The Digital Analogue Electronics Engineering Essay

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Power electronics is the application of solid-state electronics for the control and conversion of electric power 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

DC to DC (DC to DC converter)

AC to AC (AC to AC converter)

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

electronics 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

(power adapters, light dimmer). In power distribution networks AC/AC

Converters may be used to exchange power between utility frequency 50 Hz and 60Hz

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.

As efficiency is at a premium in a power electronic converter, the losses that a power

electronics 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 of thyristor

The Silicon Controlled Rectifier (SCR) or Thyristor suggested by William Shockley in 1950,

Bell Laboratories were the first to formulate a silicon-based semiconductor device called

Thyristor. Its first prototype was introduced by General Electric Company in 1957.Thyristor characteristics identical with that of thyristor are TRIAC,DIAC, Silicon Controlled Switch, PUT, GTO, RTC etc…

About thyristor

Thyristor are typically three terminal devices that have 4 layer of alternating p type and material containing its main power handling section. In contrast to the linear relation exists between load and control current in a transistor, the thyristor is bitable. The control terminal of the thyristir , called the gate electrode, its connected to an related and complex structure as a part of the device. The other two terminals called the anode and cathode handle the large applied potential and conduct the major current through the thyristor. The anode and cathode terminal are connected in series with the load to which power is to be controlled. The thyristor used to approximate ideal closed or open switches for control of power flow in a circuit. Thyristor circuit must have the capability of supplying large current and applied voltages. All thyristor type is controlled in switching from a forward-blocking state in to a forward conduction state. Most thyristor have the characteristic that after switching from a forward blocking state in to the forward-conduction state, the gate signal can be removed and the thyristor will remain in its forward-conduction mode. This property is termed 'latching' and is important difference between thyristor and other type of power electronic device. Thyristor are typically used at the highest energy levels in power condition circuit because they are designed to handle the largest currents and voltages of any device technology (voltage above 1kV or currents above 100A)

Background Theory of Thyristors

Thyristors are a broad classification of bipolar-conducting semiconductor devices having four alternating N-P-N-P layers. Thyristors include silicon controlled rectifier (SCR), TRIAC, gate turn off switch (GTO), silicon controlled switch (SCS), AC diode (DIAC), unijunction transistor (UJT), programmable unijunction transistor (PUT). SCR's are now available to handle power levels straddling watts to megawatts. The smallest devices, packaged like small-signal transistors.

Silicon controlled rectifier (SCR) doping profile-BJT equivalent circuit.

The silicon controlled rectifier is a four layer diode with a gate connection as in Figure above (a). When turned on, it conducts like a diode, for one polarity of current. If not triggered on, it is nonconducting. Operation is explained in terms of the compound connected transistor equivalent diagram above. A positive trigger signal is applied between the gate and cathode terminals. This causes the NPN equivalent transistor to conduct. The collector of the conducting NPN transistor pulls low, moving the PNP base towards its collector voltage, which causes the PNP to conduct. The collector of the conducting PNP pulls high, moving the NPN base in the direction of its collector. This positive feedback (regeneration) reinforces the NPN's already conducting state. Moreover, the NPN will now conduct even in the absence of a gate signal. Once an SCR conducts, it continues for as long as a positive anode voltage is present. For the DC battery shown, this is forever. However, SCR's are most often used with an alternating current or pulsating DC supply. Conduction ceases with the expiration of the positive half of the sine wave at the anode. Moreover, most practical SCR circuits depend on the AC cycle going to zero to cutoff or commutate the SCR.

Figure below (a) shows the doping profile of an SCR. Note that the cathode, which corresponds to an equivalent emitter of an NPN transistor is heavily doped as N+ indicates. The anode is also heavily doped (P+). It is the equivalent emitter of a PNP transistor. The two middle layers, corresponding to base and collector regions of the equivalent transistors, are less heavily doped: N- and P. This profile in high power SCR's may be spread across a whole semiconductor wafer of substantial diameter.

(a) Cross-section

(b) Silicon controlled rectifier (SCR)

(c) Gate turn-off thyristor (GTO) symbol.

The basic diode symbol indicates that cathode to anode conduction is unidirectional like a diode. The addition of a gate lead indicates control of diode conduction. The gate turn off switch (GTO) has bidirectional arrows about the gate lead, indicating that the conduction can be disabled by a negative pulse, as well as initiated by a positive pulse.

In addition to the ubiquitous silicon based SCR's, experimental silicon carbide devices have been produced. Silicon carbide operates at higher temperatures, and is more conductive of heat than any metal, second to diamond. This should allow for either physically smaller or higher power capable devices.

