Lecture Notes on Power Electronics

lecture notes on power electronics and drives and lecture notes on advanced power electronics, what is electronics power supply what is power electronics and its applications
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VEER SURENDRA SAI UNIVERSITY OF TECHNOLOGY BURLA, ODISHA, INDIA DEPARTMENT OF ELECTRICAL ENGINEERING Lecture Notes on Power Electronics Subject code – BEE1602 6th Semester B.Tech. (Electrical Engineering) Disclaimer This document does not claim any originality and cannot be used as a substitute for prescribed textbooks. The information presented here is merely a collection by the committee members for their respective teaching assignments. Various sources as mentioned at the end of the document as well as freely available material from internet were consulted for preparing this document. The ownership of the information lies with the respective authors or institutions. Further, this document is not intended to be used for commercial purpose and the committee members are not accountable for any issues, legal or otherwise, arising out of use of this document. The committee members make no representations or warranties with respect to the accuracy or completeness of the contents of this document and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. The committee members shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. (6TH SEMESTER) POWER ELECTRONICS (3-1-0) MODULE-I (10 HOURS) Thyristors, Static V-I Characteristics of SCR, TRIAC, GTO & IGBT, Turn-On & Turn-OFF Mechanism of SCR, Gate Turnoff Thyristor (GTO) .Power BJTs . Power MOSFETs - Insulated Gate Bipolar Transistors (IGBTs) - Basic Structure and VI Characteristics. Static, dynamic and thermal characteristics. Protection, cooling and mounting techniques. Series and Parallel operation of devices. Triggering and basics of driver circuits. Different types of commutation schemes: Natural and Forced commutation. MODULE-II (10 HOURS) 1-Phase Half & Full Wave Controlled Rectifier with various kinds of loads (R, R-L-E (motor)). Midpoint and Bridge type converters. Half Controlled and Fully Controlled Bridge circuits, different waveforms, Input Line Current Harmonics, Power factor, current distortion and displacement factors- Inverter Mode of Operation. Continuous and discontinuous modes, Effect of source inductance assuming constant load current. Effect of freewheeling diode. Three phase bridge converters for different types of load with constant load current, different waveforms. 180 and 120 degree operations. MODULE-III (10 HOURS) DC-DC Converters: Classification of types of choppers, One, Two and Four quadrant operations, Step up and down choppers, Analysis of Type-A chopper, Single-and two quadrant operation with DC motor load. AC-AC Converters: Single-phase mid-point and bridge types of step-up and step-down Cycloconverters. Single phase AC Voltage regulators and its basic analysis. MODULE-IV (10 HOURS) Single-phase Half and Full bridge Inverter, Pulse Width Modulated (PWM) technique for voltage control, SPWM Technique 1-phase inverters, Auxiliary Commutated (Mc-Murray) and Complementary Commutated (Mc-Murray Bedford) Inverters, Three-phase Voltage Source Bridge type of Inverters. (120 and 180 Degree conduction modes), Current Source Inverter. Applications: UPS, SMPS, Induction Heating, Electronic Ballast, AC/DC drives speed control. MODULE - 1 POWER ELECTRONICS The control of electric motor drives requires control of electric power. Power electronics have eased the concept of power control. Power electronics signifies the word power electronics and control or we can say the electronic that deal with power equipment for power control. Main power source Ref signal Control Digital Power Load Circuit Electronic Circuit circuit Feedback Signal Power electronics based on the switching of power semiconductor devices. With the development of power semiconductor technology, the power handling capabilities and switching speed of power devices have been improved tremendously. Power Semiconductor Devices The first SCR was developed in late 1957. Power semiconductor devices are broadly categorized into 3 types: 1. Power diodes (600V,4500A) 2. Transistors 3. Thyristors (10KV,300A,30MW) Thyristor is a four layer three junction pnpn semiconductor switching device. It has 3 terminals these are anode, cathode and gate. SCRs are solid state device, so they are compact, possess high reliability and have low loss. SCR is made up of silicon, it act as a rectifier; it has very low resistance in the forward direction and high resistance in the reverse direction. It is a unidirectional device. Static V-I characteristics of a Thyristor The circuit diagram for obtaining static V-I characteristics is as shown Anode and cathode are connected to main source voltage through the load. The gate and cathode are fed from source 𝐸 . 