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Postby wa2onk » Sat Nov 17, 2007 9:30 pm

I agree, sounds like an open source. The six source leads are soldered to a single brass plate, the plate is screwed to the heat sink has a ground wire screwed to it as well as the sink is screwed to the chassis.

Fets wired properly. 20v p-p measured from either 3 fet strings to ground.

Zero current. I have seperate meters for each bank of fets as well as meter monitoring total current for both banks.

I agree also that a short fet is more common.

Gotta be something I'm overlooking. But I covered the above mentioned.

Chuck...
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Postby wa2onk » Sat Nov 17, 2007 9:51 pm

I knew it had to be something I'm overlooked!! I thought I was using a 10x scope probe, it's a 1x. So I am only getting 2v p-p!!

I'll have to look at the driver tomorrow. Thanks for the replys.

Chuck...
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Postby kf1z » Sat Nov 17, 2007 9:55 pm

OH !!!!

Well THAT would do it!!!!

I was wondering how you were getting 20 volts through those transformers......

OK then.... let us know how things go tomorrow......



Also, I notice you don't have shunt caps mounted.....
Be carefull how much drain voltage you apply at first...
Check the peak drain voltage to see that it's not much more than 3.5 times the applied DC... if it is... you need more shunt C


Good luck!!
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