Unit 2 CT1 Slides PDF

 21EES101T-ELECTRICAL AND ELECTRONICSENGINEERIN EEE-UNIT 2 G 2 OVERVIEW OF SEMICONDUCTORS Depending on their conductivity, materials can be classified into three types as conductors, semiconductors and insulators. Conductor...

 21EES101T-ELECTRICAL AND ELECTRONICSENGINEERIN EEE-UNIT 2 G 2 OVERVIEW OF SEMICONDUCTORS Depending on their conductivity, materials can be classified into three types as conductors, semiconductors and insulators. Conductor is a good conductor of electricity. Insulator is a poor conductor of electricity. Semiconductor has its conductivity lying between these two extremes. Energy Band of Semiconductor In terms of energy band shown in Fig., the valence band is almost filled (partially filled) and conduction band is almost empty. 3 A comparatively smaller electric field (smaller than required for insulator) is required to push the electrons from the valence band to conduction band. At low temperatures, the valence band is completely filled and the conduction band is completely empty. Therefore a semiconductor virtually behaves as an insulator at low temperature. However even at room temperature some electrons crossover to the conduction band giving conductivity to the semiconductor. As temperature increases, the number of electrons crossing over to the conduction band increases and hence electrical conductivity increases. Hence a semiconductor has negative temperature coefficient of resistance. 4 Classifications of Semiconductors Intrinsic Semiconductor: A pure semiconductor is called intrinsic semiconductor. Extrinsic Semiconductor: Due to the poor conduction at room temperature, the intrinsic semiconductor, as such, is not useful in the electronic devices. Hence the current conduction capability of the intrinsic semiconductor should be increased. This can be achieved by adding a small amount of impurity to the intrinsic semiconductor, so that it becomes impurity semiconductor or extrinsic semiconductor. This process of adding impurity is known as doping. 5 N-type Semiconductor: A small amount of pentavalent impurities such as arsenic, antimony or phosphorus is added to the pure semiconductor (germanium or silicon crystal) to get N-type semiconductor. Thus, the addition of pentavalent impurity increases the number of electrons in the conduction band thereby increasing the conductivity of N-type semiconductor. As a result of doping, the number of free electrons far exceeds the number of holes in an N-type semiconductor. So electrons are called majority carriers and holes are called minority carriers P-type Semiconductor: A small amount of trivalent impurities such as aluminum, Gallium or boron is added to the pure semiconductor to get the P-type semiconductor. The number of holes is very much greater than the number of free electrons in a P-type material, holes are termed as majority carriers and electrons as minority carriers. 6  THEORY OF PN JUNCTION DIODE In a piece of semiconductor material, if one half is doped by P- type impurity and the other half is doped by N-type impurity, a PN junction is formed. The plane dividing the two halves or zones is called PN junction. As shown in Fig., the N-type material has high concentration of free electrons while P-type material has high concentration of holes. Therefore at the junction there is a tendency for the free electrons to diffuse over to the P-side and holes to the N-side. This process is called diffusion. 7 As the free electrons move across the junction from N-type to P- type, the donor ions become positively charged. Hence a positive charge is built. on the N-side of the junction. The free electrons that cross the junction uncover the negative acceptor ions by filling in the holes. Therefore a net negative charge is established on the P-side of the junction. This net negative charge on the P-side prevents further diffusion of electrons into the P-side. Similarly, the net positive charge on the N-side repels the holes crossing from P- side to N-side. Thus a barrier is set up near the junction which prevents further movement of charge carriers, i.e. electrons and holes. This is called potential barrier or junction barrier V0. V0 is 0.3 V for germanium and 0.72 V for silicon. The electrostatic field across the junction caused by the positively charged N-type region tends to drive the holes away from the junction and negatively charged P-type region tends to drive the electrons away from the junction. Thus the junction region is depleted to mobile charge carriers. Hence it is called depletion layer. 8 Under Forward Bias Condition When positive terminal of the battery is connected to the P-type and negative terminal to the N- type of the PN junction diode, the bias applied is known as forward bias. Under the forward bias condition, the applied positive potential repels the holes in P-type region so that the holes move towards the junction and the applied negative potential repels the electrons in the N-type region and the electrons move towards the junction. Eventually when the applied potential is more than the internal barrier potential, the depletion region and internal potential barrier disappear. 9 V–I Characteristics of a Diode under Forward Bias For VF > V0, the potential barrier at the junction completely disappears and hence, the holes cross the junction from P-type to N-type and the electrons cross the junction in the opposite direction, resulting in relatively large current flow in the external circuit. 