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Single transistor flyback driver

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Marko

Sun Mar 12 2006, 01:57PM

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

I did few 'tests' yesterday, as I saw some of people talking about how bad are bipolar transistors in use for flyback driving, or they yust cannot mach mosfets, ZVS, etc.
My idea was to use single bipolar transistor (the actual flyback circuit) to drive a flyback transformer. And I got some very interesting results.

NE555 drives push-pull to gate of a 50N06 mosfet, and it drives the base drive transformer.
It is actually a small transformer taken from old ATX SMPS supply, where it served as standby SMPS.
Mosfet pulses the primary from 12V supply and I get high-current, around 3 volts on secondary.
Fed to base of bipolar transistor it makes very good drive signal.
Driver transistor used is 2SB863 (PNP, NPN can also be used).
Duty cycle is kept very low to minimize losses, but it can be controlled with a potentiometer to get more power output.

Firstly I was using few turns of my wound primary, and got around 1,5cm sprak, then I connected flyback's internal primary and got large 3cm spark, power supply was only 20 volt.
When increased to 40 my supply saturated and voltage dropped to 28V, but arc got 4,5cm maximum.

Even then transistor keeps at abut 50 degrees on small heatsink, after long operation it can still be touched.
If I increase duty cycle I get more power but then mosfet and driver transistor heat up considerably.

I worked from my head so there is no schematic yet, and circuit will probably be retouched actual goal is to run it efficiently from 300VDC (probably flyback won't survive it

)

Pictures:

]1142171872_89_FT0_flyback.zip[/file]

uzzors2k

Sun Mar 12 2006, 07:53PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Nice. This is what I was trying to achieve, except you didn't fail.

I'd like to see how you get it working at 300 volts!

Oh and btw, I gave up on trying my single transistor design, last test was a few weeks ago. I was running it with a 60W light bulb, but I just got a firey spark from the transistor and then things just stopped. I've decided to try a half-bridge instead.

Dr. Dark Current

Sun Mar 12 2006, 08:07PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I have a flyback driver with single FET + TL494. It can make about 30kV from an old 8kVac (was used with tripler) flyback with just 20Watts in. I won't be surprised if it makes 6-8cm sparks from new DC flyback. Will try this soon (flyback hunting is planned on the next week

)
BTW: the fet stays super cool on medium sized heatsink. What I have found is that if you put a small (4.7nF) cap across the fet D-S it decreases the FET heating.

Marko

Sun Mar 12 2006, 08:17PM

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

You must feed the flyback with extremely low duty cycle, I have potentiometer so i can control it (and overall input power).
Also you must care not to saturate it at that voltages.

On low voltages its impossible to blow the transistor even on high cycles, but Il have to be more careful at 300V.

I have some BUW13A that could do there.
For now this is the schematic:

1142183689 Flyback Driver

I have a flyback driver with single FET + TL494. It can make about 30kV from an old 8kVac (was used with tripler) flyback with just 20Watts in. I won't be surprised if it makes 6-8cm sparks from new DC flyback. Will try this soon (flyback hunting is planned on the next week )
BTW: the fet stays super cool on medium sized heatsink. What I have found is that if you put a small (4.7nF) cap across the fet D-S it decreases the FET heating.

How did you drive the mosfet?
What voltage were you using, I also tried MOSFET driver but it heated far more than bipolar does now.

I'm having problems with driving BDT (mosfet push-pull driver overheats) at higher duty cycles, so im limited there.

That mosfet needs to be replaced with some better kind of base drive...

Dr. Dark Current

Sun Mar 12 2006, 08:51PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Firkragg wrote ...

How did you drive the mosfet?
What voltage were you using, I also tried MOSFET driver but it heated far more than bipolar does now.

I'm having problems with driving BDT (mosfet push-pull driver overheats) at higher duty cycles, so im limited there.

That mosfet needs to be replaced with some better kind of base drive...

The FET was driven directly from the outputs (paralleled) of the 494 with emitters to ground and collectors connected to FET gate and to 200ohm pullup resistor. Voltage is around 60V but drops to 50V under load. Supply for 494 is around 20V. Duty cycle is usually in the range of 20-35%. Try using the cap as I suggested, try a bit larger one if it doesn't help much.

EDIT: Number of primary turns was 15. I have a schematic on my PC but not on my notebook I'm using right now, if you want I can post it tomorrow.

Marko

Sun Mar 12 2006, 09:00PM

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

Poblem is with bipolar transistors that they heat up if base current is not high enough, and when we feed enough base current for saturation then it heats them up.
I used low voltage, high curent output from transformer to help here, and now driver heats much lesser than mosfet did.

I tried using that same mosfet (50N06) as driver, it gave good output (some 2cm sparks) but it dissiapted a lot of heat.
Snubber can help but bipolars still seem to win in high-current aplications.

Now im going to use the circuit for 300V, protect the transistor well and it should be OK (circuit is isolated, no more damage can be done).

Im actually more afraid to blow the flyback thathe transistor...

Dr. Dark Current

Sun Mar 12 2006, 09:07PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

If I get it right: You are trying to drive a flyback with a 2SB863 PNP Transistor, rated at 140Volts, with 300V supply voltage?

If so, then this is probably not a way to go...
IF you are going to use different transistor, it WILL see spikes well over 1kV with this supply voltage, so very good snubber is a must. I think you will never get that type of circuit work properly at 300V.

Marko

Sun Mar 12 2006, 09:09PM

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

I have some BUW13A that could do there.

Quoting myself.

Dr. Dark Current

Sun Mar 12 2006, 09:17PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Firkragg wrote ...

I have some BUW13A that could do there.

Quoting myself.

OK, sorry, I didn't see it, but anyways this transistor is rated only 1kV, so it will be a real challenge getting it to work properly.

...

Sun Mar 12 2006, 09:28PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

considering that most people get it working with a 2n3055 at 100v I would say you should be fine...

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