Re: bjts and power disapation

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jan Wagner <jwagner@xxxxxxxxx>


On Wed, 19 Jan 2005, Tesla list wrote:
> Original poster: "Anthony" <ant17@xxxxxxxxxxxxxxxx>
> hi guys i was hoping someone might be able to clear something up for me 
> with bjts is it the input power thats disipating that have watch and not 
> go over the power rating device or is it output power that you have to 
> watch out for if you could help clear this up i would be greatfull

Power dissipated _in_ that device. Which you can estimate as Pd=Vce*Ic.

Vce = voltage accross collector and emitter (or if used as a switch then 
Vce_sat which you can find in the datasheet, could be 1..2 volts), and Ic = 
collector current. If it's a high power BJT you might also add the drive 
dissipation, base current Ib times Vbe base emitter voltage (see datasheet).

Additionally, watch out that you don't exceed the maximum average collector 
current, max base current, and maximum voltages (breakdown voltages). And 
whether the maximumg dissipation specs in the datasheet are specified for 
the transistor without a heatsink, or with a heatsink. You'll need to a add 
a heatsink, anyway ;-)

cheers,
 - Jan