𝑆 A typical SCR V-I characteristic is as shown below: 𝑉 =Forward breakover voltage 𝑉 =Reverse breakover voltage 𝐼 =Gate current 𝑔 𝑉 =Anode voltage across the thyristor terminal A,K. π‘Ž 𝐼 =Anode current π‘Ž It can be inferred from the static V-I characteristic of SCR. SCR have 3 modes of operation: 1. Reverse blocking mode 2. Forward blocking mode ( off state) 3. Forward conduction mode (on state) 1. Reverse Blocking Mode When cathode of the thyristor is made positive with respect to anode with switch open thyristor is reverse biased. Junctions 𝐽 and 𝐽 are reverse biased where junction 𝐽 is 1 2 2 forward biased. The device behaves as if two diodes are connected in series with reverse voltage applied across them. ο‚· A small leakage current of the order of few mA only flows. As the thyristor is reverse biased and in blocking mode. It is called as acting in reverse blocking mode of operation. ο‚· Now if the reverse voltage is increased, at a critical breakdown level called reverse breakdown voltage 𝑉 ,an avalanche occurs at 𝐽 and 𝐽 and the reverse 1 3 𝐡𝑅 𝐡𝑅 𝐡𝑂current increases rapidly. As a large current associated with 𝑉 and hence more losses to the SCR. This results in Thyristor damage as junction temperature may exceed its maximum temperature rise. 2. Forward Blocking Mode When anode is positive with respect to cathode, with gate circuit open, thyristor is said to be forward biased. Thus junction 𝐽 and 𝐽 are forward biased and 𝐽 is reverse biased. As the forward 1 3 2 voltage is increases junction 𝐽 will have an avalanche breakdown at a voltage called 2 forward breakover voltage𝑉 . When forward voltage is less then 𝑉 thyristor offers high impedance. Thus a thyristor acts as an open switch in forward blocking mode. 3. Forward Conduction Mode Here thyristor conducts current from anode to cathode with a very small voltage drop across it. So a thyristor can be brought from forward blocking mode to forward conducting mode: 1. By exceeding the forward breakover voltage. 2. By applying a gate pulse between gate and cathode. During forward conduction mode of operation thyristor is in on state and behave like a close switch. Voltage drop is of the order of 1 to 2mV. This small voltage drop is due to ohmic drop across the four layers of the device. Different turn ON methods for SCR 1. Forward voltage triggering 2. Gate triggering 3. triggering 4. Light triggering 5. Temperature triggering 1. Forward voltage triggering 𝑑𝑑 𝑑𝑣 𝐡𝑂 𝐡𝑂 𝐡𝑅 A forward voltage is applied between anode and cathode with gate circuit open. ο‚· Junction 𝐽 and 𝐽 is forward biased. 1 3 ο‚· Juntion 𝐽 is reverse biased. 2 ο‚· As the anode to cathode voltage is increased breakdown of the reverse biased junction 𝐽 occurs. This is known as avalanche breakdown and the voltage at which this 2 phenomena occurs is called forward breakover voltage. ο‚· The conduction of current continues even if the anode cathode voltage reduces below 𝑉 till 𝐼 will not go below𝐼 . Where 𝐼 is the holding current for the thyristor. π‘Ž β„Ž β„Ž 2. Gate triggering This is the simplest, reliable and efficient method of firing the forward biased SCRs. First SCR is forward biased. Then a positive gate voltage is applied between gate and cathode. In practice the transition from OFF state to ON state by exceeding 𝑉 is never employed as it may destroy the device. The magnitude of 𝑉 , so forward breakover voltage is taken as final voltage rating of the device during the design of SCR application. First step is to choose a thyristor with forward breakover voltage (say 800V) higher than the normal working voltage. The benefit is that the thyristor will be in blocking state with normal working voltage applied across the anode and cathode with gate open. When we require the turning ON of a SCR a positive gate voltage between gate and cathode is applied. The point to be noted that cathode n- layer is heavily doped as compared to gate p-layer. So when gate supply is given between gate and cathode gate p-layer is flooded with electron from cathode n-layer. Now the thyristor is forward biased, so some of these electron reach junction 𝐽 .As a 2 result width of 𝐽 breaks down or conduction at 𝐽 occur at a voltage less than 𝑉 .As 𝐼 2 2 𝑔 increases 𝑉 reduces which decreases then turn ON time. Another important point is duration for which the gate current is applied should be more then turn ON time. This means 𝐡𝑂 𝐡𝑂 𝐡𝑂 𝐡𝑂 𝐡𝑂that if the gate current is reduced to zero before the anode current reaches a minimum value known as holding current, SCR can’t turn ON. In this process power loss is less and also low applied voltage is required for triggering. 3. dv/dt triggering This is a turning ON method but it may lead to destruction of SCR and so it must be avoided. When SCR is forward biased, junction 𝐽 and 𝐽 are forward biased and junction 𝐽 is 1 3 2 reversed biased so it behaves as if an insulator is place between two conducting plate. Here 𝐽 1 and 𝐽 acts as a conducting plate and 𝐽 acts as an insulator. 