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Registered Member #1408
Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
In the various common "Primary-Feedback" LOPT (flyback) drivers I have used a great amount of transistors and also downloaded all sorts of data sheets. I have tried to determine what is the common thread that makes for a "good transistor" in these type of drivers and I really can't find an answer. The 2N3055 seems like crap but then again I have seen a driver with two of them work very well. I have tried a 2N3773 and gotten much better results than a MJ13009 which was rater at much higher levels almost across the spectrum. What I am asking is what is the determinate factor? What makes one preform better than another?
Is this a systemic issue that also includes the level of resistors and value of the caps; so that depending on what is contained in the circuit - there is no clear cut answer for a "better transistor"?
I just recently got hold of a MJ15003! This is a monster with a 20A rating, Power dissipation of 250w & a Vceo of 140v but yet it preformed little better than a 2N3773. Could someone explain what I would look for & why in transistors for this type of driver circuit?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Firstly you should get a transistor with ca. <250V rating, this is because the higher voltage ones are made using different technique (epitaxial VS triple diffused IIRC) and have different parameters.
The parameters you are looking for in this circuit are current gain (Hfe), turn-off delay time (storage time) and fall time (and of course sufficient current and voltage rating, Vcb should be at least 5x supply voltage (approx.)). A transistor with low Hfe, long Tdoff and Tfall is likely to get very hot or even fail to oscillate in the circuit (using homemade primary; this is the case of Horizontal Output Transistors or HOTs which are commonly used to drive the flyback in the TV, these will fail to work most of the times in the single transistor driver). If you want part numbers, KD60x or KD50x works amazingly, they work maybe 2x better than 2N3055 and get only half as hot! Sadly it is impossible to obtain these today.
The MJ15003 would be my choice (if I didnt have the transistors listed above ), make sure you got an original one as these are often faked.
Another option to increase power is to simply parallel the transistors, they will not get as hot and you get more power. Don't worry about current sharing; it simply works.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
If you use the normal primary winding of the transformer, the transistor must be rated at 1500 V of maximum vce and several A of maximum current. This is the kind of transistor normally used in CRT devices. A diode of similar ratings across the transistor is a good idea. If you wind a new primary, the required voltage may be lower, but the current will be higher.
Registered Member #1408
Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
Thank you all so very much for your insight. Let me elaborate on what made me confused in the first place....There was a web page wherein a fellow elaborated on what made for a good transistor:
In it, he described the BU208A (and D) transistor which is a Horizontal Output type and upon looking them up I noticed that they showed a low base voltage and current. Check the NTE page: It's listed as the NTE238: A Horizontal Output TO-3 type with VERY high Vcex... Up until that point I was convinced that ALL parameters needed to be high as in a MJ15003 as it simply made sense due to my (mis)conception that I would always be dealing with higher voltages and current. I also believed that they all needed to be simple specific switching transistors. Some say the Horizontal Output type are great / others do not share that idea....My own brief experience with them shows them to work but they appear delicate (but that's just one example that I tried).
Both his & your outlines made for a greater understanding of the dynamics of the transistor within the flyback driver elements. But brings up the most logical issue: If the parallel transistors work - -WHY the emphasis on a single? Why NOT leave those old but cheap two 2N3055's in place and be done with it?
Re: the MJ13009....it's performance was so dismal that I was almost sure what I received may have been faked! But it was from a reputable dealer....
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Antonio wrote ...
If you use the normal primary winding of the transformer, the transistor must be rated at 1500 V of maximum vce and several A of maximum current. This is the kind of transistor normally used in CRT devices. A diode of similar ratings across the transistor is a good idea. If you wind a new primary, the required voltage may be lower, but the current will be higher.
That is not totally true. I use an irfp460 and the supply voltage is 50 volts and i use the primary winding. The mosfet has never died being run this way.
Do NOT use the horizontal output transistor(HOT) Which it the big high voltage transistor usually rated at 1200 or above(from what i have seen) and a few amps. They will not work for the single transistor design. Contact Dr. Monsterarc For the reason. He knows all about it.
Registered Member #964
Joined: Wed Aug 22 2007, 12:39AM
Location: Stockton, CA
Posts: 134
Arcstarter wrote ...
