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Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Dr. Duck wrote ...
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 curiosity?
I don't know much about them either, it is just what I have found works best for me when experimenting. Are you referring to the capacitors on the one end of the primary coil? And what do those diode's on the chip actually do?
I just used BD243B and BD244B's as that is the only ones I had.
The caps I have on the primary coil are not that physically big, they are some good Panasonic ones I got from farnell. 10,000uF 50V.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Nono, nothing is WRONG, you could've just saved some space and some components!
The UF4007 diodes are normally there to prevent back EMF spikes from killing the chip. Since discrete BJT's are a bit more hardy, they're not needed. And yes, the big 10,000 uF caps are not needed, just 1 uF caps would work; However, I wouldn't choose electrolytic ones. Anything but electrolytic/tantalum would work.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Dr. Duck wrote ...
Nono, nothing is WRONG, you could've just saved some space and some components!
The UF4007 diodes are normally there to prevent back EMF spikes from killing the chip. Since discrete BJT's are a bit more hardy, they're not needed. And yes, the big 10,000 uF caps are not needed, just 1 uF caps would work; However, I wouldn't choose electrolytic ones. Anything but electrolytic/tantalum would work.
careful here guys, the EMF explanation is only partly true, some pairs of diodes (schotky's) are there to stop the gate pulse from swinging negative ( I.E. 0.5v below ground) small short duration voltage dips for these PWM IC's can be fatal for the die. though ive never seen four diodes used for this purpose, only pairs. some circuits may be well behaved and never fail even without diodes, however i wouldnt chance it. with series inductance, stray capacitence and large swinging fast currents in those BJT's id be careful... or just use a GD IC, and you can skip many of these issues.
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Driving a gate drive transformer.