𝐽 is known as junction capacitor. 3 2 2 So if we increase the rate of change of forward voltage instead of increasing the magnitude of voltage. Junction 𝐽 breaks and starts conducting. A high value of changing current may 2 damage the SCR. So SCR may be protected from high . π‘ž = 𝑐𝑣 𝑑𝑣 𝐼 = 𝑐 π‘Ž 𝑑𝑑 𝑑𝑣 𝐼 𝛼 π‘Ž 𝑑𝑑 4. Temperature triggering During forward biased, 𝐽 is reverse biased so a leakage forward current always associated 2 with SCR. Now as we know the leakage current is temperature dependant, so if we increase the temperature the leakage current will also increase and heat dissipitation of junction 𝐽 occurs. When this heat reaches a sufficient value 𝐽 will break and conduction starts. 2 2 Disadvantages 𝑑𝑑 𝑑𝑣This type of triggering causes local hot spot and may cause thermal run away of the device. This triggering cannot be controlled easily. It is very costly as protection is costly. 5. Light triggering First a new recess niche is made in the inner p-layer. When this recess is irradiated, then free charge carriers (electron and hole) are generated. Now if the intensity is increased above a certain value then it leads to turn ON of SCR. Such SCR are known as Light activated SCR (LASCR). Some definitions: Latching current The latching current may be defined as the minimum value of anode current which at must attain during turn ON process to maintain conduction even if gate signal is removed. Holding current It is the minimum value of anode current below which if it falls, the SCR will turn OFF. Switching characteristics of thyristors The time variation of voltage across the thyristor and current through it during turn on and turn off process gives the dynamic or switching characteristic of SCR. Switching characteristic during turn on Turn on time It is the time during which it changes from forward blocking state to ON state. Total turn on time is divided into 3 intervals: 1. Delay time 2. Rise time 3. Spread time Delay time If 𝐼 and 𝐼 represent the final value of gate current and anode current. Then the delay time 𝑔 π‘Ž can be explained as time during which the gate current attains 0.9 𝐼 to the instant anode 𝑔 current reaches 0.1 𝐼 or the anode current rises from forward leakage current to 0.1 𝐼 . 𝑔 π‘Ž 1. Gate current 0.9 𝐼 to 0.1 𝐼 . 𝑔 π‘Ž 2. Anode voltage falls from 𝑉 to 0.9𝑉 . π‘Ž π‘Ž 3. Anode current rises from forward leakage current to 0.1 𝐼 . π‘Ž Rise time (𝒕 ) 𝒓 Time during which 1. Anode current rises from 0.1 𝐼 to 0.9 𝐼 π‘Ž π‘Ž 2. Forward blocking voltage falls from 0.9𝑉 to 0.1𝑉 . 𝑉 is the initial forward blocking π‘Ž π‘Ž π‘Ž voltage. Spread time (𝒕 ) 𝒑 1. Time taken by the anode current to rise from 0.9𝐼 to𝐼 . π‘Ž π‘Ž 2. Time for the forward voltage to fall from 0.1𝑉 to on state voltage drop of 1 to 1.5V. π‘œ During turn on, SCR is considered to be a charge controlled device. A certain amount of charge is injected in the gate region to begin conduction. So higher the magnitude of gate current it requires less time to inject the charges. Thus turn on time is reduced by using large magnitude of gate current. How the distribution of charge occurs? As the gate current begins to flow from gate to cathode with the application of gate signal. Gate current has a non uniform distribution of current density over the cathode surface. Distribution of current density is much higher near the gate. The density decrease as the distance from the gate increases. So anode current flows in a narrow region near gate where gate current densities are highest. From the beginning of rise time the anode current starts spreading itself. The anode current spread at a rate of 0.1mm/sec. The spreading anode current requires some time if the rise time is not sufficient then the anode current cannot spread over the entire region of cathode. Now a large anode current is applied and also a large anode current flowing through the SCR. As a result turn on losses is high. As these losses occur over a small conducting region so local hot spots may form and it may damage the device. Switching Characteristics During Turn Off Thyristor turn off means it changed from ON to OFF state. Once thyristor is oON there is no role of gate. As we know thyristor can be made turn OFF by reducing the anode current below the latching current. Here we assume the latching current to be zero ampere. If a forward voltage is applied across the SCR at the moment it reaches zero then SCR will not be able to block this forward voltage. Because the charges trapped in the 4- layer are still favourable for conduction and it may turn on the device. So to avoid such a case, SCR is reverse biased for some time even if the anode current has reached to zero. So now the turn off time can be different as the instant anode current becomes zero to the instant when SCR regains its forward blocking capability. 