V - I characteristics

An elementary circuit diagram for obtaining static V-I characteristics of a thyristor is shown in Fig. The anode and cathode are connected to main source through the load. The gate and cathode are fed from a source Es which provides positive gate current from gate to cathode. Static V-I characteristics of a thyristor. Here Va is the anode voltage across thyristor terminals A, K and Ia is the anode current. Typical SCR V-I characteristic shown in Fig. 4.2 (b) reveals that a thyristor has three basic modes of operation; namely, reverse blocking mode, forward blocking (off-state) mode and forward conduction (on-state) mode. These three modes of operation are now discussed below

Reverse Blocking Mode: When cathode is made positive with respect to anode with switch S open, thyristor is reverse biased Junctions J1 J3 are seen to be reverse biased whereas junction J2 is forward biased. The device behaves as if two diodes are connected in series with reverse voltage applied across them. A small leakage current of the order of a few milli amperes (a few microamperes depending upon the SCR rating) flows. This is reverse blocking mode, called the off-state, of the thyristor. If the reverse voltage is increased, then at a critical breakdown level, called reverse breakdown voltage VBR, an avalanche occurs at J1 and J3 and the reverse current increases rapidly. A large current associated with VBR gives rise to more losses in the SCR. This may lead to thyristor damage as the junction temperature may exceed its permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage across a thyristor does not exceed VBR. When reverse voltage applied across a thyristor is less than VBR, the device offers high impedance in the reverse direction. The SCR in the reverse blocking mode may therefore be treated as an open switch.

Note that V-I characteristic after avalanche breakdown during reverse blocking mode is applicable only when load resistance is zero,In case load resistance is present, a large anode current associated with avalanche breakdown at VBR would cause substantial voltage drop across load and as a result, V-I characteristic in third quadrant would bend to the right of vertical line drawn at VBR.

Forward Blocking Mode: When anode is positive with respect to the cathode, with gate circuit open, thyristor is said to be forward biased as shown in It is seen from this figure that junctions J1, J3 are forward biased but junction J2 is reverse biased. In this mode, a small current, called forward leakage current, flows as shown in and In case the forward voltage is increased, then the reverse biased junction J2 will have an avalanche breakdown at a voltage called forward breakover voltage VB0. When forward voltage is less than VBO, SCR offers a high impedance. Therefore, a thyristor can be treated as an open switch even in the forward blocking mode.

Forward Conduction Mode: In this mode, thyristor conducts currents from anode to cathode with a very small voltage drop across it. A thyristor is brought from forward blocking mode to forward conduction mode by turning it on by exceeding the forward breakover voltage or by applying a gate pulse between gate and cathode. In this mode, thyristor is in on-state and behaves like a closed switch. Voltage drop across thyristor in the on state is of the order of 1 to 2 V depending on the rating of SCR.That this voltage drop increases slightly with an increase in anode current. In conduction mode, anode current is limited by load impedance alone as voltage drop across SCR is quite small. This small voltage drop vT across the device is due to ohmic drop in the four layers.

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.

Applications with frequencies higher than the domestic AC mains supply (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.

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).

Type of thyristor

SCR - Silicon Controlled Rectifier

SCRs 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, means dimming, regulators and motor control etc...

ASCR - Asymmetrical SCR

RCT - Reverse Conducting Thyristor

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

BOD - Breakover Diode

TRIAC - Triode for Alternating Current

BCT - Bidirectional Control Thyristor

GTO - Gate Turn-Off thyristor

IGCT - Integrated Gate Commutated Thyristor

MCT - MOSFET Controlled Thyristor

SITh - Static Induction Thyristor, or FCTh

LASS - Light Activated Semiconducting Switch

AGT - Anode Gate Thyristor

PUT or PUJT - Programmable Unijunction Transistor

Shockley diode - Unidirectional trigger and switching device

MA-GTO - Modified Anode Gate Turn-Off thyristor

DB-GTO - Distributed Buffer Gate Turn-Off thyristor

BRT - Base Resistance Controlled Thyristor

Electric Vehicle Power System

An electric vehicle power system simply consists of three main components - Battery, controller and electric motor, an electric vehicle is powered by an electric motor, so electric vehicle are smooth and silent while driving

Working of an Electric Vehicle

For working of electric vehicle need three important sections:

Battery

Controller (Power Electronics Section)

Electric motor

.

Battery

Electric vehicle using in 12VDC/70 Ah *…………

Controller (Power Electronics Section)

•The motor controller will be mounted inside an electronics box which will reside in the engine compartment.

•This Power Electronics Section will keep combination of electronics components circuit (SMPS, Inverter, Timer, Amplifier etc...) and other electronics parts.

