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4hv.org :: Forums :: General Science and Electronics
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Simple questions about TL494 flyback driver

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mike34341
Wed Jan 27 2010, 07:18PM Print
mike34341 Registered Member #2656 Joined: Wed Jan 27 2010, 07:12PM
Location:
Posts: 2
I want to build the flyback driver on this website:

Link2

But I had a couple of questions first. The most important was which pin is the drain and which is the source on the MOSFETs? I'd assume that the bottom pin on all 4 would be the source but am not 100% sure on this.

Also, out of curiosity, I'd really like to know exactly how this circuit works. I understand the purpose of everything except the diodes and the use of two MOSFETS in series for each output. Could anyone explain what the diodes do and why there are two transistors in parallel?

Thanks!

-Mike
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Mrkva
Wed Jan 27 2010, 11:44PM
Mrkva Registered Member #2655 Joined: Wed Jan 27 2010, 04:09PM
Location: Czech Republic
Posts: 18
If you mean in schematics then yes, the bottom ones are sources. And if I can understand this schematics correctly, this is the full H-bridge schematics. At first, bottom left and top right transistors are ON and the power flows through primary coil in one direction, then they turn OFF and top left and bottom right transistors are turned ON - and the power flows through primary coil in the other direction. Related wikipedia article.
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mike34341
Thu Jan 28 2010, 05:01AM
mike34341 Registered Member #2656 Joined: Wed Jan 27 2010, 07:12PM
Location:
Posts: 2
Thanks for the reply. From what it looks like, the drain of the bottom left transistor is connected to the gate of the second transistor through a 10-ohm resistor. That doesn't look like an H-Bridge to me...it looks like the transistors are in series - one driving the other. That's what I'm trying to figure out - why is that setup like that?

I'm also curious what the purpose of the diodes are - especially the diode D2 between the drain of the bottom left MOSFET and the source of the one slightly above and to the right of it.

Thanks again,
Mike
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Turkey9
Thu Jan 28 2010, 05:58AM
Turkey9 Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
I think what's going on is that one pair of transistors is on when the tl494 output is low. The base is directly at the supply. Then the tl494 comes on and the first transistor is turned off by the second one in the pair. It acts like a pull down to the base of the first and also a sink for the current through the primary. So the current that usually turned on the first transistor is drawn away following the path of least resistance and to the collector of the bottom one and into the flyback primary. That looks like it's way too much trouble compared to a simple GDT driving an H-Bridge. After all that's what I think all those resistors and diodes are for... to isolate the bridge and use a discrete driver scheme instead of the GDT.
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