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### Author Topic: Choosing a power transistor  (Read 2647 times)

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#### sgking2

• Guest
##### Choosing a power transistor
« on: January 27, 2012, 18:15:57 PM »
What is the best logic to use in picking a power transistor?
- Choose the one closest to your actual switching current capacity needs
- Choose one that can dissipate the adequate power, or 2x or 5x expected power
- Choose on adequate hFE gain or maximum possible gain

For example. When having a 12V voltage divider with whatever power we need to drive a switching power transistor as emitter-follower to power a 12V 400ma fan (tracking the input voltage so the fan sees the same voltage with with much higher current possible), with a minimum voltage of 6V (2.4W dissipated).

- You could use a TIP31A with ratings: 3A for Ic,  10hFE min,  40W max, or
- or choose a more robust TIP142:      10A          1000 hFE    125W

1. It seems that the 31 meets the requirements, but at what cost? Just a larger Ibe current from the source? Maybe 40ma max?
2. the 142 is obviously overkill, but at what cost? a larger Vce voltage drop (darlington), but what else?

#### kam

• Hero Member
• Posts: 1849
##### Re: Choosing a power transistor
« Reply #1 on: January 28, 2012, 09:43:16 AM »
damn, i wrote the answer and lost it.... here we go again...

first, you begin with the application type. You know your input and output requirements, so you can choose the type (darlington or not), NPN or PNP and you can get an estimation of hfe.

then you choose the proper one for power dissipation, ic and vce. These are absolute limiting factors that you must not exceed. Regarding the power dissipation, some people do the mistake to think that if they need 5W of dissipation, then getting a 50W transistor will save them from using a heatsink. This is not true though. Each package type has maximum heatsink capability regardless of the dissipation capability of the collector junction.  So choose according to your maximum current, maximum voltage operation and power dissipation.

Finally, it comes down to cost. you may notice that there are small transistors (100mA) that cost much more than big transistors (10A). this is some times normal, because there are transistors that are sold in large quantities so the shop can order them in large shipments, and also there are transistors for hifi applications with special characteristics and they cost more.

Check out the attached document, it might help.

BTW: the 0,7 voltage drop is not due to vce but due to vbe. if this is a big problem for your application you may wanna go to germanium transistor which have as low as 0.3v voltage drop (0.5 for darlington) but i do not know where you will find them and at what cost.

Basically, all it takes to choose the proper transistor is experience. At the end you will come up with some 10-20 transistor types that you work, and they cover 95% of your projects... Until then, follow the guidelines above.

#### sgking2

• Guest
##### Re: Choosing a power transistor
« Reply #2 on: January 29, 2012, 12:14:49 PM »
Thanks for the spreadsheet, awesome reference table to help group eligible parts for any project.

Two more questions
1. what formula to use to make sure enough current gets to the base of a TIP142 to properly make it track the input voltage with 400ma of current?

2. Coming out of a comparator, to drive several relays, for FULL ON/OFF action, should I just use another TIP142 or a MOSFET? I think the comparator has Open Collector and can sink about 5-15ma. Do you have any basic MOSFET switch circuits already displayed, triggered from a comparator or opamp?