Alternative transistor to a 2n3055?

Discussion in 'General Electronics Chat' started by XOIIO, Mar 15, 2013.

  1. XOIIO

    Thread Starter Member

    Mar 1, 2013
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    Alright so I am making a flyback transformer driver, however, in the schematic it asks for a 2n3055 transistor. In some side notes the person said that instead they used a 2n6673 npn power transistor. I was wondering, would just any power transistor work? I have quite a few from power supplies I've been taking apart, can I use one of those?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    If it meets the voltage and current requirements for the flyback driver then it may. Try it and see. :)
     
  3. XOIIO

    Thread Starter Member

    Mar 1, 2013
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    Alright, mine are probably overspec compared to the one he used but I'll double check and see.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    If it's a flyback converter, switching speed and storage times are critical.
     
  6. takao21203

    Distinguished Member

    Apr 28, 2012
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    I don't think so if the schematic is using a 2n3055.

    would just any power transistor work?

    Actually yes. And these which don't work can be made working :)

    a 2n3055 switching circuit will be poor and very basic anyway. Most likely self-oscillating with a feedback coil. If it is using two of them, it is very likely a Royer circuit/Royer oscillator.

    http://en.wikipedia.org/wiki/Royer_oscillator

    For information have-not's, Wikipedia can be a good starting point. No offense to OP, but with the information that it is likely a Royer oscillator, you can look it up over there.
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    He said it's a flyback, did not furnish a schematic. The 2N3055 is a decent transistor, BW is typically about 3 MHz. It could be used for switching in the 50 kHz range.

    The 2N6673 is a 15 MHz device so it is superior to the 2N3055.
     
  8. takao21203

    Distinguished Member

    Apr 28, 2012
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    Yes but it is slow and low hFE. You need almost 1/10 current for the base. Think this comes from a feedback winding.

    When I built such circuits, I normally saw a few transistors would work somehow, some would not work at all, and one or two would work well (not too much heat, good eff.)

    If you don't draw much current, really most transistors will work, but if you crank it up to the margin of the transformer, you'd see differences.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Hate to break the news to you, but ALL bipolar transistors have a gain of 10 as you drive the collector current up towards the rating value.

    You said:

    You can't design circuits that way. You need to know what the max collector current is for the design, what base drive is available, and then select a device which meets those requirements.

    If you are using it in a switching circuit, you also need to know if it switches fast enough to minimize switching losses.

    As with every design: step #1 is always to figure out what it is supposed to do and what the specs are.
     
  10. takao21203

    Distinguished Member

    Apr 28, 2012
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    But if you aren't using it at the rated cuurent, a 2n3055 would be about the same as other power transistors or small signal BJTs?

    Can't do this, can't do that. A 2n3055 isn't so much used in newly profssionally engineered circuits now.

    I had a old Siemens board here from 1980, where it was used with a small cooling grid, and a LM723.

    Using transistors at the rated current for SMPS is never a good idea. They are prone to explode from inrush currents or overload.
     
  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    sure

    Then use what ever you want.

    If you can read a transistor data sheet, you should be able to pick a transistor.
     
  12. vk6zgo

    Active Member

    Jul 21, 2012
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    At a wild guess,if you can use a 2N3055,a more rugged device would be almost any BJT used as a Horizontal Output transistor in a TV set.

    A lot of these devices (H output transistors) are pretty much interchangeable.(Allowing for the higher powers needed for bigger tubes.)

    In the 27" Sony "Profeel" Monitors I used to fix,they used 2SC1413As (if I remember correctly) in both the Horizontal Output & SMPS circuits.
    When we ran out of them,we used Philips BU208Ds instead.

    Philips used them in the Horizontal Output circuits of their 27" TVs,so we knew they were up to the job.

    Of couse,the purists will go EEK! at that,but you try explaining to a TV
    Production guy why he can't have his big Monitor bacK!:D
     
  13. timescope

    Member

    Dec 14, 2011
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    I am wondering why no mention has been made of the high voltage specifications of transistors intended for horizontal output stages. For the BU508, Vces = 1500v, Vce0 = 700v and Veb0 = 10v. These high voltage ratings are required to handle the back emf of the transformer winding when the transistor is switched off producing the "flyback" of the electron beam and generating the eht. The base is usually driven by a small transformer hence the higher Veb0 rating.

    Timescope
     
  14. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The original post was vague:

    The term "flyback transformer driver" doesn't automatically mean a CRT drive application, it also means a transformer in aflyback switching converter. So, unless specified, could be either.

    The fact that the original part was a 2N3055, which is a relatively low voltage device, says to me it's not a CRT flyback application unless it's driving the primary side and has snubber clamps to protect the voltage.

    Anyway, the point was that the 2N6673 looks to me like a good replacement for the 2N3055.

    As to the question:

    Obviously NOT, you have to understand the critical requirements of the application which is impossible here because there is insufficient information.
     
  15. vk6zgo

    Active Member

    Jul 21, 2012
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    In my posting I made the following comment:-
    "In the 27" Sony "Profeel" Monitors I used to fix,they used 2SC1413As (if I remember correctly) in both the Horizontal Output & SMPS circuits."

    The SMPS in this particular Monitor also meets the description of a
    "flyback switching converter".
     
  16. takao21203

    Distinguished Member

    Apr 28, 2012
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    Depending on the input voltage and how the circuit is done, you can use ANY transistor for a voltage converter.

    You can consult the datasheet and write some pages formula but in many cases this will not be done.

    An existing circuit will be used, and substitutes which are available.

    Most of these circuits also originally only just somehow work.

    They are often not intended for max. efficiency.

    You'd sometimes see the coils are overdimensionated. Or even 50Hz transformers are used.
     
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