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Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
The 555 version is better because a lot more of the power goes though the flyback than being dissipated out though the transistors. The 555 version still isn't the best because the flyback doesn't run in flyback mode with that driver.
Registered Member #289
Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154
Assuming the 4049 is a CD4049, and you have 24vdc going in, I would replace the 7805 with a 7815, This improves the circuits signal to noise ratio on the rails and will likely make it more reliable. This is not an ideal circuit...
Registered Member #1889
Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55
@CT2
So would it be possible to use the MOSFET I linked to as a replacement for the IGBT? Because it seems like it is more than capable of doing the same job.
@hvguy
Yeah I got the same impression that this circuit isn't quite designed in the best way possible...I mean instead of using 24v input for everything and regulating it to 5v, I would have just used a small 24v/5v step down transformer with 2 small bridge rectifiers but maybe that's just me. And I love how there's no true isolatio between the IC and the rest of the circuit. I build all of my circuits with full IC isolation because especially when working with TTL they get really picky (yes i know the 4049 is CMOS). And wouldn't a 4069 be a little bit better instead of a 4049? The 4069 I thought had a far more sensitive gate input voltage thresholds, the 4049 seems to lag and even miss pulses and high operating frequencies in my experience but maybe i'm full of crap.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Myke wrote ...
The 555 version is better because a lot more of the power goes though the flyback than being dissipated out though the transistors. The 555 version still isn't the best because the flyback doesn't run in flyback mode with that driver.
It does run the flyback in flyback ( ) mode, but still a lot of power is dissipated on the MOSFET because of switching and avalanche losses. Adding a capacitor across the FET fixes these problems, but introduces new ones - the transistor can be damaged by the current spike on turn-on. The best fix for this is to add a little inductor and two diodes, like the C) schematic in this picture (hope you can load it:)
As for the "kit driver", CT2 is really right in that 5V is enough to turn that IGBT on. If you use a different one or a MOSFET however, you should put 15 volts on the gate.
Registered Member #180
Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187
I don't really think the MOSFET you linked is that good. The Rds on is very high (1.35 ohms), and the gate voltage needed to drive it cannot be had from the circuit you built (as is anyways). Look at the graphs on page 3, they will explain it all. At 5V on the gate you will barely be able to get 1-2 amps out of it, and because of the high Rds on you will have lots of power being dissapated by the MOSFET (if you were able to push some current through it). I don't think the 4049 will be supplying the full 5V output when turned on so you will be getting even less. If you want to go this route your going to need to redesign the circuit, but I don't know, you bought the kit so maybe you'd be better off figuring out the problem and fixing it. Have you made absolutely sure the IGBT is toast? Is the 555 outputting a proper waveform?
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Flyback Driver Kit Help
) mode, but still a lot of power is dissipated on the MOSFET because of switching and avalanche losses. Adding a capacitor across the FET fixes these problems, but introduces new ones - the transistor can be damaged by the current spike on turn-on. The best fix for this is to add a little inductor and two diodes, like the C) schematic in this picture (hope you can load it:) 
