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Registered Member #78
Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133
sweet stuff~ Hope it works as good as it looks. Show those stubborn FETs what your're made out of. btw, just curious, but doesn't having too high a peak current on the gate actually damage the oxide layer?
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Yes, brutalizing the gate will damage a FET. The point of this driver it so minimise any inductive ringing between the drivers and the FET. Since the transistor(s) will be in zcs/zvs situations, I don't think the gate current need be too big... There's also provision underneath the board for a gate resistor.
------ Mod edit combined double post -------
Quiet in here...
so I assembled one of the boards (with the exception of the green screwterminals, which I'm out of, and wouldn't fit anyway as I have to enlarge the holes)... it's reasonably pretty in its own right =)
If you look at the third pic, the gold pins on the left is where AC (from isolating ferrite) is dropped in. The next set of PCB pins is where an isolated drive signal is connected. The top right PCB pin is where you'd connect the FET's drain, the bottom right PCB pin is where you'd connect the FET's source.
That's a fake schottky standing up on the right, too.
NOW will you guys comment? And tell me I haven't got my board back-to-front or anything.... especially considering I just made ten.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Wow, bp made a though hole board
Looking good, lets see them put to work... I want to see the anticoil going...
I found it kind of odd that you separated them all, seems like it would be easier to leave them in banks of 4 to keep the fets organized... You could have even left a little strip on the bottom of the board to mount the transformers and even the driver on to keep everything organised... Oh well
I think you had all the right to make that double post...
Registered Member #176
Joined: Tue Feb 14 2006, 09:35PM
Location:
Posts: 44
Steve Conner wrote ...
That's exactly what I'm using for my isolated gate drivers, the Mazzilli oscillator circuit. I wanted sine waves for quite a long and complicated reason.
I designed my isolated drivers as four drive boards that mount very close to their IGBTs, and plug into the main PLL box with Cat5 network cables. So I had to feed the HF AC power down this cable along with the gate drive signal. I thought if I used a square wave, the edges might couple capacitively over into the gate drive signal.
In practice I found that when I rectify the sine wave on the gate driver board, the diode recovery spikes add lots of crud anyway. But the Mazzilli circuit is real nice and simple. I found that if I used a choke input filter on the rectifier, the voltage regulation was good enough that I could get away without regulator ICs on each board. I just used a zener to draw some current, since the voltage skyrockets if you draw less than a critical value.
I used a ferrite E core with airgap for the mazzilli circuit, with a single output winding that fed a little ferrite toroid 1:1 isolating transformer on each board. I just did that so the different cores on the Cat5 cable wouldn't have high voltages between them. If you wanted to save some hassle, it should be fine to combine the oscillator and isolation transformer into a single component with 5 windings, the way Steve Ward etc. did. Just make sure you remember what is grounded and what is floating on your DC bus voltage.
I haven't time to think through the capacitive divider thing that Avalanche mentioned, but it just sounds plain wrong. :(
Steve,
You talk about it capacitlvey coupling to the gate drive signal from the HF AC supply. I would probally be more considrdned with crosstalk. When the coil is running you could get all sorts of noise that could be coulded capacitvley and electromagnetic onto the gate drive signal.
But if you use the foil twisted pair CAT5 cable you also get a good screen thrown in and there not to expensive. It was one of the methods I was going to use to drive my CM600s but I opted to sue BNC cables and BNC connectors in the finish.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Hi all, although it's taken in excess of a month, I've done several revisions on the power supply, signal circuitry, and actual isodriver circuits. It's (tentatively) working (the power, at least), and I'll assemble some other boards and see if it'll swing a FET!
I also built a cute little current-limited (with electrolytics, don't ask!) power supply, which has greatly reduced the number of silicon fatalities during development to just a few diodes! =)
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I think they look great I also hope that the boards you are (or are not?) making for me look as good
In any case, what is the upper waveform on the scope? My best guess is that the lower trace is across the output of one of the isolated outputs from the zvs powered isolation transformer, but I hope the top trace isn't the gate drive Put a zener or something on that...
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yea, something looks not right with those waveforms. :-/ Here's what mine look like:
Top trace is gate voltage @ 10V/div Middle trace is drain voltage of same FET @ 50V/div Bottom trace is output from transformer @ 50V/div
I'm running off 21V DC and producing about 30V RMS output. The transformer secondary is centre tapped to ground, so you only see half the output voltage on the scope.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Well, fine.
I f1x0r3d mine by reducing the gate resistors to 100-ohms each, adding some more inductance to my choke, replacing the overkill IRFP450s with IRF540Ns, and changing the resonant capacitor to 100nF.
Ha!
And the sinewave isn't perfect, because it was loaded down very badly by a 16V electrolytic on one of my power supply boards (oops) which promptly exploded after I took the photo, the gush of electrolyte gas narrowly missing my eye
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isolated gate drives
But the Mazzilli circuit is real nice and simple. I found that if I used a choke input filter on the rectifier, the voltage regulation was good enough that I could get away without regulator ICs on each board. I just used a zener to draw some current, since the voltage skyrockets if you draw less than a critical value.
I also hope that the boards you are (or are not?) making for me look as good 
Put a zener or something on that...
