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Thursday, July 07, 2016

Understanding Transistor Power Dissipation

I like your page about current source circuit basic design. However, I still have Little confuse of the power dissipation calculation.

The max power dissipation of my circuit is 20.5V x 0.5A = 10.25W, but transistors I found are 2W power dissipation. But my input voltages is just varied from 18.0V to 20.5V only. Do you think the 2W power dissipation transistor is fine? Any help will be appreciated.

Mail From MT

When the Current source output is short circuited or connected to a low impedance load say 10 ohms, the full voltage of input 20V falls across the transistor but current is limited to 0.5A. which means the 2W transistor may Roast. The transistor when conducting, the watts is the volts across it and current thru it. Not your supply Voltage.

Power Electronics & Solar Energy

HeatSinks in Electronics

Power Supplies Section

Design in such a way there is no short circuit or a a power resistor in source. But then efficiency drops. The best way is to use a switching supply design or keep the input voltage across transistor to a minimum.