Basics of power electronics?

What is power electronics?

Power  electronics is nothing but  the switching of electronic circuits in order to control the electrical energy flow. The whole technology of power electronics is hidden behind the concepts of power inverters,power converters, switching power supplies and motor drives.
Power electronics is one of the core subjects in the field of electrical engineering.

First power electronics device

The first device in the field of power electronics is Mercury arc valves.This mercury arc valves is nothing but an electrical rectifier that normally converts alternating current into direct current.

What is in power electronics?

Normally the devices like transistors,diodes and GTO'S are employed in power electronics.In power electronics processing is done on the considerable amount of electrical energy.The most common device is AV/DC converter that is employed in daily routine electronic devices like battery charger,television set and computers.

Discovery of DC power

The DC power according to Thomas Edison was produced by the creation of magnetic field near the piece wire,then this magnetic field generated single directional flow of current which was directed from negative side to positive side.

Discovery of AC power

A renowned scientist Nikola Telsa gave the idea of AC power.He observed that AC power can travel larger distances as compared to DC power.To achieve this he used rotating magnet instead of steady magnet that induced the flow of electrons according to the varying position of magnet.

Differentiation between AC and DC

Direct current                                                                         Alternating current
electron flow is in single direction                                        flow is  in double direction
it cannot travel longer distances                                            it can travel longer distances
it loses energy  in large distances                                          it does not loses energy
magnetic used is steady                                                         rotating magnetic is used
frequency is 0                                                                        frequency is 50 or 60
current is constant over period of time                                  current is varying
it is used in cell phones,vehicle batteries                              AC is used in our homes

What is a rectifier?

A rectifier is just a converter that converts alternating current to direct current.This whole process is called rectification.Mercury arc valves,silicon based semiconductor switches and semiconductor diodes are various different forms of rectifiers.

What is a diode?

A diode is vacuum tube that contains two electrodes one is anode and second is cathode.
It is basically a two terminal device that permits current flow in single direction.A diode has high resistance to current in one direction and low resistance in other direction.

First semiconductor device

The first semiconductor electronic device is semiconductor diode.It is mostly made up by silicon and sometimes germanium or selenium may also be used.
Diode Applications
signal limiters
voltage regulator
switches
signal modulator
signal mixer

Difference between GTO and thyristor

A GTO has a fully controllable switch with the help of which it can be turned on and off any time.But on the other hand thyristor does not have fully
 controllable switch firstly is turned on it cannot be turned off by will.

Thyristor

A thyristor is a three terminal power electronics device that is used for the purposes of amplification and switching phenomenon of electronic signals.

Devices of power electronics

The capabilities and economy of power electronics system are determined by the active devices that are available. Their characteristics and limitations are a key element in the design of power electronics systems. Formerly, the mercury arc valve, the high-vacuum and gas-filled diode thermionic rectifiers, and triggered devices such as the thyratron and ignitron were widely used in power electronics. As the ratings of solid-state devices improved in both voltage and current-handling capacity, vacuum devices have been nearly entirely replaced by solid-state devices.

Applications of devices

Power electronic devices may be used as switches, or as amplifiers An ideal switch is either open or closed and so dissipates no power; it withstands an applied voltage and passes no current, or passes any amount of current with no voltage drop. Semiconductor devices used as switches can approximate this ideal property and so most power electronic applications rely on switching devices on and off, which makes systems very efficient as very little power is wasted in the switch. By contrast, in the case of the amplifier, the current through the device varies continuously according to a controlled input. The voltage and current at the device terminals follow a load line, and the power dissipation inside the device is large compared with the power delivered to the load.

Several attributes dictate how devices are used. Devices such as diodes conduct when a forward voltage is applied and have no external control of the start of conduction. Power devices such as silicon controlled rectifiers and thyristors (as well as the mercury valve and thyratron) allow control of the start of conduction, but rely on periodic reversal of current flow to turn them off. Devices such as gate turn-off thyristors, BJT and MOSFET transistors provide full switching control and can be turned on or off without regard to the current flow through them. Transistor devices also allow proportional amplification, but this is rarely used for systems rated more than a few hundred watts. The control input characteristics of a device also greatly affect design; sometimes the control input is at a very high voltage with respect to ground and must be driven by an isolated source.

Characteristics of devices

As efficiency is at a premium in a power electronic converter, the losses that a power electronic device generates should be as low as possible.
Devices vary in switching speed. Some diodes and thyristors are suited for relatively slow speed and are useful for power frequency switching and control; certain thyristors are useful at a few kilohertz. Devices such as MOSFETS and BJTs can switch at tens of kilohertz up to a few megahertz in power applications, but with decreasing power levels. Vacuum tube devices dominate high power (hundreds of kilowatts) at very high frequency (hundreds or thousands of megahertz) applications. Faster switching devices minimize energy lost in the transitions from on to off and back, but may create problems with radiated electromagnetic interference. Gate drive (or equivalent) circuits must be designed to supply sufficient drive current to achieve the full switching speed possible with a device. A device without sufficient drive to switch rapidly may be destroyed by excess heating.

Practical devices have non-zero voltage drop and dissipate power when on, and take some time to pass through an active region until they reach the "on" or "off" state. These losses are a significant part of the total lost power in a converter.

Some important factors

Power handling and dissipation of devices is also a critical factor in design. Power electronic devices may have to dissipate tens or hundreds of watts of waste heat, even switching as efficiently as possible between conducting and non-conducting states. In the switching mode, the power controlled is much larger than the power dissipated in the switch. The forward voltage drop in the conducting state translates into heat that must be dissipated. High power semiconductors require specialized heat sinks or active cooling systems to manage their junction temperature; exotic semiconductors such as silicon carbide have an advantage over straight silicon in this respect, and germanium, once the main-stay of solid-state electronics is now little used due to its unfavorable high temperature properties.

Semiconductor devices exist with ratings up to a few kilovolts in a single device. Where very high voltage must be controlled, multiple devices must be used in series, with networks to equalize voltage across all devices. Again, switching speed is a critical factor since the slowest-switching device will have to withstand a disproportionate share of the overall voltage. Mercury valves were once available with ratings to 100 kV in a single unit, simplifying their application in HVDC systems.

The current rating of a semiconductor device is limited by the heat generated within the dies and the heat developed in the resistance of the interconnecting leads. Semiconductor devices must be designed so that current is evenly distributed within the device across its internal junctions (or channels); once a "hot spot" develops, breakdown effects can rapidly destroy the device. Certain SCRs are available with current ratings to 3000 amperes in a single unit.Courtesy of wikipedia....