EEE421: POWER ELECTRONICS
Lecture 5
Power Semiconductor
Devices: Thyristor
INTRODUCTION
Thyristors are electronic switches used in some power electronic
circuits where control of switch turn-on is required.
The term
thyristor
often refers to a family of three-terminal
devices that includes the silicon-controlled rectifier (SCR), the
triac, the gate turnoff thyristor (GTO), the MOS-controlled
thyristor (MCT), integrated gate commutated (IGCT)and
others.
Thyristor
and
SCR
are terms that are sometimes used
synonymously.
Thyristors are capable of large currents and large blocking
voltages for use in high-power applications, but switching
frequencies cannot be as high as when using other devices
such as MOSFETs. Hence, these devices are traditionally
employed in applications such as DC transmission lines.
STRUCTURE
OPERATION
The device is a three-junction PNPN type device, which can be
represented as two bipolar junction transistors connected
through a regenerative feedback. The transistor Q1 is formed by
the n, p, and n- regions, while the second transistor Q2 is formed
by the p, n- and p layers.
The SCR is known for its symmetric voltage blocking nature, due
to its ability to block voltages in both the
forward and reverse directions.
The SCR can conduct current only when a positive voltage is
applied to the anode terminal, relative to the cathode terminal.
However, the device starts to conduct only when a positive
gate current is supplied. This current causes the transistor Q1 to
turn on.
OPERATION
Since Q1 and Q2 are connected in a regenerative feedback
manner, the collector current of Q1 provides the necessary base
current to Q2, thereby turning on Q2. Hence, a positive feedback
mechanism is created, causing minority charge carriers to be
injected into all four semiconductor layers. As a result, due to
conductivity modulation, the turn-on resistance of the device is
lowered considerably and the device is latched to the on-state.
When IG is greater than the latching current, the current flowing
through the SCR becomes independent of IG. It behaves similarly
to a conventional p-n junction diode and with the current varying
exponentially with VA. In order to turn off the SCR, a reverse
current can to be applied at the anode or a negative anode-to-
