IV. Be ready to talk about the power supply at your home.
RADIO AND RADAR
LESSON # 1
INTRODUCTION TO POWER ELECTRONICS
Power electronics refers to control and conversion of electrical power by power semiconductor devices wherein these devices operate as switches. Advent of siliconcontrolled rectifiers, abbreviated as SCRs, led to the development of a new area application called the power electronics. Prior to the introduction of SCRs, me|rcy-arc rectifiers were used for controlling electrical power, but such rectifier circuits were part of industrial electronics and the scope for applications of mercury-arc rectifiers limited. Once the SCRs were available, the application area spread to many fields such as drives, power supplies, aviation electronics, high frequency inverters and power electronics originated. Power electronics has applications that span the whole field of electrical power systems, with the power range of these applications extending from a few VA/Watts to several M VA / MW. The main task of power electronics is to control and convert electrical power from one form to another. The four main forms of conversion are:
Rectification referring to conversion of ac voltage to dc voltage,
DC-to-DC conversion and
"Electronic power converter" is the term that is used to refer to a power electronic circuit that converts voltage and current from one form to another. These converters can be classified as:
Rectifier converting an ac voltage to a dc voltage,
Inverter converting a dc voltage to an ac voltage,
Chopper or a switch-mode power supply that converts a dc voltage to another dc
Cycloconverter and cycloinverter converting an ac voltage to another ac voltage. In addition, SCRs and other power semiconductor devices are used as static switches
Rectifiers can be classified as uncontrolled and controlled rectifiers, and the on-trolled rectifiers can be further divided into semi-controlled and fully-controlled rectifiers. Uncontrolled rectifier circuits are built with diodes, and fully-controlled rectifier circuits are built with SCRs. Both diodes and SCRs are used in semi-controlled rectifier circuits.
Power rating of a single-phase rectifier tends to be lower than 10 kW. Three-phase bridge rectifiers are used for delivering higher power output, up to 500 kW at 500 V dc or even more. For low voltage, high current applications, a pair of three-phase, three-pulse rectifiers interconnected by an inter-phase transformer (IPT) is used. For a high current output, rectifiers with IPT are preferred to connecting devices directly in parallel.
DC — To — AC Conversion
The converter that changes a dc voltage to an alternating voltage is called an inverter. Earlier inverters were built with SCRs. Since the circuitry required to turn the SCR off tends to be complex, other power semiconductor devices such as bipolar junction transistors, power MOSFETs, insulated gate bipolar transistors (IGBT) and MOS-controlled thyristors (MCTs) are used nowadays. Currently only the inverters with a high power rating, such as 500 kW or higher, are likely to be built with either SCRs or gate turn-off thyristors(GTOs). There are many in- verter circuits and the techniques for controlling an inverter vary in complexity. Some of the applications of an inverter are listed below:
biergency lighting systems
AC variable speed drives,
uninterrupted power supplies,
DC - To - DC Conversion
When the SCR came into use, a dc-to-dc converter circuit was called a chopper. Nowadays, an SCR is rarely used in a dc-to-dc converter. Either a power BJT or a power MOSFET is normally used in such a converter and this converter is called a switch-mode power supply. A switch-mode power supply can be of one of the types listed below:
Step-down switch-mode power supply,
Step-up chopper, Fly-back converter
The typical applications for a switch-mode power supply or a chopper are:
Battery charger and
DC power supply.
AC - To - AC Conversion
A cycloconverter or a cycloinverter converts an ac voltage, such as the mains supply, to another ac voltage. The amplitude and the frequency of input voltage to a cycloconver-ppend to be fixed values, whereas both the amplitude and the frequency of output voltage of a cycloconverter tend to be variable. On the other hand, the circuit that converts ait
ac voltage to another ac voltage at the same frequency is known as an ac-chopper.
A typical application of a cycloconverter is to use it for controlling the speed of an ac traction motor and most of these cycloconverters have a high power output, of the order a few megawatts and SCRs are used in these circuits. In contrast, low cost, low power cycloconverters for low power ac motors are also in use and many of these circuit tend to use triacs in place of SCRs. Unlike an SCR which conducts in only one direction, a triac is capable of conducting in either direction and like an SCR, it is also I three terminal device. It may be noted that the use of a cycloconverter is not as common as that of an inverter and a cycloinverter is rarely used.