Advantages of the 3 Power Transistor used in the Automobile

Concept of Power Electronics – Power electronic systems – Power Semiconductor Devices – Principle of operation – Steady state and switching characteristics of Power diodes ‐ Power BJT ‐ Power MOSFET ‐ IGBT – Steady state and switching characteristics of SCR – DIAC – TRIAC – GTO.The three‐terminal device which is designed specifically to control high current – voltage rating and handle a large number of power levels in a device or a circuit is a power transistor.

The classification of power transistor includes the following.
Bipolar Junction Transistor (BJTs)
Metal Oxide Semiconductor Field‐Effect Transistor (MOSFETs)
Static Induction Transistor (SIT)
Insulated‐Gate Bipolar Transistor (IGBTs).

1. UNIT IV Power Electronic Devices Concept of Power Electronics – Power electronic systems – Power Semiconductor Devices – Principle of operation – Steady state and switching characteristics of Power diodes ‐ Power BJT ‐ Power MOSFET ‐ IGBT – Steady state and switching characteristics of SCR – DIAC – TRIAC – GTO.

2. A transistor is a semiconductor device, consists of three main parts Emitter (E), Base (B), and Collector (C), or a Source(S), drain (D), and gate(G ). The main function of a transistor is to Switching and amplify the weak signals A transistor compromises of semiconductor materials like silicon or germanium or gallium – arsenide. There are classified into two types based on their structure, BJT ‐ Bipolar Junction Transistor (transistors like Junction transistor, NPN transistor, PNP transistor) FET ‐ Field‐Effect Transistor ( transistors like junction function transistor and metal oxide transistor, N‐ channel MOSFET, P‐ channel MOSFET ) TRANSISTOR (BJT)

3. TRANSISTOR (BJT)

4. Transistors are composed of three parts a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply The emitter is the outlet for that supply . By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier. The same process is used to create the binary code for the digital processors but in this case a voltage threshold of five volts is needed to open the collector gate. In this way, the transistor is being used as a switch with a binary function: five volts ' ON, less than five volts ' OFF . TRANSISTOR (BJT)

5. POWER TRANSISTOR The three‐terminal device which is designed specifically to control high current – voltage rating and handle a large number of power levels in a device or a circuit is a power transistor. The classification of power transistor includes the following. Bipolar Junction Transistor (BJTs) Metal Oxide Semiconductor Field‐Effect Transistor (MOSFETs) Static Induction Transistor (SITs) Insulated‐Gate Bipolar Transistor (IGBTs).

6. Power BJT ( Bipolar Junction Transistor ) A BJT is a bipolar junction transistor, which is capable of handling two polarities (holes and electrons), it can be used as a switch or as an amplifier Power BJT is a current control device. Power transistors are Four layer & three terminal devices which are composed of semiconductor materials. They feature emitter, base and collector terminals. These devices are particularly designed to control high current – voltage rating.  The speciality of this device is when voltage or current is applied to one pair of terminals, it controls the voltage or current at the other pair of terminals.

7. Construction of Power NPN transistor

8. Basics of Power BJT Power BJT - Current control device It has large current handling and Power handling capacity compared to normal BJT. It offers high voltage resistance in OFF state than BJT . It offers high current handling in ON state than BJT . It has vertically oriented structure . High gain is maintained by enhancing doping level of emitter several times that of base. Due to more emitter doping, ẞ current gain will decrease . It handles more current, so it has large metallic body to dissipate power as heat.

9. Where the emitter terminal is connected to highly doped n‐ type layer, below which a moderately doped p‐layer of 10 16 cm ‐3 concentration is present, a lightly doped n‐ layer of 10 14 cm ‐3 concentration, which is also named as collector drift region , where the collector drift region decides the breakover voltage of the device A t the bottom, it has an n+ layer which is highly doped n + type layer of 10 19 cm ‐3 concentration, where the collector is etched away for user interface. Construction of Power NPN transistor

10. Symbol of Power BJT

11. Power Transistor biasing circuit

12. Operation of Power Transistor Power Transistor ( BJT ) works in 4 regions of operation they are Cut off region Active region Saturation region Quasi saturation region Hard saturation region. CUT‐OFF : Base emitter - reverse biased collector base – reverse biased ACTIVE : Base emitter - Forward biased collector base – reverse biased SATURATED : Base emitter - Forward biased collector base – Forward biased