10 Under Reverse Bias Condition When the negative terminal of the battery is connected to the P-type and positive terminal of the battery is connected to the N-type of the PN junction, the bias applied is known as reverse bias. Under applied reverse bias, holes which form the majority carriers of the P-side move towards the negative terminal of the battery and electrons which form the majority carrier of the N-side are attracted towards the positive terminal of the battery. Hence the width of the depletion region which is depleted of mobile charge carriers increases. Thus the electric field produced by applied reverse bias, is in the same direction as the electric field of the potential barrier. Hence, the resultant potential barrier is increased, which prevents the flow of majority carriers in both directions. Therefore, theoretically no current should flow in the external circuit. But in practice, a very small current of the order of a few microamperes flows under reverse bias. 11 V–I Characteristics of a Diode under Reverse Bias For large applied reverse bias, the free electrons from the N-type moving towards the positive terminal of the battery acquire sufficient energy to move with high velocity to dislodge valence electrons from semiconductor atoms in the crystal. These newly liberated electrons, in turn, acquire sufficient energy to dislodge other parent electrons. Thus, a large number of free electrons are formed which is commonly called as an avalanche of free electrons. This leads to the breakdown of the junction leading to very large reverse current. The reverse voltage at which the junction breakdown occurs is known as breakdown voltage. 12 13 APPLICATIONS OF PN JUNCTION DIODE RECTIFIERS, CLIPPERS, CLAMPERS ect.. RECTIFIERS-Rectifier is defined as an electronic device used for converting ac voltage into dc voltage Half-wave Rectifier It converts an ac voltage into a pulsating dc voltage using only one half of the applied ac voltage. The rectifying diode conducts during one half of the ac cycle only. Figure shows the basic circuit and waveforms of a half wave rectifier. 14 15 16 17 18  BIPOLAR JUNCTION TRANSISTOR [BJT] A Bipolar Junction Transistor (BJT) is a three terminal semiconductor device in which the operation depends on the interaction of both majority and minority carriers and hence the name Bipolar. It is used in amplifier and oscillator circuits, and as a switch in digital circuits. It has wide applications in computers, satellites and other modern communication systems. 19 TRANSISTOR BIASING Usually the emitter-base junction is forward biased and collector-base junction is reverse biased. Due to the forward bias on the emitter-base junction an emitter current flows through the base into the collector. Though, the collector-base junction is reverse biased, almost the entire emitter current flows through the collector circuit. 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 Types of power converters 46 1-AC to DC Converters 1A-Diode Rectifiers: This rectifier circuit changes applied ac input voltage into a fixed dc voltage. Either a single-phase or three-phase ac signal is applied at the input. These are mainly used in electric traction and in electrochemical processes like electroplating along with in battery charging and power supply. These are also used in welding and UPS related services. 1B-Phase Controlled Rectifiers: Unlike diode rectifiers, phase-controlled rectifiers are designed to convert a fixed value of ac signal voltage into a variable dc voltage. Here line voltage operates the rectifier hence these are sometimes known as line commutated ac to dc converters. Similar to diode rectifiers, here also the applied ac signal can be a single-phase or three-phase ac signal. Its major applications are in dc drives, HVDC systems, compensators, metallurgical and chemical industries as well as in excitation systems for synchronous machines. 2-DC to DC Converters The converters that convert the dc signal of fixed frequency present at the input into a variable dc signal at the output are also known as choppers. Here the achieved output dc voltage may have a different amplitude than the source voltage. Generally, power transistors, MOSFETs, and thyristors are the semiconductor devices used for their fabrication. The output is controlled by a low power signal that controls these semiconductor devices from a control unit. 47 Here forced commutation is required to turn off the semiconductor device. Generally, in low power circuits power transistors are used while in high power circuits thyristors are used. Choppers are classified on the basis of the type of commutation applied to them and on the basis of the direction of power flow. Some major uses of choppers are in dc drives, SMPS, subway cars, electric traction, trolley trucks, vehicles powered by battery, etc. 3-DC to AC Converters The devices that are designed to convert the dc signal into ac signal are known as inverters. The applied input is a fixed dc voltage that can be obtained from batteries but the output obtained is variable ac voltage. The voltage and frequency of the signal obtained are of variable nature. Here the semiconductor device i.e., the thyristor is turned off by using either line, load, or forced commutation. Thus, it can be said that by the use of inverters, a fixed dc voltage