Power electronics is the application of solid-state electronics to the control and conversion

of electric power. It also refers to a subject of research in electronic and electrical

engineering which deals with the design, control, computation and integration of nonlinear,

time-varying energy-processing electronic systems with fast dynamics.

The first high power electronic devices were mercury-arc valves. In modern systems the

conversion is performed with semiconductor switching devices such

as diodes,thyristors and transistors, pioneered by R. D. Middlebrook and others beginning

in the 1950s. In contrast to electronic systems concerned with transmission and processing

of signals and data, in power electronics substantial amounts of electrical energy are

processed. An AC/DC converter (rectifier) is the most typical power electronics device found

in many consumer electronic devices, e.g. televisionsets, personal computers, battery

chargers, etc. The power range is typically from tens of watts to several hundred watts.

In industry a common application is thevariable speed drive (VSD) that is used to control

an induction motor. The power range of VSDs start from a few hundred watts and end at

tens of megawatts.

The power conversion systems can be classified according to the type of the input and

output power

AC to DC (rectifier)

DC to AC (inverter)

DC to DC (DC-to-DC converter)

AC to AC (AC-to-AC converter)

Contents

1 History 2 Devices 3 Solid-state devices 4 DC/AC converters (inverters)

o 4.1 Single-phase half-bridge invertero 4.2 Single-phase full-bridge invertero 4.3 Three-phase voltage source invertero 4.4 Current source inverterso 4.5 Multilevel inverters

5 AC/AC converters 6 Simulations of power electronic systems 7 Applications

o 7.1 Inverterso 7.2 Smart grid

7.2.1 Grid voltage regulation 8 Notes 9 References 10 External links

History[edit]

Power electronics started with the development of the mercury arc rectifier. Invented

by Peter Cooper Hewitt in 1902, it was used to convert alternating current (AC) into direct

current (DC). From the 1920s on, research continued on applyingthyratrons and grid-

controlled mercury arc valves to power transmission. Uno Lamm developed a mercury valve

with grading electrodes making them suitable for high voltage direct current power

transmission. In 1933 selenium rectifiers were invented.[1]

In 1947 the bipolar point-contact transistor was invented by Walter H. Brattain and John

Bardeen under the direction ofWilliam Shockley at Bell Labs. In 1948 Shockley's invention

of the bipolar junction transistor (BJT) improved the stability and performance of transistors,

and reduced costs. By the 1950s, higher power semiconductor diodes became available

and started replacing vacuum tubes. In 1956 the Silicon Controlled Rectifier (SCR) was

introduced by General Electric, greatly increasing the range of power electronics

applications.[2]

By the 1960s the improved switching speed of bipolar junction transistors had allowed for

high frequency DC/DC converters. In 1976 power MOSFETs became commercially

available. In 1982 the Insulated Gate Bipolar Transistor (IGBT) was introduced.

Devices[edit]

See also: Power semiconductor device

This section includes a list of references, but its sources remain

unclear because it has insufficient inline citations. Please help

to improve this article by introducingmore precise citations. (December

2013)

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 ignitronwere 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.

Power electronic devices may be used as switches, or as amplifiers.[3] 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.

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.

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 assilicon 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 HVDCsystems.

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.

Solid-state devices[edit]

Main article: Power semiconductor device

Device Description Ratings

Diode

Uni-polar, uncontrolled, switching device used in

applications such as rectification and circuit

directional current control. Reverse voltage

blocking device, commonly modeled as a switch in

series with a voltage source, usually 0.7 VDC. The

model can be enhanced to include a junction

resistance, in order to accurately predict the diode

voltage drop across the diode with respect to

current flow.

Up to 3000

amperes and 5000

volts in a single

silicon device. High

voltage requires

multiple series

silicon devices.

Silicon-

controlled

rectifier(SCR)

This semi-controlled device turns on when a gate

pulse is present and the anode is positive

compared to the cathode. When a gate pulse is

present, the device operates like a standard diode.

When the anode is negative compared to the

cathode, the device turns off and blocks positive or

negative voltages present. The gate voltage does

not allow the device to turn off.[4]

Up to 3000

amperes, 5000 volts

in a single silicon

device.

Thyristor

The thyristor is a family of three-terminal devices

that include SCRs, GTOs, and MCT. For most of

the devices, a gate pulse turns the device on. The

device turns off when the anode voltage falls below

a value (relative to the cathode) determined by the

device characteristics. When off, it is considered a

reverse voltage blocking device.[4]

Gate turn-off

thyristor(GTO)

The gate turn-off thyristor, unlike an SCR, can be

turned on and off with a gate pulse. One issue with

the device is that turn off gate voltages are usually

larger and require more current than turn on levels.

This turn off voltage is a negative voltage from

gate to source, usually it only needs to be present

for a short time, but the magnitude s on the order

of 1/3 of the anode current. A snubber circuit is

required in order to provide a usable switching

curve for this device. Without the snubber circuit,

the GTO cannot be used for turning inductive loads

off. These devices, because of developments in

IGCT technology are not very popular in the power

electronics realm. They are considered controlled,

uni-polar and bi-polar voltage blocking.[5]

Triac

The triac is a device that is essentially an

integrated pair of phase-controlled thyristors

connected in inverse-parallel on the same chip.[6] Like an SCR, when a voltage pulse is present on

the gate terminal, the device turns on. The main

difference between an SCR and a Triac is that

both the positive and negative cycle can be turned

on independently of each other, using a positive or

negative gate pulse. Similar to an SCR, once the

device is turned on, the device cannot be turned

off. This device is considered bi-polar and reverse

voltage blocking.

Bipolar junction

transistor(BJT)

The BJT cannot be used at high power; they are

slower and have more resistive losses when

compared to MOSFET type devices. To carry high

current, BJTs must have relatively large base

currents, thus these devices have high power

losses when compared to MOSFET devices. BJTs

along with MOSFETs, are also considered unipolar

and do not block reverse voltage very well, unless

installed in pairs with protection diodes. Generally,

BJTs are not utilized in power electronics switching

circuits because of the I2R losses associated with

on resistance and base current requirements.[4] BJTs have lower current gains in high power

packages, thus requiring them to be set up in

Darlington configurations in order to handle the

currents required by power electronic circuits.

Because of these multiple transistor configurations,

switching times are in the hundreds of

nanoseconds to microseconds. Devices have

voltage ratings which max out around 1500 V and

fairly high current ratings. They can also be

paralleled in order to increase power handling, but

must be limited to around 5 devices for current

sharing.[5]

Power MOSFET The main benefit of the power MOSFET is that the

base current for BJT is large compared to almost

zero for MOSFET gate current. Since the MOSFET

is a depletion channel device, voltage, not current,

is necessary to create a conduction path from

drain to source. The gate does not contribute to

either drain or source current. Turn on gate current

is essentially zero with the only power dissipated at

the gate coming during switching. Losses in

MOSFETs are largely attributed to on-resistance.

The calculations show a direct correlation to drain

source on-resistance and the device blocking

voltage rating, BVdss.

Switching times range from tens of nanoseconds

to a few hundred microseconds, depending on the

device. MOSFET drain source resistances

increase as more current flows through the device.

As frequencies increase the losses increase as

well, making BJTs more attractive. Power

MOSFETs can be paralleled in order to increase

switching current and therefore overall switching

power. Nominal voltages for MOSFET switching

devices range from a few volts to a little over

1000 V, with currents up to about 100 A or so.

Newer devices may have higher operational

characteristics. MOSFET devices are not bi-

directional, nor are they reverse voltage blocking.[5] ||