ELECTRICAL AND POWER ELECTRONICS NOTE.pptx

AN ADVANCED NOTE ON POWER

1. POWER ELECTRonICS ELE436

2. What is power electronics Power electronic is the application of solid state devices or electronics to the control and conversion of electric power, such devices can be diodes, thyristors, power transistors such as BJJ, Power MOSFET and IGBT.

3. What is power electronics The major components of power electronic system is shown in form of block diagram below The 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 .

4. Power electronic system Power electronic can be use to change the characteristics of V&I either converting AC to DC (diode rectifier) or DC to AC (inverter) or change AC to another level of AC ( cyclo conveter ) or DC to DC (chopper) e.g

5. Application of power electronics Aerospace: space shuttle power supply, satellite power supply and aircraft power system. Commercial: advertising, heating, air conditioning, central refrigerators, computer and office equipment, uninterruptible power supply etc Telecommunication: battery chargers, power supply (dc and ups) Transportation: Due to the continuous increase in the world wide economy there is a more demand for transportation which is the most signification source of carbon emission and in order to reduce that

6. Application of power electronics the concept of using electric systems to replace other systems that utilize convention of energy resource such as fossil fuels the subsystems utilizing, hydraulic, mechanical and prieumatic power that were previously used in conventional airplanes are replaces by electrical systems and such enhance the reduction of: Fuel consumption Operating Cost Weight Noise Size of equipment Such advancement result to the production of Electric air craft Electric air ship Electric air vehicle

7. Power Electronics contributes in the advancement of industrial technology in the following ways Grid Modernization: This is a process whereby power quality/control is enhance and renewable generation integration using solid state transformer. Minimized power loss and condition and interface for increasing intelligent and complex energy and transport system Proper time management: Since this components time is reduced Full automation is obtained: Jobs that were done manually, can now be Automated. Enhance safety: Since this component can handle high power and voltage, they are used in conjunction with interfacing devices to enhance control

8. potentiality of power diodes in modern technology Due to the quest for control of equipments , there is a need to get active components and one of the earlier type is the diode which emerged as a result of bring together to metalloid materials P and N to form a PN Junction device called a diode and after further research it is discovered that our semiconductor components can be gotten from the diode e.g , thyristor, transistor, integrated circuit etc.

9. ideal characteristics of a power device Breakdown voltage is ɱ is 100% PF is unity (1) T on & T off = 0 Power handling =  

10. Power semiconductor devices Based on turn-on and turn-off characteristics and Gate signal requirements the power semiconductor device can be classified into Diodes: these are uncontrolled rectifying devices. There on and off are controlled by power supply Thyristors: these have control turn on by a gate signal. Controllable switches: this devices are turn-on and turn-off by a control signal. These devices are BJT, MOSFET, IGBT etc

11. Types of power electronics converter A power electronics converters is made up of some power semiconductors devices controlled by integrated circuits. They include Diode rectifiers AC to DC converters( phased control) DC to DC converters (DC choppers) DC to AC converters( inverters) AC to AC converters( AC voltage regulators and cycloconverters Static switches

12. Power diode Power diodes are similar to low-power p-n junction diodes called signal diodes but the voltage, current and power rating of power diodes are much higher than the signal diodes.

13. (a). The P-and N-type regions are referred to as anode and cathode respectively. (b). arrow-head indicates the conventional direction of current flow when forward-biased. It is the same direction in which hole flow takes place. Commercially available diodes usually have some means to indicate which lead is P and which lead is N. Standard notation consists of type numbers preceded by ‘IN’ such as IN 240 and IN 1250.

14. Working A P-N junction diode is one-way device offering low resistance when forward-biased and behaving almost as an insulator when reverse-biased. Hence, such diodes are mostly used as rectifiers i.e. for converting alternating current into direct current .

15. I/V characteristics Forward Characteristic When the diode is forward-biased and the applied voltage is increased from zero, hardly any current flows through the device in the beginning. It is so because the external voltage is being opposed by the internal barrier voltage V B whose value is 0.7 V for Si and 0.3 V for Ge.

