Confused about transistor connections/heat sink

Discussion in 'General Electronics Chat' started by bowrider, Jun 14, 2013.

  1. bowrider

    Thread Starter New Member

    Mar 6, 2009
    2
    0
    It seems to me, most power transistors with a metal tab have the tab connected to the collector. What I don't understand is how the transistor is intended to be used. For example, I want to break the ground connection to an inductive load with the load connected to the collector and the emitter connected to ground. Nothing new, see it done everywhere, and it works, but I have to insulate either my heatsink from ground, or the transistor from the heatsink. It seems it would be so much simpler if the tab were the emitter and I could bolt it directly to a large structural ground/heatsink, but from what I can see transistors are not made that way. Why not, and what am I missing about the use and connection of transistors and heat sinks? I know this is really a beginner question, which I am, but I have a real mental block about this so please educate me. Thank you
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    You can get isolated tab transistors.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    It's due to the way transistors are constructed. For the most best transistor performance it happens that the substrate of the chip becomes the collector. It would be possible to build a transistor with the emitter being substrate instead, but its performance would be much poorer. Thus when the chip is soldered to the case. the case becomes the collector connection. So you either need to isolate the transistor or heat sink, or use an isolated tab transistor as Dodgydave suggested.
     
  4. bowrider

    Thread Starter New Member

    Mar 6, 2009
    2
    0
    Thank you for the explanations. I was aware insulated tabs were available, so I guess my next question is "Would an insulted tab package or a mica/film pad give better thermal transfer to the heat sink?
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    I would guess the insulated tab would be better, but you'd have to compare the relative thermal resistance from the thermal tab transistor data sheet you choose, compared with the combined (sum) thermal resistance of a regular transistor and the mica/film pad thermal resistance.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    In general proper heatsink design is more important than the thermal connection between the semiconductor (thyristor, mosfet, transistor etc) and the heatsink.

    That is the cooling environment, sizing orientation and so on.
     
Loading...