2.0 Core Semiconductor Switching Devices
Semiconductor switching devices are the core components of modern power converters. Their operational characteristics, such as power handling capability and switching speed, directly dictate the performance, efficiency, and physical size of the converter. Understanding the function of these key devices is essential to grasping how different converter topologies operate.
2.1 Silicon Controlled Rectifier (SCR)
[SCR Symbol]
Anode
+
/ \
/—\
| | <– Gate
\-/
|
Cathode
An SCR, also known by the generic term thyristor, is a four-layer (P-N-P-N), three-terminal solid-state current-controlling device. It is primarily used in applications requiring the control of high voltage and power, making it ideal for medium to high AC power operations like motor control. The SCR operates in three distinct modes: a forward blocking (OFF) state, a forward conducting (ON) state, and a reverse blocking state. When a positive voltage is applied from anode to cathode, the SCR remains off until a positive pulse is applied to the gate terminal. Once conducting, the gate signal is no longer required to maintain the ON state; the device turns off only when the load current falls below a minimum holding current.
2.2 Insulated Gate Bipolar Transistor (IGBT)
[IGBT Symbol]
Collector
|
/
Gate —|
\
|
Emitter
The IGBT is a three-terminal semiconductor device that strategically combines the high-current and low-saturation-voltage capabilities of a Bipolar Junction Transistor (BJT) with the isolated gate structure of a Metal Oxide Semiconductor Field Effect Transistor (MOSFET). Known for its high efficiency and fast switching, the IGBT is a critical component in modern appliances like variable frequency drives (VFDs) and electric cars. It requires only a small voltage at the gate to control a large unidirectional current flow from the collector to the emitter.
2.3 Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
[MOSFET Symbol (N-Channel Enhancement)]
Drain
|
|—-
Gate –| |– Source
|—-
|
The MOSFET is a four-terminal transistor (source, drain, gate, body), though the body is often connected internally to the source, making it functionally a three-terminal device. MOSFETs are widely used for switching electronic signals. Their primary advantage is the extremely low current required at the gate to switch the device ON, while still being capable of delivering a high current to a load. MOSFETs exist in two main forms: the Depletion state, where the device is normally ON, and the Enhancement state, where the device is normally OFF and requires a gate voltage to turn ON.
2.4 Bipolar Junction Transistor (BJT)
[BJT Symbol (NPN)]
Collector
|
/
Base —|
\
|>
Emitter
The BJT is a transistor whose operation relies on two types of charge carriers: holes and electrons. Its structure consists of two P-N junctions connected back-to-back, forming either a P-N-P or N-P-N configuration with three terminals: emitter, base, and collector. A BJT can function as a switch or an amplifier. Its performance is defined by two key parameters that describe its ability to amplify current: the Transport Factor (α), the ratio of collector current to emitter current, and the Current Gain (β), the ratio of collector current to base current.
- Transport Factor (α):
- Current Gain (β):
The integration of these individual switching devices into specific circuit configurations, or topologies, allows for the precise conversion of electrical power, beginning with AC-DC conversion.