16. I/V characteristics Forward Characteristic As soon as V B is neutralized, current through the diode increases rapidly with increasing applied battery voltage. It is found that as little a voltage as 1.0 V produces a forward current of about 50 mA. A burnout is likely to occur if forward voltage is increased beyond a certain safe limit

17. I/V characteristics Reverse Characteristic When the diode is reverse-biased, majority carriers are blocked and only a small current (due to minority carriers) flows through the diode. As the reverse voltage is increased from zero, the reverse current very quickly reaches its maximum or saturation value I0 which is also known as leakage current.

18. I/V characteristics Reverse Characteristic It is of the order of nanoamperes ( nA ) for Si and microamperes ( μA ) for Ge. The value of I (or I s ) is independent of the applied reverse voltage but depends on temperature, degree of doping and physical size of the junction.

19. The forward and reverse characteristics have been combined into a single diagram below

20. Load Line Analysis Let’s see how to use a diode in a circuit Use KVL for this circuit V ss = Ri D + v D This equation is plotted on the same graph as the diode VI characteristics.

21. Load Line Analysis

22. Ideal Diode Basically, a switch – Forward Bias: any current allowed, diode on – Reverse Bias: zero current, diode off – No reverse breakdown region

23. Equation of the Static Characteristic These characteristics can be described by the analytical equation called Boltzmann diode equation given below where I = diode reverse saturation current V = voltage across junction − positive for forward bias and negative for reverse bias. k = Boltzmann constant = 1.38 × 10−23 J/ºK T = crystal temperature in ºK η = 1 – for germanium = 2 – for silicon

24. Equation of the Static Characteristic Hence, the above diode equation becomes

25. Diode Parameters The diode parameters of greatest interest are as under : 1. Bulk resistance ( r B ) It is the sum of the resistance values of the P-and N-type semiconductor materials of which the diode is made of Usually, it is very small. It is given by It is the resistance offered by the diode well above the barrier voltage i.e. when current is large. Obviously, this resistance is offered in the forward direction.

26. Diode Parameters 2. Junction resistance ( r J ) Its value for forward-biased junction depends on the magnitude of forward dc current. rj = 25 mV/I F (mA) – for Ge = 50 mV/I F (mA) – for Si Obviously, it is a variable resistance. 3. Dynamic or ac resistance 4. Forward voltage drop It is given by the relation forward voltage drop = power dissipated/forward dc current

27. Diode Parameters 5. Reverse saturation current ( I ). 6. Reverse breakdown voltage ( V BR ). Reverse dc resistance R R = reverse voltage / reverse current EXAMPLE Using analytical expression for diode current, calculate the dynamic slope resistance of a germanium diode at 290 K when forward biased at current of ( i ) 10μA and (ii) 5 mA. SOLUTION T= 290K, Is = ( i ) 10 (ii) 5mA I = = dI =  

28. Diode Parameters I = 10 = 10 x 10 -6 = 10 -5 A = 2500 I = 5mA = 5 x 10 -3 A = 5  

29. Diode Parameters Example2 Using approximate Boltzmann’s diode equation, find the change in forward bias for doubling the forward current of Germanium semiconductors at 290K. T = 290K, Change in forward bias ∆V for doubling the forward current of a germanium semiconductor at 290K is I = but let I s = I o and q = e I 1 = = and I 2 = when n = 1 or Since I2 = 2I 1 or  

30. Power transistors Power transistors are devices that turned on when a current signal is given to the base, or control terminal. The transistor remains in the on-state so long as the signal is present. When this control signal is removed the power transistor turned off. Types of power transistors Bipolar Junction Transistor (BJT) Metal-oxide-semiconductor field-effect transistors (MOSFETs) Insulated gate bipolar transistors (IGBTs)

31. Bipolar junction transistors The term ‘bipolar’ denotes that current flow in the device is due to the movement of both the holes and electrons. Consider the diagram bellow

32. Bipolar junction transistors A BJT has three terminals named collector, emitter and base. An emitter is indicated by an arrowhead indicating the direction of emitter current. No arrow is associated with base or collector. steady-state characteristics : out of the three possible circuit configurations, for a transistor, common-emitter arrangement is more common in switching application. So npn transistor will be considered