is changed into an ac voltage of variable frequency. Generally, the semiconductor devices used for its fabrication are power transistors, MOSFETs, IGBT, GTO, thyristors, ect Inverters mainly find applications in induction motor and synchronous motor drives along with UPS, aircraft, and space power supplies. In high voltage dc transmission system, induction heating supplies as well as low power systems of mobile nature like flashlight discharge system in photography camera to very high power industrial system. Like choppers, in inverters also conventional thyristors are used in high power applications and power transistors are used in low power applications 48 4-AC to AC Converters An ac to ac converter is designed to change the ac signal of fixed frequency into a variable ac output voltage. There are two classifications of ac to ac converters which are as follows: 4A-Cycloconverters: A cycloconverter is a device used for changing ac supply of fixed voltage and single frequency into an ac output voltage of variable voltage as well as different frequency. However, here the obtained variable ac signal frequency is lower than the frequency of the applied ac input signal. It adopts single-stage conversion. Generally, line commutation is mostly used in cycloconverters however forced or load commutated cycloconverters are also used in various applications. These mainly find applications in slow-speed large AC traction drives such as a rotary kiln, multi MW ac motor drives, etc. 4B-AC Voltage Controllers (AC voltage regulators): The converters designed to change the applied ac signal of fixed voltage into a variable ac voltage signal of the same frequency as that of input. For the operation of these controllers, two thyristors in an antiparallel arrangement are used. Line commutation is used for turning off both the devices. It offers the controlling of the output voltage by changing the firing angle delay. The major applications of ac voltage controllers are in lighting control, electronic tap changers, speed control of large fans and pumps as well. 49 Commutation Commutation is the process of turning off a conducting thyristor. There are two methods for commutation viz. natural commutation and forced commutation. Natural Commutation In natural commutation, the source of commutation voltage is the supply source itself. If the SCR is connected to an AC supply, at every end of the positive half cycle, the anode current naturally becomes zero (due to the alternating nature of the AC Supply). As the current in the circuit goes through the natural zero, a reverse voltage is applied immediately across the SCR (due to the negative half cycle). These conditions turn OFF the SCR. This method of commutation is also called as Source Commutation or AC Line Commutation or Class F Commutation. This commutation is possible with line commutated inverters, controlled rectifiers, cyclo converters and AC voltage regulators because the supply is the AC source in all these converters. 50 Forced Commutation In case of DC circuits, there is no natural current zero to turn OFF the SCR. In such circuits, forward current must be forced to zero with an external circuit (known as Commutating Circuit) to commutate the SCR. Hence the name, Forced Commutation. This commutating circuit consist of components like inductors and capacitors and they are called Commutating Components. These commutating components cause to apply a reverse voltage across the SCR that immediately bring the current in the SCR to zero. Depending on the process for achieving zero current in the SCR and the arrangement of the commutating components, Forced Commutation is classified into different types. They are: Class A – Self Commutation by Resonating the Load Class B – Self Commutation by Resonating the Load Class C – Complementary Commutation Class D – Auxiliary Commutation Class E – Pulse Commutation This commutation is mainly used in chopper and inverter circuits. 51 Linear Voltage Regulator Electronic systems usually receive a power-supply voltage that is higher than the voltage required by the system’s circuitry. For example, a 9 V battery might be used to power an amplifier that needs an input range of 0 to 5 V, In such case, we need to regulate the input power using a component that accepts a higher voltage and produces a lower voltage. One very common way to achieve this type of regulation is to incorporate a linear voltage regulator. The simplest regulators are called 3-pin regulators, which output a stable fixed voltage just by inserting an input capacitor (CIN) between the VIN and the GND pins, and an output capacitor (COUT) between the VOUT and the GND pins. The figure below illustrates that the controlling circuit supervises the output voltage and regulates the resistance value of the variable resistor so that the IC can output the set fixed voltage. For instance, if the input voltage (VIN) is fixed, a linear regulator can maintain a stable output voltage by keeping the ratio between the variable resistance value and the load resistance value fixed according to the changing rate of the load resistance value. The input voltage is divided by the two resistors, so linear regulators generate a lower output voltage than their input voltage. 52 The difference between the higher input voltage and lower output voltage will generate heat which is called waste heat. The current flowing inside the load resistor goes on to flow to the variable resistor, where the electricity is consumed with some heat generated. 