Power switches

The idea

To control a motor you need a switch that turns power on and off, often thousands of times a second. A relay does it mechanically. Transistors do it electronically: a small control signal on one leg lets a large current flow through the other two. The key difference between transistor types is what the control signal is, a current or a voltage.

The transistors

BJT (bipolar junction transistor): current-controlled. Legs: base (B), collector (C), emitter (E). A small base current $I_B$ lets a large collector current flow, roughly $I_C=\beta I_B$. The drawback is that the base needs current all the time it’s on, which wastes power. NPN and PNP types are biased opposite ways round.

MOSFET: voltage-controlled. Legs: gate (G), drain (D), source (S). The gate is insulated from the channel by a thin layer of silicon dioxide, so almost no gate current flows. Raise the gate–source voltage $V_{GS}$ above the threshold $V_{GS(th)}$ and current flows drain → source. Fully on, it behaves like a small resistor, $R_{DS(on)}$.

IGBT (insulated gate bipolar transistor): a hybrid. It has a MOSFET’s insulated, voltage-driven gate and a BJT’s high-power output stage. Same structure as a MOSFET plus an extra P layer.

Comparison

Switch	Controlled by	Strengths	Weaknesses	Typical use
Relay	Coil current moves a contact	Cheap, takes surges, true isolation, cool, several poles	Slow, wears out, sparks, vibration-sensitive	Occasional mains switching
SSR (solid-state relay)	Small input signal	Long life, fast, silent, no arcing, tiny control power	Gets hot, costs more, leaks a little current, surge-sensitive	Frequent switching, heaters
BJT	Base current	Simple, cheap	Needs continuous drive current, slower	Small signals
MOSFET	Gate voltage	Very fast, easy to drive	$R_{DS(on)}$ rises in high-voltage parts	Robots and motor drives, below ~700 V
IGBT	Gate voltage	High voltage and current	Slower switching	Above ~1000 V: trains, grid, large inverters

Rule of thumb

MOSFET below about 700 V, IGBT above about 1000 V. In between, pick whichever best fits the size, efficiency and cost.

Reading a MOSFET datasheet

Value	Section	What it tells you	IRFP140	IRFP250
$V_{DSS}$	Absolute max	Most voltage it can block when off	100 V	200 V
$I_D$	Absolute max	Most continuous current (falls when hot)	31 A	30 A
$R_{DS(on)}$	Static	Resistance when fully on, for conduction loss	0.077 Ω	0.075 Ω
$t_{d(on)},t_r$	Dynamic	Turn-on delay and rise time	11, 44 ns	14, 43 ns
$t_{d(off)},t_f$	Dynamic	Turn-off delay and fall time	53, 43 ns	41, 33 ns
$R_{thJC},R_{thCS},R_{thJA}$	Thermal	For heatsink sizing	0.83, 0.24, 40 °C/W	0.7, 0.24, 40 °C/W

Check yourself

1. Why does a MOSFET need almost no drive power once it's on?

Its gate is insulated, so it only needs a voltage, with no continuous current. A BJT’s base needs current the whole time.

2. You need to switch 48 V at 20 A, 20 000 times a second. Relay, BJT, MOSFET or IGBT?

A MOSFET: low voltage, fast switching. A relay is far too slow, and an IGBT is aimed at much higher voltages.