Examining And Understanding Power Electronics Engineering Essay

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Power electronic converters can be found wherever there is a need to modify a form of electrical energy. The power range of these converters is from some mille watts to hundreds of megawatts. With "classical" electronics, electrical currents and voltage are used to carry information, whereas with power electronics, they carry power. Thus, the main metric of power electronics becomes the efficiency.

The first very 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. 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. television sets, personal computers, battery chargers, etc. The power range is typically from tens of watts to several hundred watts. In industry the most common application is the variable speed drive (VSD) that is used to control an induction motor. The power range of VSDs starts 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)

Applications

Power electronic systems are found in virtually every electronic device.

DC/DC converters are used in most mobile devices to maintain the voltage at a fixed value whatever the voltage level of the battery is. These converters are also used for electronic isolation and power factor correction.

AC/DC converters (rectifiers) are used every time an electronic device is connected to the mains (computer, television etc.). These may simply change AC to DC or can also change the voltage level as part of their operation.

AC/AC converters are used to change either the voltage level or the frequency (international power adapters, light dimmer). In power distribution networks AC/AC converters may be used to exchange power between utility frequency 50 Hz and 60 Hz power grids.

DC/AC converters (inverters) are used primarily in UPS or emergency lighting systems. When mains power is available, it will charge the DC battery. If the main fails, an inverter will be used to produce AC electricity at mains voltage from the DC battery.

Switching

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. The instantaneous dissipated power of a device is equal to the product of the voltage across the device and the current through it (). From this, one can see that the losses of a power device are at a minimum when the voltage across it is zero (the device is in the On-State) or when no current flows through it (Off-State). Therefore, a power electronic converter is built around one (or more) device operating in switching mode (either On or Off). With such a structure, the energy is transferred from the input of the converter to its output by bursts.

Thyristor

History

The Silicon Controlled Rectifier (SCR) or Thyristor proposed by William Shockley in 1950, Bell Laboratories were the first to fabricate a silicon-based semiconductor device called Thyristor. Its first prototype was introduced by General Electric Company in 1957.These semiconductor device, which their characteristics identical with that of thyristor are TRIAC, DIAC, Silicon Controlled Switch, PUT, GTO, RTC etc…

Function

The thyristor is a four-layer, three terminal semiconducting devices, with each layer consisting of alternately N-type or P-type material, for example P-N-P-N. The main terminals, labelled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to p-type material near to the cathode. (A variant called an SCS-Silicon Controlled Switch-brings all four layers out to terminals.) The operation of a thyristor can be understood in terms of a pair of tightly coupled bipolar junction transistors, arranged to cause the self-latching action:

Thyristors have three states

Reverse blocking mode - Voltage is applied in the direction that would be blocked by a diode

Forward blocking mode - Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not yet been triggered into conduction

Forward conducting mode - The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the "holding current"

Switching characteristics

In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state (i.e. does not need a continuous supply of gate current to conduct), providing the anode current has exceeded the latching current (IL). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (IH).

V - I characteristics.

A thyristor can be switched off if the external circuit causes the anode to become negatively biased. In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This method is called forced commutation.

After a thyristor has been switched off by forced commutation, a finite time delay must have elapsed before the anode can again be positively biased and retain the thyristor in the off-state. This minimum delay is called the circuit commutated turn off time (tQ). Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by the remaining charge carriers (holes and electrons) that have not yet recombined.

For applications with frequencies higher than the domestic AC mains supply (e.g. 50 Hz or 60 Hz), thyristors with lower values of tQ are required. Such fast thyristors are made by diffusing into the silicon heavy metals ions such as gold or platinum which act as charge combination centres. Alternatively, fast thyristors may be made by neutron irradiation of the silicon.

