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Transistors

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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.:)
 

MikeMl

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Power transistors like you are describing depend on being mounted to their heatsink with the lowest possible thermal resistance. Did you use clean thermal compound, the right mica washers, torque the mounting screws to the recommended torque, etc?

Heat is what kills transistors. Not being able to conduct the heat out of a high-power transistor causes thermal runaway, and is the root cause of failures in power supplies, amplifiers, etc.

I have an printed copy of an old ECG Semiconductor Master Replacement Guide, which has a useful cross reference. i dont know if it is on line somewhere.
 
Yes, the Transistors were properly mounted to the heatsinks, but this wouldn't cause them to dissipate more / less heat? The thermal compound I used were the same as what I've used on my Processor. Yes, they were new.

The original D4515's blew because of a surge which occured the moment I switched on the PSU. This is of course a switching power supply, switching at very high frequencies for lots of output amps, using BJT's. Because it's a Computer ATX PSU, it does have a fan inside, blowing directly on the heatsinks of the transistors, but with the BUW13's, the fan didn't even help cooling the heatsink down, as they dissipated so much heat that the heatsinks was damn hot... The 2SC3320's doesn't get nearly as hot as they did, even without the fan... With the 2SC3320's the PSU was also more energy efficient.
 

MikeMl

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Most Helpful Member
Yes, the Transistors were properly mounted to the heatsinks, but this wouldn't cause them to dissipate more / less heat? ...
No, but if the heat cannot be conducted out of the transistor package to the underlying heatsink, the transistor junction gets hotter than normal. It is usual for the transistor junction to be operated at up to 85 deg C. A slight increase in thermal resistance between the case and the heat sink can send the transistor over the edge.
 
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