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Problem with Flyback Driver

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Desmogod

Thu Mar 09 2006, 12:45AM

Registered Member #139 Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358

Mike wrote ...

Max frequency is 5khz@10V

Just had a quick scan through, and not sure if anyone else has said anything, but isn't 5Khz just a bit too slow for a flyback?
I would have thought more like 20-25Khz.
After you have sorted the circuit of course.

Mike

Thu Mar 09 2006, 02:15AM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

Sorry, If you saw one of my posts towards the middle you would have known I got it to working at 15-25khz output on pin 3 however Voltage is only 4v, and this is off a 12v Power Supply.
Im still having troubles getting the Transistor to run though. here is the 555 Schem I am using since i couldnt get the other going:

Desmogod

Thu Mar 09 2006, 03:20AM

Registered Member #139 Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358

Sorry mate, only had a quick scan. And you'll only ever get 5V out of a 555 if I'm correct.
Honestly, a ZVS or similiar would be a better option for a flyback driver, So many people have had drama's with 555 driven units. you would probably have more luck with a basic 2n3055 circuit, even if it is lossy.
Although if you are getting arcs out of the flyback, then it might just be freq adjustment needed.
Are you running your own primary winding, or using the potted one in the flyback.
you might want to try your own primary.

teravolt

Thu Mar 09 2006, 05:49AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

the standard 555 ocilator that you are using is normaly high at pin 3 and each pulse out is low. You want to invert to a low going to a high. After inversion you will need to drive your transistor. you can do this all with an inverting driver IC or transistors. when you do this you need about 330 ohms between your driver and the base of your transistor to limit the base current.

Hazmatt_(The Underdog)

Thu Mar 09 2006, 06:16AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Q: I substituted X for Y and it doesn't work ...what do I do?
A: don't substitute. Use the specs the designer gives you and it will work if the designer bothered to build it and test it.

Q: the 555 won't do the job
A: the 555 works great if you use it right.

Q: Why do my transistors get hot in my flyback circuit..I'm doing it all correct!
A: Because a flyback is not a 50% duty device. On time is very very short compared to off time. If you use my driver it works fine IF you make no substitutions AND if you adjust the on time to minimum and use the off time to tune to operating frequency. It is very simple.
Read up about mark-space generators and the 555, its all there for you.

Dr. Shark

Thu Mar 09 2006, 08:46AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

OK sorry, seems I stand corrected. What I was meaning to object to was the practice of driving a 2n3055 directly from the output of a NE555 or from a feedback winding - thats what seems to cause trouble for a lot of people (including me, in the old days).
Better transistors with more current gain seem to be very expensive, especially compared to the dropping prices of MOSFETs.
Thus, if you dont have a very good reason to do otherwise, I suggest using the current-fed parallel-resonant ZVS Flyback driver with MOSFETs - it seems to work _much_ better for most people.

Desmogod

Fri Mar 10 2006, 12:36AM

Registered Member #139 Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358

And the ZVS is a lot less lossy, and an altogrther easier circuit to construct.
5 minutes on a bit of vero...

Steve Conner

Fri Mar 10 2006, 01:37AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Mike: The 555 can't really output enough current to fully drive a big power transistor. I guess it is trying its hardest, but the heavy load of the transistor is dragging the 555's output voltage down and that's why you only see 4v. I have seen people add extra booster transistors to that circuit to really pound the main transistor with lots of base current.

Driving directly from the 555, it will probably carry on working OK, although the 555 might blow sooner than it should

Mike

Fri Mar 10 2006, 03:47AM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

I am not gonna bother with this circuit, i am using a tl494 /UCC halfbridge circuit now.
Thanks, Thread can be closed.

Marko

Sat Mar 11 2006, 06:13PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Well I designed a small circuit that can use bipolars, NPN or PNP and mosfets.
I used small transformer from ATX SMPS as GDT, driven by NE555 trough mosfet and push-pull preamplifier.

Driver is 2SB863, hifi PNP transistor that seemed to outpreform 2N3055 well, obivously because of higher amplification (lower base current needed for saturation).
From 20V supply I got some 3cm sparks, image is bad, camera doesnt like such closures.
I drew cm ruler on paper as it is not clearly visible.

cheers..

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