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Driving a gate drive transformer.

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Alex M

Tue Dec 27 2011, 03:39PM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Hi, having built a halfbridge plasma speaker based on this

diagram I now have a decent sounding plasma speaker that is reasonably loud, well at-least compared to my 555 timer based one.

Video of it running

Now the problem I had at first was the chip and mosfets were heating up pretty bad so I put in some totem pole transistors to buffer the SG3525's output. It seems to be working in keeping the chip cool and the mosfets cooler (they still heat up after a while).

But the problem is that those buffer transistors get quite hot. I am using BD244B and BD243B (they were the only type I had enough for 2 pairs) in a totem pole configuration to drive the GDT.

I am using 470 ohm resistors right now becuase I do not know what value I should use. I have tried some 1k and 10k ones though and although they make the heating slower the MOSFET's get hotter as the high base resistors mean the GDT isn't putting out as much voltage.

Any suggestions would be greatly appreciated,

Thanks. Below is where I have inserted the transistors.

Edit: I made a mistake in the schematics, the transistors go AFTER the 12v voltage regulator.

Inducktion

Tue Dec 27 2011, 05:39PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

I wanna say you have two problems here;

1. Your transistors are too slow, their turn off times are around 1 uS, which is a bit excessive.

2. Your base driving resistors are much too large. You want your transistors to be driven into saturation, so try going with a much lower, say, 10 ohms, or none at all.

If you can get some 2N3904 and 2N3906 transistors, they'll work a lot better, and I believe they shouldn't heat up as much.

Also, you don't need those UF4007 diodes in there if you're using the totem poles.

Alex M

Tue Dec 27 2011, 08:32PM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Dr. Duck wrote ...

I wanna say you have two problems here;

1. Your transistors are too slow, their turn off times are around 1 uS, which is a bit excessive.

2. Your base driving resistors are much too large. You want your transistors to be driven into saturation, so try going with a much lower, say, 10 ohms, or none at all.

If you can get some 2N3904 and 2N3906 transistors, they'll work a lot better, and I believe they shouldn't heat up as much.

Also, you don't need those UF4007 diodes in there if you're using the totem poles.

How do I go about working out base resistor values for transistors? And aren't the 2n390x transistors only rated for 200mA max?

I have no idea what the diodes actually do, I just put them there as the schematic said to use them. What do they actually do?

Thanks.

Dr. Dark Current

Tue Dec 27 2011, 09:29PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Hi,
your transistors are missing reverse diodes, use a Schottky diode on each transistor (eg. 1N5819).
I would use a BD139/140 or BD137/138 complementary pair transistors, they are 1.5 amp and are fast.
No base resistors are needed.

PS. I don't see a 100nF ceramic DC block cap on your 12V supply, do you use one?

Inducktion

Tue Dec 27 2011, 11:25PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Alex1M6 wrote ...

That's their max continuous current. Because you're driving them with a pulse, they can handle a lot more current; probably around 1 amp peak.

Daedronus

Tue Dec 27 2011, 11:59PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

Wouldn't you need something more like 6 to 10A peak for gate driving? Especially if you are driving multiple mosfets at the same time (with the GDT)

Inducktion

Wed Dec 28 2011, 06:00AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Daedronus wrote ...

Wouldn't you need something more like 6 to 10A peak for gate driving? Especially if you are driving multiple mosfets at the same time (with the GDT)

Since it's only a half bridge, and the mosfets are small, I think 1 - 2 amps peak is fine. 9 - 10 amps are for much larger mosfets... It won't hurt of course but it's overkill beyond a certain point.

Alex M

Wed Jan 04 2012, 04:07PM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Could a moderator please delete this post, I messed up on the image.

Thanks.

Alex M

Wed Jan 04 2012, 04:11PM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

I have now made an updated schematic of what seems to be working well. Note the MOSFET heatsinks still get quite warm after a while but I have added a thermistor controlled fan to cool it down as needed.

The heating only seems to appear when I audio modulate the arc.

Sorry about the messy schematic, can anyone recommend a good free program that I can make schematics with?

Inducktion

Wed Jan 04 2012, 04:49PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

ALSO; the half bridge does NOT need such excessive capacitance! Gren designed that circuit when he didn't know much about half bridge designs.

I've built it with much much smaller capacitance and it works just as well, and plus, you don't need a huge bank of electrolytics.

1 uF caps work fine!

You also do not need the UF4007 diodes in there, since you're driving the GDT with the BJT's.

Also, you can use expressPCB to make schematics as well as designing PCB boards if you ever get into etching. What kind of transistors are you using for the BJT driver, out of curiousity?

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