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BJT Transistors Question

Discussion in 'General Electronics Discussion' started by Carel, Jun 21, 2009.

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  1. Carel

    Carel

    11
    0
    Jun 21, 2009
    Ok, so I’m about to ask weird / odd questions randomly, but please, help is needed…

    Q1.) Does anyone know of a website with a search engine where I can just enter a Transistor part number & it will output a list of all the Equivalents, containing different brands, etc? Datasheet archive had this feature, but for some reason they took it away! So, where can I search for transistor equivalents now?

    Q2.) What EXACTLY is the “Collector Power Dissipation”, “Turn-On Time”, “Storage Time”, “Rise Time” & “Fall Time” of a NPN Power Transistor & what does it do & are higher or lower values better?

    Q3.) My Computer’s power supply had blown a long time ago, it had 2x “D4515” NPN Power Transistors & both of them have blown. These Transistors are rated 15A Continuous & 30A Pulsed Current with a voltage of 400V & Power Dissipation of 120W. So, I didn’t know what transistors to use in the place of these… Eventually I bought 2x “BUW13A” NPN Power Transistors, Rated 15A, >800V & 175W. I tested this monsterious PC of me at full load, pulling a Quad-Core Processor & a Radeon HD 4800 at full load. So, after about 2 minutes, the two “BUW13A” got damn hot. After another two minutes, one of them collapsed (short circuit, but didn’t explode). So, I eventually got 2x “2SC3320” Transistors from an old PSU. I’ve put them in & the PSU performed. Again, I pulled full load from the PSU & after about 5 minutes I felt that the Heatsinks wasn’t even warn, it was little lowerish. So, I left the PC on for more than 15 Minutes & after that I touched the sinks again & they were not nearly as hot as those 2x “BUW13”’s. In fact, those 2x 2SC3320’s temperature was totally acceptable, they were not warm, but only low. This tickled me, because why did the “BUW13A” get hotter than the “2SC3320”. Eventually I used a few of these 2SC3320’s in parallel & the PSU is able to handle an overload without blowing. Does this maybe have something to do with the Transistor’s other specs, except for the Amps, Volts & Pd? I’ve experimented a bit & found that some, even though they’ve got the same Voltage & Amp Rating, some transistors just simply do the job better than others, because some will get way much hotter than others, even if they are used in the exact same circuit, with the exact same load attached to them. Why?

    I even tested the reliability of the 2SC3320’s by putting them at overload in a PSU & by causing surges (to check how they’ll handle it), but they just seem to be the best in this circuit… Better than D4515 & BUW13.

    Please, I really want to understand these things better, any help would be appreciated. Question 1 & 2 are high priority...
     
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    I noticed you had this old unreplied-to post so I decided to bring it back up now that it's kind of quiet.

    I have noticed one of the datasheet providers have a cross-reference (or equivalent) button a bit down on their result pages. Search - and I'm sure you'll find something.

    Collector dissipation is the term used because that is the side of the chip that is bonded to the heatsink tab. It consists of c-e voltage * collector current + b-e voltage * base current.

    “Turn-On Time” is the time it takes from you apply base current and until collector actually conducts.
    “Storage Time” is the time it takes from you remove base current and until collector actually ceases to conduct. (The base kind of stores electrons.)
    “Rise Time” is the time it takes for collector voltage to rise (from 10 to 90%) once it has started to rise after removal of base current.
    “Fall Time" is the time it takes for collector voltage to fall (from 90 to 10%) once it has started to fall after application of base current.

    There are of course also other data crucial to performance.
    Safe Operating Area (SOA) which includes Secondary Breakdown. Often you cannot use the full collector dissipation at higher collector voltages.
    Gain, varying at different voltages & currents.
    Saturation Voltage. There's a big difference in dissipation between 1.5V and 0.15V.

    I'm guessing that Rise & Fall times and Saturation Voltage is what makes the 2SC's better than the BUW's.
    Scoping the waveforms can reveal this, if the datasheets doesn't.
     
  3. neon

    neon

    1,325
    0
    Oct 21, 2006
    The rise time is not when the transistor start to conduct but rather is the time that it takes to rreach saturation.
    the power dissipation will be greater during transition not at saturation status.
     
  4. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Whoever said that first part? Your definition is also incomplete, and wrong I believe.
    That will depend on transistor type, Vsat, switching speeds, switching frequency, duty-cycle, voltages, & currents in the actual circuit - so one can't make a blanket statement like that.
     
  5. chagz

    chagz

    7
    0
    Jan 4, 2010
    Try datasheetpro.com. It offers a few somewhat 'equivalent' parts. I think it's best if you'd try it for yourself.
     
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