Motherboard power transistor?

I am working on a computer problem.  I have found a broken transistor on the motherboard.  It is a large TO-220 transistor rated at 10A.

My question is what is a large TO-220 transistor being used for on a motherboard?  

Hopefully the answer to this will help me to better understand why this thing has blown.

Karl
KarlAiblingAsked:
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RoadWarriorCommented:
Are you sure it's not a voltage regulator?
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joe_hCommented:
Probably the main transistor in a switching power supply (voltage converter) - provided it is a transistor. Any markings on it?

Modern processors consume lots of power (around 10-30W, in peak load even more), and their core voltage is often not supplied by the PC power supply. So, the motherboard needs to have a voltage regulation circuit on it that makes the required voltage. On cheap boards, this circuit is not over-rated, and often "barely makes it" in terms of its maximum power rating compared to the power needed by the processor. Sometimes, it just dies.. (My MB died on me this way.)
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jhanceCommented:
What's the part number on it?  If it's a transistor the number may be something like "2Nxxxx" or "2SCxxxx".  If it's another type of device, regulator, MOSFET, PWM, it would have a different type of number.  Perhaps a number like "LMxxx" or "Sxxx".

Chances are, however, that it's a standard part and a replacement can be found from the usual sources:

MCM Electronics
DigiKey
Jameco
JDR

or sometimes even Radio Shack.

BTW, how do you know it's rated at 10A?
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KarlAiblingAuthor Commented:
RoadWarrior:  It has device number on it.  It is a National Semiconductor (now a Fairchild Semiconductor product), number D45H2A.

Although it is a PNP transistor, it COULD be wired as a voltage regulator, although I can't find any zener diodes to control the base voltage.

joe h:  A computer power supply is self contained.  However, the second part of your answer is probably closer to the correct answer.  

ihance: See above.  A replacement part should be forth coming.  I also cross referenced it to an NTE transistor that might work, except it has a lower forward gain rating than the D45H2A.  (100 for the D45 and 60 for the NTE.  

I am rejecting all answers because I need to know what a power transistor would be used for on a motherboard.  The question is still open for others to comment on.

Thanks gentlemen.

Karl
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jhanceCommented:
A computer power supply is indeed self contained BUT the motherboard may need a voltage other than one supplied by a standard power supply.  For example, it's not uncommon these days to require 3.3V at high current to power the CPU and memory.  An AT-type power supply supplies only -12, -5, +5, and +12 volts.  If there is a need for 3.3V, it get regulated down (usually) from the +5V supply.

The Pentium CPU VRM (Voltage Regulator Module) is in fact just such a circuit.  It's a programmable switching power supply that takes the +5V and generates an output voltage commanded by the Pentium chip itself.
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joe_hCommented:
Well, in my previous comment, there is a slight confusion in terminology:

PC power supply = the metal box in the case that converts 230V/110V AC to +5V, -5V, +12V, -12V (all DC) and sometimes (ATX) +3.3V.

power supply (or voltage converter) = any circuit that converts input voltage (either AC or DC) to a different output voltage.

The motherboard needs to convert the 5V or 3.3V coming from the PC power supply to 1.something V -  3.something V, as required by the processor. (For example, AMD K6-2 requires 2.2 or 2.4V for its core, and this voltage is not provided by the PC power supply). As I said before, theprocessor tends to consume lots of power, thus a power transistor is needed at the output of such a voltage conversion circuit - an integrated regulator usually can't handle such a large current (e.g. AMD K6 specifications require 7.5A) by itself.

BTW, you don't need a Zener diode for voltage reference. Most likely, you will find nearby an IC that controls the voltage converter and has a voltage reference inside it. You should also find an inductor (a coil) with torroid-type core (unless your motherboard uses a switched-capacitor design, which I seriously doubt). In any case you will find quite big capacitors near the transistor.
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KarlAiblingAuthor Commented:
Joe h:

I find your answer and explaination quite adequate!  Thank you.  The transistor has a 470 ufd electrolytic cap nearby.  I don't think 470 ufd is very large but really don't know how it is used.  However, I do know that the core voltage of a CPU is usually smaller than the +5V supplied by the main power supply.  My hope now is that the CPU isn't gone.  This transistor burned up the first time the board was powered up and the distributor replaced it.  Now here it is again.  However, I did find the heat sink very loose on the transistor.  I will correct that!

I hance:

You are in the same ball park as Joe h but his detailed explanation gets him the cigar.  Thank you for your input.  Good answer.

Karl
 

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