Electric motor

A series wound DC motor with the following characteristics will be used:

â- 20 Horse Power

â- Series wound

â- 96 VAC

Application of Thyristor in Electric Vehicle power system

In above section discus about Electric Vehicle power system, so we know the main part of the electric vehicle system is controller section (Power Electronics). The hart of the controller is step up current and voltage section, switching circuit is doing this conversation. Inverter will help for low voltage to high voltage conversation. Thyristor is the main part of the high voltage (12v to 400v) circuit. Thyristor voltage & current rating is very high

An inverter is an electrical device that converts direct current (DC) to alternating current (AC),

The converted AC can be at any required voltage and frequency with the use of appropriate switching, and control circuits, transformers.

Solid-state inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in electronics device to high electric voltage direct current applications of transmission bulk power. Inverters are commonly used to supply from DC to AC sources such as solar panels or batteries in use of purpose.

A battery electric vehicle is a type of electric vehicle (EV) that uses chemical energy stored in

rechargeable battery and convert the electric power to mechanical power.

Most standard appliances are designed to accept only AC voltages because that's how electricity is supplied from the grid. To run an electronic device from a DC voltage source you need to transform DC into AC. A power inverter is a device that converts electrical energy from DC form to AC form using electronic circuits. Its typical application is to convert a 12V battery voltage into conventional household voltage means above 200VAC. Inverters are used in a wide variety of applications from small car adapters to large grid-tie systems that can supply AC power to a high power application.

There are three basic types of dc-ac converters: square wave, modified sine wave, and pure sine wave (see the diagram below). The square wave is the simplest and cheapest type, but nowadays it is practically not used commercially because of low power quality. The modified sine wave topologies (which are actually modified square waves) provide square waves with some dead spots between positive and negative half-cycles. They are suitable for many electronic

A sinewave inverter produces voltage with low total harmonic distortion. It is the most expensive type of AC power source, which is used when there is a need for clean sinusoidal output for some sensitive devices such as medical equipment, laser printers etc. there is a number of topologies used in the inverter circuits. Simple square wave circuits suitable primarily for hobbyists' projects may use just a push-pull converter with a step-up transformer. Commercially manufactured models use a multi-stage concept. With such technique, first a switch mode power supply (SMPS) steps down voltage from an input source to another DC voltage corresponding to the peak value of the desired sinusoidal voltage. The output stage then generates an AC. This stage usually uses a full-bridge or half-bridge configuration. If a half-bridge is used, the DC-link voltage should be more than twice the peak of the generated output. Input to output galvanic isolation is provided by either a high-frequency transformer in the SMPS pre-regulator, or by a large low-frequency output transformer. If a low-frequency transformer is used, the sinusoidal voltage is generated on its primary side and transformed to the secondary side. The output can be controlled either in square-wave mode or in pulse width-modulated (PWM) mode. Sine wave circuits use PWM mode, in which the output voltage and frequency are controlled by varying the duty cycle of the high frequency pulses. Chopped signal then passes through a low pass LC-filter to supply a clean sinusoidal output. Although such approach is more expensive, it is usually employed in the backup devices for home or business use, which require high quality of AC power.

Power inverters for cars often come with a jack that can be plugged directly into the cigarette lighter. Note, however, thatthe cigarette lighters are protected by a fuse rated typically between 15 and 20 A. If you need touse an electrical device that consumes more than 12VÃ-(fuse amperage)Ã-0.95 volt-amps, where0.95 is a typical efficiency of an auto inverter, your unit has to be connected directly to the carbattery. That's why the models above 200 VA usually don't even provide a plug for cigarettelighters, and instead includes jumper cables that can be connected directly to the battery terminals.

Note the industrial inverters are commonly used in volt-amps (VA). The real power (watts) we can supply will depend on the power factor of the load,

Watts=VA*PF.

Other applications

Battery charger circuit

Starter circuits

Logic and digital circuit

Modern electronic dimmer

Electrical transmission system

Control of induction heat

Relay control

Diesel motor

Latest Thyristor Technologies

SIDAC

The SIDAC is a high voltage bilateral trigger device that extends the trigger capabilities to significantly higher voltages and currents than have been before obtainable, thus letting new, cost-effective applications. Being a bilateral device, it will switch from a blocking

state to a conducting state when the applied voltage of either polarity exceeds the break over voltage. As in other trigger devices, (SBS, Four Layer Diode), the SIDAC switches through a negative resistance region to the low voltage on-state (Figure 4.1) and will remain on until the main terminal current is interrupted or drops below the holding current .SIDAC's are available in the large MKP3V series and economical, easy to insert, small MKP1V series axial lead packages. Breakdown voltages ranging from 104 to 280 V are available. The MKP3V devices feature bigger chips and provide much greater surge capability along with somewhat higher RMS current ratings. The high-voltage and current ratings of SIDACs make them ideal for high energy applications where other trigger devices are unable to function alone without the aid of additional power boosting components. The basic SIDAC circuit and waveforms, operating off of ac. Note that once the input voltage exceeds V(BO), the device will switch on to the forward on-voltage VTM of typically 1.1 V and can conduct as much as the specified repetitive peak on-state current ITRM of 20 A (10 μs pulse, 1 kHz repetition frequency).