33. Input characteristics Consider the diagram below

34. Input characteristics As seen in the graph, when collector-emitter voltage V CE2 is more than V CE1 base current decreases. the graph is a input characteristics between base current and base emitter voltage OUTPUT CHARACTERISTICS a graph between collector current I C and collector emitter voltage V CE gives the output characteristic of transistor. For zero base current i.e I B =0 as is V CE increased, a small collector current, exist as shown in the figure below. The base current is increased from I B =0 to I B1 I B2 etc , the collector current also raises as shown in Fig. 2.4(c)

35. Input characteristics Fig 2.5(a) shows two of the output characteristics curves, 1 for I B =0 and 2 for I b not = 0. the initial part of curve 2, characterized by low V CE is called the saturation region. In this region the transistor acts like a switch. The flat part of the curve 2, indicated V CE by increasing and almost constant I C is the active region. In this region, transistor acts like an amplifier. Almost vertically rising curve in the breakdown region which must be avoided at all costs .

36. Input characteristics

37. Equation of load line This is the equation of load line. It is shown as line AB in fig. 2.5 (a). A load line is the locus of all possible operating points. Ideally, when transistor is on, V CE is zero and This collector current is shown by point A on the vertical axis. When the transistor is off, or in the cut-off region, V CC appears across collector-emitter terminals and there is no collector current. This value is indicated by point B on the horizontal axis.

38. Relationship between α & β Collector current though less than emitter current is almost equal to emitter current. A symbol α is used to indicate how close in value these two currents are. Here α , called forward current gain, is defined as The ratio of collector (output) current to base (input) current is known as current gain β

39. As IB is much smaller, beta is much more than unity; its value varies from 50 to 300. in another system of analysis, called h parameters, hFE is used in the place of beta.

40. Transistor switching performance Transistor with resistive load Switching waveforms for npn transistor

41. Transistor switching performance (a) When base-emitter voltage VBE is applied, the base current rises to IBS; the collector current, however remains zero or equal to the collector leakage current ICE as shown. After some time delay td, called delay time, the collector current begins to rise. This delay is due to the time required to charge base-emitter capacitance VBES=0.7. after this delay td, collector current rises to the steady state value Ics in time tr, which is known as rise time. Therefore turn-on time for BJT is ton= td + tr

42. Power mosfets A MOSFET has three terminals called drain, source and gate in place of the corresponding three terminals collector, emitter and base for BJT. Circuit symbol of a power MOSFET Here arrow indicates the direction of electron flow. A BJT is a current controlled device whereas a power MOSFET is a voltage-controlled device. As it operation depends upon the flow of majority carriers only, MOSFET is a unipolar device.

43. Mosfet characteristics The basic circuit diagram for n-channel power MOSFET is shown below It is seen from the graph that there is a threshold voltage Vgst below which the device is off. the magnitude of Vgst is of the order of2to 3 V.

44. Output characteristics Power MOSFET output characteristics indicates the variation of the drain current Id as a function of drain-source voltage as a parameter. For low value of Vds , the graph between Id – Vds is almost linear; this indicates a constant value of on-resistance. For given Vgs , if Vds is increased, the output characteristics relatively flat indicating that drain current is nearly constant. A load line intersects the output characteristics at A and B. Here A indicates fully-on condition and B fully-off state.

45. Switching characteristics. The switching characteristic is influenced by Internal capacitance of the device. Internal impedance of the gate drive circuit. Total turn on time is divided into 1.Turn on delay time 2.Rise time Turn on time is affected by impedance of gate drive source. During turn on delay time gate to source voltage attends its threshold value 𝑉𝐺𝑆𝑇. After 𝑡𝑑𝑛 and during rise time gate to source voltage rise to 𝑉𝐺𝑠𝑝, a voltage which is sufficient to drive the MOSFET to ON state. The turn off process is initiated by removing the gate to source voltage. Turn off time is composed of turn off delay time to fall time.

