3.1
The power diode is the simplest,
uncontrollable power electronic switch. A power diode is forward biased (on) when its current is positive and reverse biased (off) when its voltage is negative.
A thyristor is a controllable three-terminal device.
If a current pulse is applied to its gate, the thyristor can be turned on and conducts current from its anode to cathode, providing a positive anode-to-cathode voltage. However, in order to turn a thyristor on, the gate current must be above a minimum value IGT. After the thyristor turns on, if its current (i.e., anode to cathode) reaches above a minimum value called latching current IL, the gate current is no longer required. The thyristor will continue to conduct until its current falls below a minimum value called holding current IH.
A diac is a two-terminal power electronic device.
When the voltage across the terminals reaches the diac specific voltage, the diac is turned on and conducts current from the positive terminal to the negative terminal. The voltage across terminals decreases to a small value which is the voltage drop while diac is on. A diac is a bidirectional device.
A triac is a three-terminal, controllable power electronic switch.
The operation of a triac is equivalent to two thyristors which are parallel in opposite directions. Therefore, a triac has the capability of conducting current in both directions. Gate current can also be positive or negative. As a result, a triac has four different operating modes.
Power transistors have the characteristics of conventional transistors.
However, they have the capability of conducting higher collector current. They have also higher breakdown voltage Vceo. Power transistors are designed for high-current, high-voltage, and high-power applications. They are usually operated either in the fully on or fully off state.
Power MOSFETs are voltage-controlled devices.
They are usually N-channel and of the enhancement type. Most power MOSFETs are off when Vgs < 2 V and are on when Vgs > 4 V. When a power MOSFET is on there is a small resistance, i.e., less than 1 Q, between drain and source, and when it is off there is a large resistance (almost open circuit) between drain and source.
Isolated gate bipolar transistors (IGBTs) are equivalent to power transistors whose bases are driven by MOSFETs. Similar to a MOSFET, an IGBT has a high impedance gate, which requires only a small amount of energy to switch the device. Like a power transistor, an IGBT has a small on-state voltage.
Unijunction transistors (UJTs) are three-terminal devices with one emitter and two bases. It can be assumed that between the two bases two resistors are connected in series. If in the forward-biased mode the emitter voltage reaches the voltage divided between the two resistors, emitter current suddenly increases and the device conducts.
Pulse transformers are similar to conventional transformers.
However, they are designed for high-frequency and very low-power applications. They are not for transforming power from the primary to the secondary. They are used for isolating control circuits from power circuits in power electronic applications.
