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ragnar

Fri Apr 06 2007, 01:25PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Without a DC blocking capacitor, if you fire up the power to the gatedrivers without having a signal applied, they're as good as toast.

The series cap allows them to idle, since the capacitor lets only a small initial pulse through. An appropriately sized cap (1uF) will have no effect whilst they're switching AC (high frequency, at least).

Steve Conner

Fri Apr 06 2007, 01:45PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Seriously though, if Shawn is using the circuit he gave in his first post, where on earth is he going to put the DC block capacitor?

Marko

Fri Apr 06 2007, 02:08PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
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Yes, push-pull is sometimes troublesome because it doesn't support a dc block cap. It may not be important although because gates themselves are capacitive.

Oscillator driven coil in megahertz range is poor choice.

I would strongly suggest you to get some UCC's and use self resonant scheme.
Your current driver doesn't look like it would drive anything but tinyest fet's, and still with huge delays, it looks slow to me even with that 270pF load...

Adding external diodes will also increase parasitic capacitances over the mosfet.

I believe your breadboard is partly messing things up. If you really feel like continuing with this driver, rebuild it on a PCB with shortest possible connections and good decoupling.

ShawnLG

Fri Apr 06 2007, 02:14PM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
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"Without a DC blocking capacitor, if you fire up the power to the gatedrivers without having a signal applied, they're as good as toast."

I see the problem now. This would cause a problem during an interrupt. I can try putting a capacitor in series with the gates of the MOSFETs with a pull down resistor between the capacitor and the gate.

thedatastream

Fri Apr 06 2007, 09:55PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

Putting series capacitance in the gate will maintain the gate drive amplitude but will introduce a DC shift at the MOSFET gate. Assuming a 50% duty cycle and a 10V drive, you will see Â±5V on the gate.

This doesn't get around the problem of an uneven duty cycle causing flux walking in the primary of the core - if one side is on for longer than the other then there is a DC flux component that will slowly build until the top part of the B-H curve hits saturation causing excessive driver currents.

You can't couple the centretap because it has a necessary DC component. You can't couple the collectors of the transistors because the DC component of the push pull is still present in the winding. Coupling the outputs is the same.

All the other clever schemes I can think of all require the use of external FETs or transistors to drive the primary. I would go for a dual MOSFET driver in a SO8 package - maxim do some.

James

Marko

Fri Apr 06 2007, 10:08PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Flux walking is generally main concern when talking about this.
I don't know any way it could be prevented except minimizing the duty cycle, and so I don't think this as a good topology for mosfet drive.

Proportional base drive transformers use push-pull but only trough a few kiloohm resistor in series with positive supply, and during ''off'' time both transistors are on and keep both primaries shorted together to ground.

There is also a current winding seriesed with halfbridge output wich actually provides the main base drive power, and wich I believe for also keeps the transformer safe from flux walking since there is a DC blocking cap in series with that winding.

Anyway I don't believe this would be much of use for mosfets. They generally wouldn't like their gate voltage to be proportional to drain current, it must be within limits of few volts!

ShawnLG

Tue Apr 24 2007, 07:28AM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
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It lives!!!

I used a pair of IRF510 to drive the primery. They had a low enuff gate capacitance to not overheat the 2N7000s. But the IRF510s are heating up, even with heat sinks and a fan blowing on them. The SMT 220s would let their smoke out in no time. This could be fixed with an interupter. The coil is also hooked up in DR mode, which give better output. Below are some pics in CW mode. By the way, RF burns hurt!

uzzors2k

Tue Apr 24 2007, 11:41AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Awesome work ShawnLG! Looks like it's working better than the RSGTC did.

Marko

Tue Apr 24 2007, 11:50AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Especially considering what is he using to drive it!

I see the variable cap.. are you rinning it class E??

ragnar

Tue Apr 24 2007, 01:40PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Posts: 1425

ShawnLG wrote ...

The coil is also hooked up in DR mode, which give better output. Below are some pics in CW mode. By the way, RF burns hurt!

Hmm, what do you mean "DR mode" (DR as in dual resonant? How?)

RF burns are nasty. Sure, they happen when you just want to grab a tool to see if you can draw an arc during testing or prototyping, but try not to make them a regular habit. They're worse than blood blisters.

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