𝑑 =𝑑 +𝑑 π‘ž π‘Ÿπ‘Ÿ π‘žπ‘Ÿ Where, 𝑑 is the turn off time,𝑑 is the reverse recovery time, 𝑑 is the gate recovery time π‘ž π‘Ÿπ‘Ÿ π‘žπ‘Ÿ At 𝑑 anode current is zero. Now anode current builds up in reverse direction with same 1 slope. This is due to the presence of charge carriers in the four layers. The reverse recovery current removes the excess carriers from 𝐽 and 𝐽 between the instants 𝑑 and𝑑 . 1 3 1 3 At instant 𝑑 the end junction 𝐽 and 𝐽 is recovered. But 𝐽 still has trapped charges which 3 1 3 2 decay due to recombination only so the reverse voltage has to be maintained for some more time. The time taken for the recombination of charges between 𝑑 and 𝑑 is called 3 4 gate recovery time 𝑑 . Junction 𝐽 recovered and now a forward voltage can be applied π‘žπ‘Ÿ 2 across SCR. The turn off time is affected by: 1. Junction temperature 𝑑𝑖 2. Magnitude of forward current during commutation. Turn off time decreases with the increase of magnitude of reverse applied voltage. GTO(Gate turn off thyristor) 𝑑𝑑 𝑑𝑑 𝑑𝑣A gate turn off thyristor is a pnpn device. In which it can be turned ON like an ordinary SCR by a positive gate current. However it can be easily turned off by a negative gate pulse of appropriate magnitude. Conventional SCR are turned on by a positive gate signal but once the SCR is turned on gate loses control over it. So to turn it off we require external commutation circuit. These commutation circuits are bulky and costly. So due to these drawbacks GTO comes into existence. The salient features of GTO are: 1. GTO turned on like conventional SCR and is turned off by a negative gate signal of sufficient magnitude. 2. It is a non latching device. 3. GTO reduces acoustic and electromagnetic noise. It has high switching frequency and efficiency. A gate turn off thyristor can turn on like an ordinary thyristor but it is turn off by negative gate pulse of appropriate magnitude. Disadvantage The negative gate current required to turn off a GTO is quite large that is 20% to 30 % of anode current Advantage It is compact and cost less Switching performance 1. For turning ON a GTO first TR1is turned on. 2. This in turn switches on TR2 so that a positive gate current pulse is applied to turn on the GTO. 3. Thyristor 𝑇 is used to apply a high peak negative gate current pulse. 1 Gate turn-on characteristics 1. The gate turn on characteristics is similar to a thyristor. Total turn on time consists of delay time, rise time, spread time. 2. The turn on time can be reduced by increasing its forward gate current. GATE TURN OFF Turn off time is different for SCR.Turn off characteristics is divied into 3 pd 1. Storage time 2. Fall time 3. Tail time Tq=ts+tf+tt At normal operating condition gto carries a steady state current.The turn off process starts as soon as negative current is applied after t=0. STORAGE TIME During the storagepd the anode voltage and current remains constant.The gate current rises depending upon the gate circuit impedance and gate applied voltage.The beginning of pd is as soon as negative gate current is applied.The end of storage pd is marked by fall in anode current and rise in voltage,what we have to do is remove the excess carriers.the excess carriers are removed by negative carriers. FALL TIME After ts, anode current begins to fall rapidly and anode voltage starts rising.After falling to a certain value,then anode current changes its rate to fall.this time is called fall time. SPIKE IN VOLTAGE During the time of storage and fall timethere is achange in voltage due to abrupt current change. TAIL TIME During this time ,the anode current and voltage continues towards the turn off values.The transient overshoot is due to the snubber parameter and voltage stabilizes to steady state value. THE TRIAC As SCR is a unidirectional device,the conduction is from anode to cathode and not from cathode to anode. It conducts in both direction.It is a bidirectional SCR with three terminal. TRIAC=TRIODE+AC Here it is considered to be two SCRS connected in anti parallel.As it conducts in both direction so it is named as MT1,MT2 and gate G. SALIENT FEATURES 1.Bi directional triode thyristor 2.TRIAC means triode that works on ac 3.It conduct in both direction 4.It is a controlled device 5.Its operation is similar to two devices connected in anti parallel with common gate connection. 6.It has 3 terminals MT1,MT2 and gate G Its use is control of power in ac. POWER BJT Power BJT means a large voltage blocking in the OFF state and high current carrying capability in the ON state. In most power application, base is the input terminal. Emitter is the common terminal. Collector is the output terminal. SIGNAL LEVEL OF BJT n+ doped emitter layer ,doping of base is more than collector.Depletion layer exists more towards the collector than emitter

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