13. Cutoff‐Region‐of‐Power‐Transistor A power transistor is said to be in a cut off mode if the n‐p‐n power transistor is connected in reverse bias where Case(I): The base terminal of the transistor is connected to negative and emitter terminals of the transistor is connected to positive, Case(ii): The collector terminal of the transistor is connected to the negative and base terminal of the transistor is connected to positive that is base‐emitter and collector emitter is in reverse bias. Hence there will be no flow of output current to the base of the transistor where IBE = 0, and also there will be no output current flowing through the collector to emitter since IC = IB = 0 which indicates transistor is in off state that is a cut off region. But a small fraction of leakage current flows throw the transistor from collector to emitter

14. Active‐Region‐of‐Power‐Transistor The current from collector to emitter is proportional to the current flowing into the base . A transistor is said to be in active state only when the base‐emitter region is forward bias and collector‐base region reverse bias. Hence there will be a flow of current IB in the base of transistor and flow of current IC through the collector to emitter of the transistor . When IB increases IC also increases.

15. Saturation‐Region‐of‐Power‐Transistor Saturation is the ON mode of a transistor . A transistor in saturation mode acts like a short circuit between collector and emitter . In saturation mode both of the "diodes" in the transistor are forward biased. A transistor is said to be in the quasi saturation stage if base‐emitter and collector‐base are connected in forwarding bias. In the quasi saturation region, the base current still retains some control over the collector current although the value of β decreases significantly A transistor is said to be in hard saturation if base‐emitter and collector‐base are connected in forwarding bias. In the hard saturation region, the base current loses control over the collector current and the value of β decreases rapidly.

16. V‐I Output or steady state Characteristics of a Power Transistor The output characteristics can be calibrated graphically as shown below, where the x‐axis represents VCE and the y‐axis represents IC .

17. The below graph represents various regions like the cut‐off region, active region, hard saturation region, quasi saturation region . For different values of V B E , there are different current values IB0, IB1, IB2, IB3, IB4, IB5, IB6 . Whenever there is no current flow, it means the transistor is off. But few current flows which are I CE0 For increased value of IB = 0, 1,2, 3, 4, 5. Where IB0 is the minimum value and IB6 is the maximum value. When V CE increases I C also increases slightly. Where I C = β I B , hence the device is known as a current control device. Which means the device is in active region, which exists for a particular period. [β – current gain ] V‐I Output or steady state Characteristics of a Power Transistor

18. Once the I C has reached to maximum the transistor switches to the saturation region. Where it has two saturation regions quasi saturation region and hard saturation region . Quasi saturation region A transistor is said to be in a quasi saturation region if and only if the switching speed from on to off is fast. Since On state resistance is much less and it can be turned off (Cut off mode) quickly as a result it can be used high ‐frequency application. Hard saturation region In a hard saturation region the transistor requires a certain amount of time to switch from on to off or off to on state. When compare with quasi saturation, resistance offered by hard saturation will be slightly much less. Power loss also very less This type of saturation is observed in the low ‐frequency applications. V‐I Output or steady state Characteristics of a Power Transistor

19. Switching Characteristics of a Power BJT BJT cannot turned ON Instantly because of the presence of internal capacitor When we give base voltage( V b ) pulse, base current rises to I BS (Base Current Saturation) Even I B = I BS , collector current I c = I CE0(Leakage current) is called delay time ( t d) When V BE > 0.7v then I C starts to rise t on = t d + t r t off = t s + t f

20. Switching Characteristics of a Power BJT

21. Applications of Power Transistor (BJT) SMPS Power amplifier DC to AC Inverter Relay Power control circuit

22. Field‐Effect Transistors (FET) A field effect transistor has only two layers of semiconductor material , one on top of the other. Current flows through one of the layers, called the channel and voltage connected to the other layer, called the gate, interferes with the current flowing in the channel . The voltage connected to the gate controls the strength of the current in the channel . There are two basic varieties of field effect transistors: Junction Field Effect Transistor (JFET). Metal Oxide Semiconductor Field Effect Transistor (MOSFET ).

