Overview of Power Semiconductor Switches Presently available power semiconductor switches can be divided into three groups according to their degree of controllability: Diodes: ON and OFF states controlled by power circuits Thyristors: latched on by a control signal but turned OFF by the power circuit Controllable switches: turned ON and OFF by control signals

Diodes On and off states controlled by the power circuit Forward biased – conduction Reverse biased – small leakage current flow until break down voltage reached

Thyristors Semi-controlled device Latches ON by a gate-current pulse if forward biased Turns-off if current tries to reverse

Thyristor in a Simple Circuit For successful turn-off, reverse voltage required

Generic Switch Symbol Idealized switch symbol When on, current can flow only in the direction of the arrow Instantaneous switching from one state to the other Conduct large current with zero voltage drop in on-state Block large forward and reverse voltages with zero current flow when off Infinite voltage and current handling capabilities

Bipolar Junction Transistors (BJT) BJT is a current-controlled device A sufficiently large base current will turn the device ON Base current must be supplied continuously to keep it in the ON state Used commonly in the past Now used in specific applications, replaced by MOSFETs and IGBTs

Various Configurations of BJTs dc gain is in the order of 5-10 of one BJT To achieve larger current gain, these devices are sometimes connected in the above configurations.

MOSFETs MOSFET is a voltage-controlled device Easy to control by the gate – continuous application of vGS required to keep the device in the ON state Faster switching speed (in the nanosecond range) than BJTs Switching loss is lower compared to BJTs

Gate-Turn-Off Thyristors (GTO) GTO as an ON/OFF switch Once forward biased GTO can be turned ON by a gate pulse GTO will stay ON However, can be turned off by applying a negative gate-cathode voltage Used at very high power levels Require elaborate gate control circuitry

IGBT High impedance gate – requires small amount of energy to switch the device Current rating: ~1700 A Voltage rating: 2~3 kV

Comparison of Controllable Switches

Review of Basic Electrical and Magnetic Circuit Concepts

Sinusoidal Steady State

Three-Phase Circuit

Steady State in Power Electronics Voltage produced by an inverter in an ac motor drive Often line currents drawn from the utility by the power electronic circuits are highly distorted as shown in b

Fourier Analysis

Phasor Representation

Response of L and C

Inductor Voltage and Current in Steady State In steady-state, the average inductor voltage (over one time period) must be zero.

Capacitor Voltage and Current in Steady State In steady-state, the average capacitor current (over one time period) must be zero.