46. Switching characteristics. Turn off delay time To turn off the MOSFET the input capacitance has to be discharged . During 𝑡𝑑𝑓 the input capacitance discharge from 𝑉1to 𝑉𝐺𝑠𝑝. During 𝑡𝑓 , fall time ,the input capacitance discharges from 𝑉𝐺𝑠𝑝 to 𝑉𝐺𝑆𝑇. During 𝑡𝑓 drain current falls from 𝐼𝐷 to zero. So when 𝑉𝐺𝑠 ≤ 𝑉𝐺𝑆𝑇 , MOFSET turn off is complete. Fig. Switching waveform of power MOSFET

47. Comparison of mosfet with bjt Power MOSFET has lower switching losses but its on-state and conduction losses are more. A BJT has higher switching losses but lower conduction loss. Power MOSFET is voltage control device whereas BJT is current controlled device. MOSFET has positive temperature coefficient for resistance. This makes the parallel operation of MOSFETs easy. A BJT has negative temperature coefficient, so current-sharing resistors 4. Power MOSFETs in higher voltage ratings have more conduction loss 5. The state of the art MOSFETs are available with ratings upto 500 V, 140A whereas BJTs are available with ratings up to1200 V, 800 A

48. Insulated gate bipolar transistor ( igbt ) IGBT is a new development in the area of power MOSFET technology. This device combined the advantage of both MOSFET and BJT. So IGBT has high input impedance like MOSFET and low-state-on power lost like BJT. IGBT Characteristics Here the controlling parameter is gate emitter voltage As IGBT is a voltage controlled device. When 𝑉𝐺𝐸 is less than 𝑉𝐺𝐸𝑇 that is gate emitter threshold voltage IGBT is in off state.

49. Switching characteristics Figure below shows the turn ON and turn OFF characteristics of IGBT

50. Switching characteristics Turn on time Time between the instants forward blocking state to forward on -state . Turn on time = Delay time + Rise time Delay time = Time for collector emitter voltage fall from 𝑉𝐶𝐸 to 0.9𝑉𝐶𝐸 𝑉𝐶𝐸=Initial collector emitter voltage 𝑡𝑑𝑛=collector current to rise from initial leakage current to 0.1Ic Ic = Final value of collector current Rise time Collector emitter voltage to fall from 0.9𝑉𝐶𝐸 to 0.1𝑉𝐶𝐸. 0.1Ic to Ic After 𝑡𝑜𝑛 the device is on state the device carries a steady current of Ic and the collector emitter voltage falls to a small value called conduction drop 𝑉𝐶𝐸

51. Switching characteristics Turn off time 1) Delay time 𝑡𝑑𝑓 2) Initial fall time 𝑡𝑓1 3) Final fall time 𝑡𝑓2 𝑡𝑂𝑓𝑓 =𝑡𝑑𝑓 + 𝑡𝑓1+ 𝑡𝑓2 𝑡𝑑𝑓 = Time during which the gate emitter voltage falls to the threshold value 𝑉𝐺𝐸𝑇. Collector current falls from Ic to 0.9Ic at the end of the 𝑡𝑑𝑓 collector emitter voltage begins to rise. Turn off time = Collector current falls from 90% to 20% of its initial value Ic OR The time during which collector emitter voltage rise from 𝑉𝐶𝐸 to 0.1𝑉𝐶𝐸. 𝑡𝑓2 = collector current falls from20% to 10% of Ic . During this collector emitter voltage rise 0.1𝑉𝐶𝐸 to final value of 𝑉𝐶�

52. IGBT AND MOSFET CHARACTERISTIC SUMMARIZED S/No IGBT MOSFET   Combination of MOSFET & BJT Single MOSFET only     Voltage rating >1kv Voltage rating<1kv   Current rating low <200A High >500A   Operating frequency < 100kHz < 1mHz   High i/p impedance High i /p impedance

53. Thyristors – Silicon Controlled Rectifiers (SCR’s) 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.

54. 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 𝐸𝑆 ..

