Power Electronics

Power electronics is a subject that deals with apparatus or equipment’s working on the principle of Electronics but at high power level than signal power level. The power electronics era was started with the invention of silicon controlled rectifiers (SCR) in 1956.

You can say that in modern power electronics systems, the power semiconductor devices can be regarded as muscles and the microelectronics as the intelligent brain. Power electronics in electrical world is used to control flow of energy from source to load by varying voltage, current or frequency.

Block Diagram of Power Electronics System

The block diagram of power electronics system is shown below.

The block system consists of command signal to command the operation of the system as desired. The control unit and digital circuit manipulated the command signal in order to drive the power electronic circuit. The power electronic circuit is driven by an external main power source as it works on higher power level than that of the signal levels of the basic electronics components. Finally the load is driven by the power electronic circuit and a feedback mechanism is used to improve the performance of the power electronics control system.

Role of Power Electronics

Let’s consider a DC motor being supplied by an input 200 Volts and the motor is running at 500 rpm (revolution per minute). If we want to reduce the speed of the motor from 500 rpm to 250 rpm with a back emf of E=190 Volts.

Then we have,

Form this relation it is quiet clear that to reduce the speed we have to reduce the E. So, to reduce the speed to half i.e. from 500 rpm to 250 rpm we have to reduce the back emf E by halve its value.

The new value of E required would be half of 190 Volts which will be equal to 95 Volts. To reduce this voltage we can use a suitable value of external resistance R_ext as shown in the figure below.

But this method of introducing a resistor in the circuitry will introduces additional losses in the R_ext and thereby reduces the overall efficiency of the system.

So, instead of using external resistor we can use power electronics devices in order to reduce the losses and improve the efficiency. In order to control speed of fan in our houses, nowadays TRIAC is used which is a PE (Power Electronics) device. Before that resistors were used to control the speed which increase the overall size of regulator due to large heat sink requirement and were less efficient due to losses in the resistors.

Power Electronics Devices

All semiconductor devices in power electronics system act as switches. If switch is on, power semiconductor device handles large current with negligible voltage drop and when switch is off it handles negligible current with full voltage across it. So, they can be classified as

Unidirectional switch: It allows the flow of current in one direction only. Example- SCR, diode.

Bi- directional switch: Allows the flow of current in both directions. Example- TRIAC.

Uncontrolled switch: In these switches we cannot control the ON and OFF state both. They are controlled by polarity of supply. Example- diode.

Semi controlled switch: Which can be controlled in only one state, either ON state or OFF state. Example: SCR

Fully controlled switch: It can be controlled in both ON and OFF state. Example- GTO, BJT, MOSFET and IGBT.

Unipolar switch: Block only one polarity of voltage. Example- diode.

Bipolar switch: Block both polarity of voltage. Example- SCR

DEVICES

PROPERTIES

SAFE OPERATING AREA

DIODE

Unidirectional: as current flows from anode to cathode only.

Uncontrolled Unipolar: Block reverse voltage only.

* As it blocks reverse voltage so put cross on –V axis and it allows forward current only so put cross on +I axis then make the rectangle to get safe operating area.

Diode Safe Operating Region

SCR

Unidirectional: as current flows from anode to cathode only.

Semi controlled: On state can be controlled by gate signal.

Bipolar: Block forward voltage in forward blocking mode and reverse voltage in reverse blocking mode.

SCR Safe Operating Region

GTO (Gate Turn on Thyristor)

Unidirectional: as current flows from anode to cathode only.

Fully controlled: On and OFF

state both can be controlled

Bipolar: Block forward voltage in forward blocking mode and reverse voltage in reverse blocking mode.

GTO Safe Operating Region

TRIAC

Bi-directional: as current flows in both directions.

Fully controlled: On and OFF state both can be controlled

Bipolar: Block forward voltage in forward blocking mode and reverse voltage in reverse blocking mode.

TRIAC Safe Operating Region

TRANSISTOR (BJT, FET, IGBT)

Unidirectional: as current flows in one direction.

Fully controlled: On and OFF

state both can be controlled

Unipolar: Block forward voltage in forward blocking mode.

Transistor Safe Operating Region

MOSFET/IGBT with body diode

Unidirectional: due to diode it conducts in reverse bias also.

Fully controlled: On and OFF

state both can be controlled

Unipolar: Block forward voltage in forward blocking mode.

MOSFET Safe Operating Region

Advantages and Disadvantages

The advantages of power electronics are:

High Efficiency.
High Reliability.
Long Life and less maintenance.
Fast dynamic response compared to electromechanical systems.
Small size and less weight which contributes towards its lower installation cost.

The disadvantages of power electronics are:

Power electronics converter circuits have tendency to generate harmonics.
Ac to dc and ac to ac converters operate at low input power factor under certain operating conditions.
They have low overload capacity. That’s why they are rated for taking momentary loads.