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Improved single transistor flyback driver

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Antonio

Wed May 14 2008, 01:04AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

About high-voltage transistors, in all the monitors that I have seen the flyback is driven by a transistor rated at 1500 V, several A. Just find the transistor connected to the flyback primary winding where you found the flyback. Take also the diode also rated at 1500 V that is in parallel with it, and the high voltage capacitor (several nF, 1600 V) that is usually there too.

quicksilver

Wed May 14 2008, 03:12PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

I could be wrong about this but the transistor that you are taking about IS rated at 1500v but at the low current (ma) level level. I was looking up a great many transistors on my day off and I am very well acquainted with ripping guts from old stuff and re-using it. Unfortunately the transistor (equivalent to a C4634) is a 1500v but low current according to data sheet. You may have gotten lucky; but I'd check again because the high voltage/low current part is common.

However if you have a monitor that you want to use for parts there is a large fellow that's 400v / 12A & it's the C4423. There are also a few MJE13005, MJE13007 & 9's available. Those fellows are common.

I'm of the opinion that there are so many vast differences in flybacks & their construction that not one transistor or methodology is the answer. However I've found some commonalities..... If you use well made, evenly spaced coils (even and well constructed) with a thicker well insulated wire, you may have better arcs. The idea of an 8/4 winding is what I have had success with also. BUT, I've also had fantastic success with a weird 10/3 winding that if I changed it....but fall to sh*t....

The best transistor for a variety of transformers that I have personally used (over a broad spectrum of flybacks) is the 2N3773. Best performance over a broader range.....I have ripped apart a very large amount of stuff; for a really large selection of parts. Another great source for power transistors are larger, broken amplifiers for home stereos, musical instruments, etc.

I get a monitor (or TV or MO - whatever) and really pull every part off that bastard. Using the traces and looking closely at the circuit board helps a lot in understanding a great many things! THEN finding out what I've got in the way of transistors becomes a real challenge. Because a transistor MAY be used for horizontal diffusion in a TV (let's say) but it could still be used in HV applications....IF you check the specs and don't simply make a generalization based on superficialities.

It's been a good learning process for me. The parts are there! There are very few things that need to be bought. But I am beginning to see th value in a big breadboard.

dingo27

Wed May 14 2008, 03:59PM

Registered Member #890 Joined: Tue Jul 10 2007, 10:06PM
Location: Slovakia
Posts: 180

that diode you are talking about is BY329 series as i writed few posts sooner. Usually looks like TO220 but only with 2 legs...

that diode is perfect for most applications

Mads Barnkob

Wed May 14 2008, 05:23PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Another thought on optimization.

I used 0,75 mm^2 (awg 18-19) pvc insulated wire to wind primary and feedback, would it be better to use heavy enamaled magnet wire?

I have now written about my version of it and pictures can be seen at http://kaizerpowerelectronics.dk/high-voltage/2n3055-flyback-driver/

for now its in danish till I have time to translate it, but there is still pictures to enjoy

Antonio

Thu May 15 2008, 12:09AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

This is the biggest flyback driver transistor that I have, found in a monitor:

Another, less powerful, but easier to find. It has many equivalents:

Transistors are really limited to low currents, if they are sustaining high voltage at the same time and for long periods. Power dissipation limits this. But in these flyback drivers they operate ideally as switches, with high current at low voltage or no current at high voltage, and only at switching instants there is significant power dissipation.
Another useful component that you can find in monitors are switching power supply controllers. Some are very simple to use, and are more flexible that the 555s that people is using to control the drivers. An example:

quicksilver

Thu May 15 2008, 05:11AM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

@MadsKaizer: I've had VERY good responses w/ the insulated 18. In fact, that's all I use now. I can honestly tell a difference between that and the enameled thinner magnet material. But what I've found very important is that simply by the insulation, my coils are tighter and even and that may be a serious issue (IMO). In addition, if the core has sharp corners, it can shave off the enamel.

@ Antonio: Posting what you have found is a GREAT help. The basic problem is that often (depending upon the year of manufacture and maker) the nomenclature (naming conventions) can differ. But generally a Cxxxx is common enough to get most folks close. Good stuff!

dingo27

Fri May 16 2008, 07:35PM

Registered Member #890 Joined: Tue Jul 10 2007, 10:06PM
Location: Slovakia
Posts: 180

whoah, i tried thet schematics with diode and cap. and what i found is, that when i use that, i blow every 2n3055 in it. I blowed 2 and don't want to blow another... What capacitance i should use?

aaah... i was really angry for that, i trashed circuit board to wall...

it was working once, few seconds...

Dr. Dark Current

Fri May 16 2008, 07:44PM

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

dingo27 wrote ...

whoah, i tried thet schematics with diode and cap. and what i found is, that when i use that, i blow every 2n3055 in it. I blowed 2 and don't want to blow another... What capacitance i should use?

aaah... i was really angry for that, i trashed circuit board to wall...

it was working once, few seconds...

If you mean the resonant capacitor, it should be between 200nF to 500nF (approx.) and it MUST be non-polarised film or foil type, (PP/MKP type preferred).

-try to start with lower voltages and if nothing happens, reverse polarity of one of the coils.

dingo27

Sat May 17 2008, 09:18AM

Registered Member #890 Joined: Tue Jul 10 2007, 10:06PM
Location: Slovakia
Posts: 180

it worked, it worked... i used 0.47uF (470n, right?) foil from power supply clearing circuit...

but nothing with it was working as without it. Maybe i am using BAD transistor, because first manufacturer was (i dunno name, some stupid logo i newer saw yet... maybe china stuff), but as i am using ones with ISC on it, i burn them as with ot without C and D.

Dont buy 2n3055 with ISC on it! next time i will request another manufacturer from local radio shack.

Berni

Sat May 17 2008, 10:23AM

Registered Member #1132 Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84

I once got my hands on some fake 2N3055.

They could handle as much power as a signal transistor.Then i grinded it open and saw a tiny die in there that was coverd in some white stuff.These fake trasistor had its partnumber printed on it in red, then found out all these red 2N3055 are fake

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