55. MODE OF OPERATION A typical SCR V-I characteristic is as shown below

56. 𝑉𝐵𝑂=Forward breakover voltage 𝑉𝐵𝑅=Reverse breakover voltage 𝐼𝑔=Gate current 𝑉𝑎=Anode voltage across the thyristor terminal A,K. 𝐼𝑎=Anode current It can be inferred from the static V-I characteristic of SCR. SCR have 3 modes of operation: Reverse blocking mode 2. Forward blocking mode ( off state) 3. Forward conduction mode (on state)

57. Reverse Blocking Mode When cathode of the thyristor is made positive with respect to anode with switch open thyristor is reverse biased. Junctions 𝐽1 and 𝐽2 are reverse biased where 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 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. Now if the reverse voltage is increased, at a critical breakdown level called reverse breakdown voltage 𝑉𝐵𝑅,an avalanche occurs at 𝐽1 and 𝐽3 and the reverse current increases rapidly. As a large current associated with 𝑉𝐵𝑅 and hence more losses to the SCR. This results in Thyristor damage as junction temperature may exceed its maximum temperature rise

58. Forward Blocking Mode When anode is positive with respect to cathode, with gate circuit open, thyristor is said to be forward biased. Thus junction 𝐽1 and 𝐽3 are forward biased and 𝐽2 is reverse biased. As the forward voltage is increases junction 𝐽2 will have an avalanche breakdown at a voltage called forward breakover voltage 𝑉𝐵𝑂. When forward voltage is less then 𝑉𝐵𝑂thyristor offers high impedance. Thus a thyristor acts as an open switch in forward blocking mode

59. 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.

60. Forward Blocking Mode Now considering the anode is positive with respect to the cathode, with gate kept in open condition. The thyristor is now said to be forward biased as shown the figure below. As we can see the junctions J1 and J3 are now forward biased but junction J2 goes into reverse biased condition. In this particular mode, a small current, called forward leakage current is allowed to flow initially as shown in the diagram for characteristics of thyristor. Now, if we keep on increasing the forward biased anode to cathode voltage

61. 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: 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.

62. Different turn ON methods for SCR 1.Forward voltage triggering 2. Gate triggering 3. 𝑑𝑣/𝑑𝑡 triggering 4. Light triggering 5. Temperature triggering

63. Two transistor analogy of SCR When the transistors are in off state, the relation between the collector current and emitter current is shown below

64. Two transistor analogy of SCR

65. Two transistor analogy of SCR

66. CONVERTERS RECTIFIER Rectifier are used to convert A.C to D.C supply. Rectifiers can be classified as single phase rectifier and three phase rectifier. Single phase rectifier are classified as 1-Փ half wave and 1-Փ full wave rectifier. Three phase rectifier are classified as 3-Փ half wave rectifier and 3-Փ full wave rectifier. 1-Փ Full wave rectifier are classified as1-Փ mid point type and 1-Փ bridge type rectifier. 1-Փ bridge type rectifier are classified as 1-Փ half controlled and 1-Փ full controlled rectifier. 3-Փ full wave rectifier are again classified as 3-Փ mid point type and 3-Փ bridge type rectifier. 3-Փ bridge type rectifier are again divided as 3-Փ half controlled rectifier and 3-Փ full controlled rectifier.

67. Single phase half wave circuit with R-L load Output current 𝑖𝑜 rises gradually. After some time 𝑖𝑜 reaches a maximum value and then begins to decrease. At π, 𝑣𝑜=0 but 𝑖𝑜 is not zero because of the load inductance L. After π interval SCR is reverse biased but load current is not less then the holding current. At β>π, 𝑖𝑜 reduces to zero and SCR is turned off. At 2π+β SCR triggers again α is the firing angle.

68. CHOPPER A chopper is a static device that converts fixed DC input voltage to variable output voltage directly. Chopper are mostly used in electric vehicle, mini haulers. Chopper are used for speed control and braking. The systems employing chopper offer smooth control, high efficiency and have fast response.

69. TYPES OF CHOPPER: FIRST QUADRANT OR TYPE A CHOPPER: SECOND QUADRANT OR TYPE B CHOPPER TWO QUADRANT TYPE A CHOPPER OR, TYPE C CHOPPER:

70. INVERTERS The device that converts dc power into ac power at desired output voltage and frequency is called an inverter. Single phase voltage source inverters