23. Field‐Effect Transistors (FET)

24. N-Channel FET

25. N-Channel FET

26. Characteristics of N-Channel FET

27. Introduction to MOSFET A MOSFET is a four‐terminal device having source(S), gate (G), drain (D) and body (B) terminals. In general, the body of the MOSFET is in connection with the source terminal thus forming a three‐terminal device such as a field‐effect transistor . MOSFET is generally considered as a transistor and employed in both the analog and digital circuits .

28. Symbol of MOSFET

29. WORKING OF MOSFET

30. Introduction to MOSFET From the MOSFET structure, the functionality of MOSFET depends on the electrical variations happening in the channel width along with the flow of carriers (either holes or electrons ). The charge carriers enter into the channel through the source terminal and exit via the drain . The width of the channel is controlled by the voltage on an electrode which is called the gate and it is located in between the source and the drain . It is insulated from the channel near an extremely thin layer of metal oxide .

31. Enhancement MOSFET Depletion MOSFET It is Normally OFF device at zero Gate to Source voltage It is Normally ON device at zero Gate to Source voltage At OFF condition it cannot conduct electrical current. At OFF condition it can conduct electrical current. Positive gate voltage (more than source voltage) is required to turn on this type of MOSFET Negative gate voltage (less than threshold voltage) is required to turn off this type of MOSFET In this type of MOSFET, the gate voltage directly proportional to the Drain Current. In this type of MOSFET, the gate voltage Inversely proportional to the Drain Current. There is no permanent channel; a temporary channel is produced when a voltage applied across it. There is a permanent fabricated channel available. Generally, the N‐ Type channel Enhancement MOSFET has leakage current and diffusion current concept. Depletion MOSFET does not have any leakage current and diffusion current concept. The advantage of Enhancement MOSFET is ultra‐fast switching capability with high current conduction. The advantage of Depletion MOSFET is, it can be used as a variable resistive load.

32. Disadvantages of power BJT More gate drive power requirement (Low input impedance) Low frequency of operation (Bipolar) Secondary breakdown happen due to the presence of minority charge carrier Disadvantages of Normal MOSFET As voltage increase, On state resistance R ON increase (Size of the MOSFET increase) Due to the increase of R ON, losses also increase, power dissipation increase Limited to 500v, 150A only

33. Basics of Power MOSFET The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistor is a semiconductor device that is widely used for switching purposes and for the amplification of in power electronic devices. A MOSFET is either a core or integrated circuit where it is designed and fabricated in a single chip because the device is available in very small sizes . Power MOSFET are having high switching frequency up to about 100kHz Power BJT is a current controlled device, The collector current depends on the base current. Hence the current is highly depends on the junction temperature, which is not there in Power MOSFET Power MOSFET is a voltage controlled device

34. Features of Power MOSFET Insulated Gate FET (IGFET) Low input current It is voltage controlled device High switching speed They don’t have problem of second breakdown due to absence of minority carrier High input impedance Active device

35. Symbol of Power MOSFET N-Channel Power MOSFET P-Channel Power MOSFET

36. WORKING OF n-CHANNEL POWER MOSFET

37. Biasing circuit of N-channel Power MOSFET

38. Transfer characteristics of Power MOSFET

39. Output characteristics of Power MOSFET

40. MOSFET Regions of Operation Cut‐off Region – It is the region where the device will be in the OFF condition and there zero amount of current flow through it . Here, the device functions as a basic switch and is so employed as when they are necessary to operate as electrical switches. Saturation Region – In this region, the devices will have their drain to source current value as constant without considering the enhancement in the voltage across the drain to source . This happens only once when the voltage across the drain to source terminal increases more than the pinch‐off voltage value. In this region, the device functions as a closed switch where a saturated level of current across the drain to source terminals flows. Due to this, the saturation region is selected when the devices are supposed to perform switching. Linear/Ohmic Region – It is the region where the current across the drain to source terminal enhances with the increment in the voltage across the drain to source path . When the MOSFET devices function in this linear region, they perform amplifier functionality.

