What Is a Power Transistor?
A power transistor is a transistor designed to switch or amplify high current and power (it can be a BJT, MOSFET or IGBT).
What Does It Mean?
While ordinary signal transistors work at the milliamp level, power transistors are built to carry current on the order of amperes and tens of watts of power, and come in large metal-body packages.
Package and Cooling
| Package | Feature |
|---|---|
| TO-220 | Common, medium-high power |
| TO-247 | Higher power |
| TO-3 / D2PAK | Very high power / SMD |
Most power transistors are mounted on a heat sink to dump the heat.
Power Loss
Conduction resistance and switching produce heat in the transistor. The junction temperature must not exceed the datasheet limit, so the power loss is calculated and cooling chosen accordingly.
Where Is It Used?
Power supplies, motor drives, audio power stages, inverters and high-current load switching.
Safe Operating Area (SOA)
A power transistor can't be exposed to high voltage and high current at the same time; the SOA (Safe Operating Area) graph in the datasheet shows this limit. Going outside this area destroys the transistor suddenly, even if cooling is adequate.
Common Mistakes
- Using it without a heat sink: at high power it quickly overheats and fails.
- Forgetting the insulating washer: the body is usually tied to one terminal; it must not be mounted to the heat sink without insulation.
- Exceeding the SOA: if the safe operating area (current+voltage limit) is exceeded, the transistor breaks down.
Frequently Asked Questions
Why does a power transistor have a large metal body?+
Which type should I choose?+
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