Over Voltage Circuit Protection

Thyristor can be employed for protecting equipments being affected from over voltage due to the fast switching action. The thyristors in this form are connected in parallel with the load when the voltage exceeds the specified unit the gate get energized and trigger the thyristor a large amount of current is drawn  from the supplies and voltage is reduced across the load . in this two thyristors or SCR are used one in the positive half cycle and other for negative half cycles. Resistor R! Limits the short circuit current when the thyristors are fired a voltage sensing device is constituted with diode D% and resistors R1 and R2.

 

The valve's electric design allows a fully optimized thyristor. The semiconductors are optimized for each HVDC project with respect to voltages and especially the on-state losses. Very high voltage thyristors have been available for sometime. Today it is use these in commercial HVDC plants. Improved processing techniques and more implimenting wafer design have increased the capability of handling voltage and current derivatives. Today some company uses 9 kV devices for plants with current ratings in the lower range. A higher voltage rating means a thicker silicon wafer, for a given area and current, the increase in thickness means more losses and reduced surge current capability. In addition, increased DC current demands result in higher valve short circuit current. These demands combined have led to demands for a larger wafer area.

A simple battery charger based on SCR is shown here.Here the SCR rectifies the AC mains voltage to charge the battery.When the battery connected to the charger gets discharged the battery voltage gets dropped.This inhibits the forward biasing voltage from reaching the base of the transistor Q1 through R4 and D2.This switches off the transistor.When the transistor is turned OFF,the gate of SCR (H1) gets the triggering voltage via R1 & D3.This makes the SCR to conduct and it starts to rectify the AC input voltage.The rectified voltage is given to the battery through the resistor R6(5W).This starts charging of the  battery.

When the battery is completely charged the base of Q1 gets the forward bias signal through the voltage divider circuit made of R3,R4,R5 and D2.This turns the transistor ON.When the Q1 is turned ON the trigger voltage at the gate of SCR is cut off and the SCR is turned OFF.In this condition a very small amount of charge reaches the battery via R2 and D4 for trickle charging.Since the charging voltage is only half wave rectified ,this type of charger is suitable only for slow charging.For fast charging full wave rectified charging voltage is needed.

 

Conclusion

Thyristors used to control the power of the components and are comprised of multiple layers of P and N layers. Various types of thyristors included are the silicon controlled rectifier (SCR), TRIAC and DIACs. Hence thyristors are device used to carry the over current because the sensing circuit is very essential forms. Thyristors are used in many essential of large capacity with low ON state voltage characteristics. In thyristors process semiconductor switches are required mainly to control large amounts of power while compatible to challenging power loss requirements. Such switches are being used in motor control systems, uninterrupted power supplies, high-voltage DC transmissions and in many other high power applications. Thyristor are also a very well power semiconductor switch which is used to permit large currents to be switched off at high voltages. The anode and the cathode terminals are connected between the current carrying circuit and ground. The circuit current is sensed by gate terminal the value of the resistor is set up to threshold of current before the thyristor which is before the thyristor is gated into the conduction in order to oppose the over current from the conductor to the ground. Silicon controlled rectifier is one of the type of thyristor is also a semiconductor which is having alternative four layers of P and N semiconductor layers. With the supply of voltage connected in series and the supply voltage is being less than the voltage of the device and no trigger signal is applied to the gate. In this state the thyristor remains in OFF-state, it is because of the P and N type configurations which is comprised of polarity of P and N junctions. The junction has a small capacitance known as the junction capacitance.  The other one is the TRIAC in this the functions are bidirectional alternating current switch and the functions of these is unidirectional switch which permits the current flow only in one direction.  TRIAC and silicon controlled rectifier (SCR) are mainly found in light dimmers, motor speed controllers and other power controlling devices.

        Thyristors are used in motor control devices, CFI lights, home appliances, office equipment, telecommunication tools, dimmer switches, and engine ignition system and also in many other types of equipment. The gate trigger current must be at least 50% more than the maximum rated gate trigger current. Thyristors are driven in many ways such as directly from transistors or logic families by isolated TRIAC & DIAC drivers.

Thyristors are very useful for the designers because they offer many considerable benefits which are as follows:

Drive circuit with low consumption

Switching OFF is done automatically

Reverse polarity protection

It is a bidirectional switch- current and voltage

It has high ruggedness

It is easy to drive

Referencing

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