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Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Which means your mosfets will be driven with +/- 8 volts. If that is under the saturation voltage of your mosfets you'll be driving them in teh linear range, which means alot of heat dissipation. When using push-pull output stages most people use a 1:2:2 ratio, or doubled supply voltage. Also put some small resistors on the gates to prevent LC oscillations. (Small high frequency LC oscillations between gate C and gdt L)
Registered Member #522
Joined: Thu Feb 15 2007, 01:04AM
Location: The Netherlands
Posts: 33
Well, i forget that the gdt was 1 tot 1:1 wind .. So i rewind the gdt to 1:2:2 en set the voltage on the chip to 18 volts. I just want a litle fun from this thing, i now it is far from perfect. But it is perfect to learn, how i have the use my first o-scope! So when i set the voltage on the fets to 40v ,the output will then swing +/- 20v correct? Can i then drive a big fet like a irfp460 direct from the output of this ugly bridge? or sould i use a gdt with a cap between it?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I don't think you are going to do much with UC38**. You need to manually set duty cycle to 50%, somehow mimic current mode operation and have no deadtime.
Since you are actually using halfbridge, GDT primary sees 1/2 supply voltage so 1:2:2 is fine. Your idea about 40V on mosfet gate is wrong, you will blow it that way. Voltage on gate can make 40V swing but never over 20V in any direction, and since AC is always spoken about as effective value it's said that maximum gate voltage is 20V.
Don't use UC3842, switch to SG3525 instead. You'l be much happier.
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
You can DC restore the gate drive signals with a coupling capacitor and a clamp diode which removes the 50% duty cycle problem.
As for mimicing current mode operation, you can inject a portion of the ramp into the Isense pin - there are application notes on how to do this in the 3842 datasheet.
I would be inclined to use a TL494 or a 3525 instead to save you sooooooo much work
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low budget low voltage uc3842 h-bridge