41. Switching characteristics of MOSFET Ideal Switch Characteristics For a semiconductor device, like a MOSFET, to act as an ideal switch, it must have the following features: During ON state , there should not be any limit on the amount of current it can carry. (should not resist) In OFF state , there should not be any limit on the blocking voltage. (should not conduct) When the device is in ON state , there should be zero voltage drop. (100 % efficiency) OFF state resistance should be infinite. (zero conduction, high resistance) Operating speed of the device has no limits.

42. Switch Characteristics of power MOSFET

43. IGBT‐ Insulated Gate Bipolar Transistor IGBT is the short form of Insulated Gate Bipolar Transistor. It is a three‐terminal semiconductor switching device that can be used for fast switching with high efficiency in many types of electronic devices. These devices are mostly used in amplifiers for switching/processing complex wave patter n s with pulse width modulation (PWM). The typical symbol of IGBT along with its image is shown below.

44. IGBT‐ Insulated Gate Bipolar Transistor An IGBT is a fusion between a BJT and MOSFET. The symbol of the IGBT also represents the same, as you can see the input side represents a MOSFET with a Gate terminal and the output side represents a BJT with Collector and Emitter. The Collector and the Emitter are the conduction terminals and the gate is the control terminal with which the switching operation is controlled.

45. Symbol of IGBT

46. IGBT IGBT can be constructed with the equivalent circuit that consists of two transistors and MOSFET, as the IGBT posses the output of the below combination of the PNP transistor, NPN transistor, and MOSFET. IGBT combines the low saturation voltage of a transistor with the high input impedance and switching speed of a MOSFET . The outcome obtained from this combination delivers the output switching and conduction characteristics of a bipolar transistor, but the voltage is controlled like a MOSFET. Since IGBT is the combination of MOSFET and BJT they are also called by different names. Insulated Gate Transistor( IGT), Metal Oxide Insulated Gate Transistor (MOSIGT), Gain Modulated Field Effect Transistor (GEMFET), Conductively Modulated Field Effect Transistor (COMFET).

47. Structure of IGBT

48. IGBT is a Four Layers (P‐N‐P‐N) device . Body Region , Drif t L a y e r , Injectio n l a y e r , and Metal layer are shown in the figure. The metal layer of the gate terminal is insulated from the semiconductors by Silicon Dioxide (SiO 2 ) metallic layer. There are two emitter terminals connected together and two junctions are present . The junction between P substrate and n‐ layer is called Junction 2 (J2) and the junction between the P layer and n‐ layer is called Junction 1 (J1 ). Internal Structure of IGBT

49. Working of IGBT

50. Working of IGBT

51. Working of IGBT

52. Initially, consider that there is no voltage applied to the Gate terminal , at this stage the IGBT will be in a non‐conductive state (OFF state). Now if we increase the applied gate voltage, due to the capacitance effect on the SiO 2 layer the negative ions will get accumulated on the upper side of the layer and the positive ions will get accumulated on the lower side of the SiO 2 layer . This will cause the insertion of negative charge carriers in the p region, higher the applied voltage V G greater the insertion of negatively charged carriers. This will lead to a formation of the channel between the J2 junction which allows the flow of current from collector to emitter. The flow of current is represented as the current path in the picture, when the applied Gate voltage VG increases the amount of current flow from the collector to the emitter also increases . Working of IGBT

53. Transfer Characteristics of IGBT

54. Output Characteristics of IGBT

55. IGBT Characteristics When the gate voltage(VGE) is zero, then the device is in the off condition, this is called Cut‐off Region . When the gate voltage increased but below the threshold voltage , there will be a small leakage current but then also the device will be in the cutoff region . When the gate voltage increased above the threshold voltage, the device will be turned on and goes into Active Region . In this active region, the current will flow through the device and the flow of current can be increased by increasing the gate voltage . When the device goes into the saturation region then the flow of current will be constant it will not increase even increasing the gate voltage .

56. Switching Characteristics of IGBT

57. IGBT Advantages IGBT or Insulated Gate Bipolar Transistor provides very high efficiency . IGBT can work with a very high switching speed . IGBT can handle high current, voltage, and power . As the gate isolated, so it provides electrical safety during high voltage operation.