53 SMPS-Switched Mode Power Supply [SMPS] Various electrical and electronic loads are provided power using batteries. But batteries do not provide regulated power as they offer voltages of value either very high or very low. So, to obtain regulated dc output, SMPS is used. Unlike linear power supply, which uses the standard linear method of voltage regulation, a switch mode power supply is a device that performs voltage regulation of unregulated signal by using semiconductor switching methods. It is considered to be highly efficient because it lessens power consumption thereby showing a decrease in the amount of heat dissipated. Thus, has replaced traditional linear power supply units. SMPS includes a switching transistor (power MOSFET) for the purpose of voltage regulation. During operation, the transistor switches between on state and off state in a way that when it is on, it fully conducts current with the negligible voltage drop across it. While when it is off, it tries to completely block the flow of current. Thus, switching between on state (saturated) and off state (cut-off) occurs at high frequency, and in this way, the device acts as an ideal switch. It is to be noted here that if the transformer operates at high frequency, so the device size is reduced. Hence, the overall size of the SMPS is small with less weight which is another advantage over linear power supplies. 54 Block Diagram and Working of SMPS The major components that constitute SMPS are as follows: 1. Input rectifier and Filter (Diode rectifier and capacitor filter) 2. High-frequency switch (Power transistor or MOSFET) 3. Power transformer 4. Output rectifier and Filter (Diode rectifier and capacitor filter) 5. Control circuit (comparator and pulse width modulator) Initially, the unregulated ac input signal from the source is provided to the input rectifier and filter circuit. Here the ac input signal is rectified to generate a dc signal and further smoothened to remove high-frequency noise component from it. The dc output (still in unregulated form) is fed to the power transistor that acts as a high- frequency switch. 55  Here the dc signal undergoes chopping (switching). This circuit acts as an ideal switch i.e., when the power transistor (chopper circuit) is in on state, current passes through it with negligible voltage drop, and dc signal is obtained at the output terminal of the transistor. However, under the off state of the power transistor, no current passes through it and leading to cause maximal voltage drop within it. Thus, at the output side, no voltage will be present. Hence, according to the switching action of the power transistor dc voltage will be obtained at its output side. The chopping frequency plays a crucial role in maintaining the desired dc voltage level. The obtained dc signal at the output of the chopper circuit is then fed to the primary winding of the high-frequency power transformer. Here the step-down transformer converts the high voltage signal into a low voltage level which is further provided as input to the output rectifier and filter unit. This simply filters out the unwanted residuals from the signal in order to provide a regulated dc signal as the output. The control circuitry present here acts as the feedback circuit for the complete unit. This involves a comparator along with a pulse width modulator (PWM). The dc output from the rectifier and filter is fed to the control circuit where the error amplifier which acts as a comparator, compares the obtained dc voltage with the reference value. 56  If the dc output is greater than the reference value then the chopping frequency is to be decreased. The decrease in chopping frequency will reduce the output power and so the dc output voltage. However, if the dc output is less than the reference value then the chopping frequency is increased. When chopping frequency is raised then the dc output voltage will get increased. The pulse width modulator in the above circuit is responsible for generating a fixed frequency pulse width modulated waveform whose duty cycle controls the chopping frequency. Basically, the duty ratio is the ratio of on-time to the overall cycle time (i.e., on + off) time. Hence, by making necessary adjustments in the width of the pulses, the chopping frequency gets adjusted hence, regulated dc output can be obtained. Advantages 1. It is highly efficient than linear power supplies. Typically, the efficiency of SMPS lies between 60% – 95%. 2. Due to the high-frequency operation of the device, the overall size is small and less bulky. Thus, is compact. 3. It is inexpensive because heat dissipation is less. 4. The obtained output voltage can be more or less than the supply input. 57 Disadvantages 1. The transient spike generation due to switching action is one of the major issues. This may lead to cause RF interference thus, isolation is mandatory. 2. The circuit is complex. Also, voltage regulation (controlling) is tricky. 3. Proper filtration is necessary to deal with noise and spikes. Applications of SMPS The devices invented under the latest technologies require a highly efficient power supply which is offered by SMPS. Thus, it finds applications in various power amplifiers, personal computers, security and railway systems, television sets, motor drives, etc. 58