Power electronics devices

Power electronics switches:

Power electronics switches are known as solid state switches or semiconductor switches. These switches can operate without any mechanical movement. Solid-state devices are completely made from a solid material and their flow of charges is confined to this solid material.

These switches are classified into two parts according to number of terminals;

     1)    2-terminal device

•     Diode

     2)    3-terminal device

•     MOSFET
•     IGBT
•     BJT
•     Thyristor

Let’s get detail information about switches;

Power Diode:

The diode is a two-terminal P-N junction semiconductor device, with terminals anode (A) and cathode (C). The symbol of the Power diode is as shown in below figure.

If anode terminal is at higher potential compared to cathode terminal, the device is said to be in forward biased and forward current will flow through the anode to the cathode. Here, very small amount of voltage drop occurs across the device is called as forward voltage drop. If you consider this diode as an ideal switch, then this voltage drop will be considered as zero.

If anode terminal is at lower potential compared to cathode terminal, the device is said to be in reverse biased. In reverse bias, very small amount of current will flow across the diode and this current is known as leakage current.

The reverse recovery characteristics of the Power diode is shown in the following figure. We can understand the turn off characteristic of the diode from the figure. The Reverse recovery time is the time interval between the application of reverse recovery voltage and the reverse current dropped to 0.25 of I_RR.

MOSFET: 

The MOSFET is known as Metal Oxide Semi-conductor Field Effect Transistor. The MOSFET is a voltage controlled device. Generally, MOSFET is used for the switching and amplifying of electric signal in electronics devices. Two types of MOSFET are used; n-type and P-type MOSFET.

MOSFET can work in two mode;

1) Depletion mode: When there is no voltage on the gate, the channel shows its maximum conductance. As the voltage on the gate is either positive or negative,  the channel conductivity decreases.

2) Enhancement mode: When there is no voltage on the gate the device does not conduct. More is the voltage on the gate, the better the device can conduct.

MOSFETs can operate in three region;

1) Cut-off region:  the gate-source voltage is much lower than the transistors threshold voltage so the MOSFET transistor is switched “fully-OFF” thus, Drain current is zero, with the transistor acting like an open switch.

2) Linear region: When Gate-source voltage is grater than threshold voltage and Drain-source voltage is less than Gate-source voltage the transistor is in its constant resistance region behaving as a voltage-controlled resistance whose resistive value is determined by the gate voltage, VGS level.

3) Saturation region: When Gate-source voltage is greater than threshold voltage the transistor is in its constant current region and is therefore “fully-ON”. The Drain current Drain current is Maximum with the transistor acting as a closed switch.

IGBT:

IGBT stands for Integrated Gate Bi-polar Transistor. IGBT has hybrid combination of MOSFET and BJT. IGBT has high input impedance and switching frequency like MOSFET and low saturation of voltage like BJT. IGBT is mostly used in power electronics application like inverter, converter.  

Structure and V-I characteristic of IGBT is as shown in below figure;

Advantages:

• lower switching loss
• better safe operating area
• high input impedance 
• voltage controlled device
• lower gate drive requirement

Disadvantages:

• Cost is high
• high turn off time compared to MOSFET

BJT: 

BJT stands for Bi-polar junction transistor. BJT has three terminals; Gate, emitter and junction. The transistor’s ability to change between these two states enables it to have two basic functions: “switching” (digital electronics) or “amplification” (analogue electronics). Then bipolar transistors have the ability to operate within three different regions:

1. Active Region   –   the transistor operates as an amplifier 
2. Saturation   –   the transistor is “Fully-ON” operating as a switch 
3. Cut-off   –   the transistor is “Fully-OFF” operating as a switch 

Bipolar Transistors are current regulating devices that control the amount of current flowing through them from the Emitter to the Collector terminals in proportion to the amount of biasing voltage applied to their base terminal, thus acting like a current-controlled switch. As a small current flowing into the base terminal controls a much larger collector current forming the basis of transistor action.

Two types of BJT are available; NPN transistor and PNP transistor. Symbol, construction and operation is as shown in below figures. 

Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output. Each method of connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement.

1) Common Base Configuration   –   has Voltage Gain but no Current Gain. (figure-a)

2) Common Emitter Configuration   –   has both Current and Voltage Gain. (figure-b)

3) Common Collector Configuration   –   has Current Gain but no Voltage Gain. (figure-c)

Thyristor:

thyristor is a uni-directional devise. It means that, current can flow only in one direction across the thyristor. function of thyristor is almost same as diode. but in case of thyristor, turn on control is given. But thyristor turned off control is not given. 

thyristor has three terminals; gate, anode and cathode. When gate pulse is applied and anode terminal is at higher potential compared to cathode terminal than it can turn off. once thyristor is turned on and gate signal is not given, in this condition also it remains turned on. 

If gate signal is available but anode terminal voltage is less than compare to cathode terminal voltage, than also thyristor can not turned on.

Symbol of thyristor is as below figure;

Application: 

Thyristors, or silicon controleld rectifiers, SCRs are used in many areas of electronics where they find uses in a variety of different applications. Some of the more common applications for them are outlined below:

• AC power control (including lights, motors,etc).
• Overvoltage protection crowbar for power supplies.
• AC power switching.
• Control elements in phase angle triggered controllers.