Antonio wrote ...
If you use the normal primary winding of the transformer, the transistor must be rated at 1500 V of maximum vce and several A of maximum current. This is the kind of transistor normally used in CRT devices. A diode of similar ratings across the transistor is a good idea. If you wind a new primary, the required voltage may be lower, but the current will be higher.
That is not totally true. I use an irfp460 and the supply voltage is 50 volts and i use the primary winding. The mosfet has never died being run this way.
Do NOT use the horizontal output transistor(HOT) Which it the big high voltage transistor usually rated at 1200 or above(from what i have seen) and a few amps. They will not work for the single transistor design. Contact Dr. Monsterarc For the reason. He knows all about it.
Oh, that explains why my single-transistor didn;t work... Time to go get a stinking 2n3055 from radioshack..
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
Arcstarter wrote ...
Antonio wrote ...
If you use the normal primary winding of the transformer, the transistor must be rated at 1500 V of maximum vce and several A of maximum current. This is the kind of transistor normally used in CRT devices. A diode of similar ratings across the transistor is a good idea. If you wind a new primary, the required voltage may be lower, but the current will be higher.
That is not totally true. I use an irfp460 and the supply voltage is 50 volts and i use the primary winding. The mosfet has never died being run this way.
Do NOT use the horizontal output transistor(HOT) Which it the big high voltage transistor usually rated at 1200 or above(from what i have seen) and a few amps. They will not work for the single transistor design. Contact Dr. Monsterarc For the reason. He knows all about it.
The irfp460 is a very robust MOSFET, almost impossible to destroy with a simple flyback driver. Not a bipolar transistor. But it may get hot due to the relatively low 500 V maximum voltage. It has an avalanche diode across it, rated to limit the voltage in a safe way. The horizontal output transistors may require some changes in the design of the driver, due to the quite low current gain and are not direct replacements for a 3055, that has a higher current gain (an insufficient maximum voltage). For a driver with a separated oscillator, they are the right transistors to use, since they are made for this purpose.
Registered Member #1408
Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
I set up a very unique experiment with the highest value transistor I have available to me - a MJ15003 and several others, a 2N3773, a MJE13009, and a C4433 (NTE2308). I used the standard "capacitor, diode loop" driver. The arcs were VERY nearly the same! The input was 15vdc 3A. I altered this to 19vdc 3A and saw minor increases, etc; but they all showed that level of increase.
It was when I started changing the values of the resistors that I saw differences. I even altered the first cap value from 4000+ uf to as low as 1000uf & THAT did not make as serious a difference as the resistance values.
My conclusions were (thus far) that it's the systemic design of the circuit and not the transistor, per se' that determines the arc intensity.
I have the materials to make a Mazelli driver but I have read that the input demands 10 amps! At THAT level I can see that there would be a substantial difference no matter the driver!
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Antonio wrote ...
Arcstarter wrote ...
Antonio wrote ...
If you use the normal primary winding of the transformer, the transistor must be rated at 1500 V of maximum vce and several A of maximum current. This is the kind of transistor normally used in CRT devices. A diode of similar ratings across the transistor is a good idea. If you wind a new primary, the required voltage may be lower, but the current will be higher.
That is not totally true. I use an irfp460 and the supply voltage is 50 volts and i use the primary winding. The mosfet has never died being run this way.
Do NOT use the horizontal output transistor(HOT) Which it the big high voltage transistor usually rated at 1200 or above(from what i have seen) and a few amps. They will not work for the single transistor design. Contact Dr. Monsterarc For the reason. He knows all about it.
The irfp460 is a very robust MOSFET, almost impossible to destroy with a simple flyback driver. Not a bipolar transistor. But it may get hot due to the relatively low 500 V maximum voltage. It has an avalanche diode across it, rated to limit the voltage in a safe way. The horizontal output transistors may require some changes in the design of the driver, due to the quite low current gain and are not direct replacements for a 3055, that has a higher current gain (an insufficient maximum voltage). For a driver with a separated oscillator, they are the right transistors to use, since they are made for this purpose.
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Determining A Useful Transistor
), make sure you got an original one as these are often faked.