Type of Thyristor

SCR - Silicon Controlled Rectifier

ASCR - Asymmetrical SCR

RCT - Reverse Conducting Thyristor

LASCR - Light Activated SCR, or LTT - Light triggered thyristor

BOD - Breakover Diode 

Shockley diode - Unidirectional trigger and switching device

Dynistor - Unidirectional switching device

DIAC - Bidirectional trigger device

SIDAC - Bidirectional switching device

TRIAC - Triode for Alternating Current

BCT - Bidirectional Control Thyristor

GTO - Gate Turn-Off thyristor

IGCT - Integrated Gate Commutated Thyristor

MA-GTO - Modified Anode Gate Turn-Off thyristor

DB-GTO - Distributed Buffer Gate Turn-Off thyristor

MCT - MOSFET Controlled Thyristor

BRT - Base Resistance Controlled Thyristor

SITh - Static Induction Thyristor, or FCTh 

LASS - Light Activated Semiconducting Switch

AGT - Anode Gate Thyristor

PUT or PUJT - Programmable Unijunction Transistor

SCS - Sylicon Controlled Switch or Thyristor Tetrode

Reverse conducting thyristor

A reverse conducting thyristor (RCT) has an integrated reverse diode, so is not capable of reverse blocking. These devices are advantageous where a reverse or freewheel diode must be used. Because the SCR and diode never conduct at the same time they do not produce heat simultaneously and can easily be integrated and cooled together. Reverse conducting thyristors are often used in frequency changers andinverters.

Construction of SCR

An SCR consists of four layers of alternating P and N type semiconductor materials. Silicon is used as the intrinsic semiconductor, to which the proper dopants are added. The junctions are either diffused or alloyed. The planar construction is used for low power SCRs (and all the junctions are diffused). The mesa type construction is used for high power SCRs. In this case, junction J2 is obtained by the diffusion method and then the outer two layers are alloyed to it, since the PNPN pellet is required to handle large currents. It is properly braced with tungsten ormolybdenum plates to provide greater mechanical strength. One of these plates is hard soldered to a copper stud, which is threaded for attachment of heat sink. The doping of PNPN will depend on the application of SCR, since its characteristics are similar to those of the thyratron. Today, the term thyristor applies to the larger family of multilayer devices that exhibit bistable state-change behaviour, that is, switching either ON or OFF.

Modes of operation

In the normal "off" state, the device restricts current to the leakage current. When the gate-to-cathode voltage exceeds a certain threshold, the device turns "on" and conducts current. The device will remain in the "on" state even after gate current is removed so long as current through the device remains above the holding current. Once current falls below the holding current for an appropriate period of time, the device will switch "off". If the gate is pulsed and the current through the device is below the holding current, the device will remain in the "off" state.

If the applied voltage increases rapidly enough, capacitive coupling may induce enough charge into the gate to trigger the device into the "on" state; this is referred to as "dv/dt triggering." This is usually prevented by limiting the rate of voltage rise across the device, perhaps by using a snubber. "dv/dt triggering" may not switch the SCR into full conduction rapidly and the partially-triggered SCR may dissipate more power than is usual, possibly harming the device.

SCRs can also be triggered by increasing the forward voltage beyond their rated breakdown voltage (also called as break over voltage), but again, this does not rapidly switch the entire device into conduction and so may be harmful so this mode of operation is also usually avoided. Also, the actual breakdown voltage may be substantially higher than the rated breakdown voltage, so the exact trigger point will vary from device to device. This device is generally used in switching applications.

Reverse Bias

SCR are available with or without reverse blocking capability. Reverse blocking capability adds to the forward voltage drop because of the need to have a long, low doped P1 region. Usually, the reverse blocking voltage rating and forward blocking voltage rating are the same. The typical application for reverse blocking SCR is in current source inverters.

SCR incapable of blocking reverse voltage are known as asymmetrical SCR, abbreviated ASCR. They typically have a reverse breakdown rating in the 10's of volts. ASCR are used where either a reverse conducting diode is applied in parallel (for example, in voltage source inverters) or where reverse voltage would never occur (for example, in switching power supplies or DC traction choppers).

Asymmetrical SCR can be fabricated with a reverse conducting diode in the same package. These are known as RCT, for reverse conducting thyristor.

Application of SCR

SCRs are mainly used in devices where the control of high power, possibly coupled with high voltage, is demanded. Their operation makes them suitable for use in medium to high-voltage AC power control applications, such as lamp dimming, regulators and motor control.

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