powerelectronics-switch.pptx for Engineering students
- 1. Power Semi – Conductor Devices Engr.Kashif Iqbal Department of Electrical Engineering & Technology Kashif.iqbaluet@gmail.com
- 2. Contents: Power Diodes Thyristors Power Transistors Snubber Circuit Comparison between Ideal and Practical Switch Bipolar Junction Transistor (BJT) BJT Darlington Pair Metal Oxide Silicon Field Effect Transistor MOSFET Insulated Gate Bipolar Transistor (IGBT) GTO 2
- 3. Power Semi – Conductor Devices These devices act as switches without any mechanical movement. Few of the Power Devices are 1. Power Diodes 2. Power Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) 3. Power Bipolar -Junction Transistor(BJT) 4. Insulated-Gate Bipolar Transistor (IGBT) 5. Thyristors (SCR, GTO, MCT, IGCT) 3
- 4. Power Semi – Conductor Devices Ratings 4
- 6. Comparison between Ideal and Practical Switch Ideal Switch Practical Switch Block arbitrarily large forward and reverse voltage with zero current flow when off. Finite blocking voltage with small current flow during turn-off. Conduct arbitrarily large currents with zero voltage drop when on. Finite current flow and appreciable voltage drop during turn-on (e.g. 2-3V for IGBT). Switch from on to off or vice versa instantaneously when triggered. Requires finite time to reach maximum voltage and current. Requires time to turn on and off. Very small power required from control source to trigger the switch. In general voltage driven devices (IGBT, MOSFET) require small power for triggering. GTO requires substantial amount of current to turn off. 6
- 7. Ideal Characteristics of a Power Device Arbitrarily large forward and reverse voltages are blocked with zero currentflow during OFF condition. No conduction losses i.e. large current flows with no voltage drop duringON condition. ON – OFF switching is instantaneous without any delay. For triggering action, negligible power is required. 7
- 8. Power Diode: Power Diode is a two terminal P – N junction semiconductor device. The two terminals are anode (A) and cathode (K/C). If terminal A experiences a higher potential compared to terminal K, the device is said to be forward biased and a forward current will flow from anode tocathode. This causes a small voltage drop across the device (<1V) called as forward voltage drop(V f), which under ideal conditions is usuallyignored. By contrast, when a diode is reverse biased, it does not conduct and the diode then experiences a small current flowing in the reverse direction called the leakage current. It is shown below in the V-I characteristics of the diode. The structure of a power diode is different from a signal diode. 8
- 9. Power Diode – Structure : The complexity in power diode is to accommodate for the high power applications. Power diodes find applications in rectifier circuits, freewheeling or fly backdiodes. There is heavily doped n+ substrate (cathode) with doping level of1019/cm3 Lightly doped n – epitaxial layer is grown, also known as driftregion. Anode is formed by the heavily doped p+ region. The breakdown voltage depends on the thickness of the n –layer. The n – layer is absent in signal diodes. 9
- 10. Power Diode: Diodes block voltage in reverse direction and allow current in forwarddirection. They start conduction once the voltage in the forward direction goes beyond a certain value. I – V Characteristics and Reverse Recovery Characteristicsschematic 10
- 11. Power Diode When a diode is forward biased, it conducts current with a small forward voltage of 0.2 – 3 V across it. When reversed, a negligible small leakage current (µA to mA) flows until the reverse breakdown occurs. When a diode is switched quickly from forward to reverse bias, it continues to conduct due to the minority carriers which remains in the p – n junction. The minority carriers require finite time i.e. t rr (reverse recovery time) to recombine with opposite charge and neutralize. 11
- 12. Thyristor 4 – layer semiconductor device of alternating p- and n- material. Two – states: ON & OFF. Silicon Controlled Rectifier: SCR Trade Name of Thyristors commercialized by General Electric in 1957. 4-layered 3-terminal device. Have the highest power handling capability. Rating of 1200V / 1500A. Switching Frequency: 1KHz to 20KHz. 12
- 13. Symbol & Construction Alternately N-type or P-type material, for example P-N-P-N. The control terminal, called the gate, is attached to p-type material near to the cathode. 13
- 14. Characteristics of Thyristors When the anode is at a positive potential VAK with respect to the cathode with no voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place. Now if VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and the thyristor startsconducting. If a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value of VG, the thyristor can be switched into the on statesuddenly. 14
- 15. Thyristors Specifications: 15 Forward-break over voltage, VBR (F): This is the voltage at which the SCR enters the forward-conduction region. Holding current, IH: This is the value of anode current below which the SCR switches from the forward- conduction region to the forward- blocking region. Gate trigger current, IGT: This is the value of gate current necessary to switch the SCR from the forward- blocking region to the forward- under specified conduction region conditions.
- 16. Characteristic Curve – Thyristor 16
- 17. Ideal Switch An ideal switch behaves like a diode. 3 – states: Forward Blocking; Forward Conduction; Reverse Blocking. Gate signal switches ON the thyristor. 17
- 18. Types of Thyristors Unidirectional thyristor: The thyristors which conduct in forward direction only are known as unidirectional thyristors Example: SCR- Silicon Controlled Rectifier; LASCR-Light Activated Silicon Controlled Rectifier Bidirectional Thyristor: The thyristors which can conduct in forward as well as in reverse direction are known as bidirectional thyristor Ex: TRIAC - TRIode ACswitch Triggering Devices: The devices which generate a control signal to switch the device from non-conducting to conducting state are known as triggeringdevices. Ex: Diode AC Switch-DIAC, 18
- 19. Static Characteristics Blocking when reverse biased, no matter if there is gate current or not. Conducting only when forward biased and there is triggering current applied to the gate. Once triggered on, will be latched on conducting even when the gate current is no longer applied. Turning off: decreasing current to a near zero with the effect of external powercircuit. If VAK is further increased to a large value, the reverse biased junction will breakdown due to avalanche effect resulting in a large current through the device. The voltage at which this phenomenon occurs is called the forward breakdown voltage (VBO) 19
- 20. Two Important Current Terms LATCHING CURRENT (IL) After the SCR has switched on, there is a minimum current required to sustain conduction even if the gate supply is removed. This current is called the latching current. associated with turn on and is usually greater than holdingcurrent. HOLDING CURRENT (IH) After an SCR has been switched to the on state a certain minimum value of anode current is required to maintain the Thyristor in ON state. If the anode current is reduced below the critical holding current value, the Thyristor cannot maintain the current through it and turns OFF. 20
- 21. Effects on Gate current on Forward Blocking Voltage 21
- 22. V – I Characteristics 22
- 23. Gate Triggering Positive signal is applied at Gate terminal. By this, thyristor can be triggered much before break over voltage is reached. Conduction period can be controlled by varying Gate signal. Signal is applied between Gate and Cathode. When a positive voltage is applied at the gate terminal, charge carriers are injected in the inner P-layer, thereby reducing the depletion layer thickness. As the applied voltage increases, the carrier injection increases, therefore the voltage at which forward break-over occurs decreases. 23
- 24. Types of Gate Triggering Three types of signals are used for gate triggering. 1. DC Gate Triggering 2. AC Gate Triggering 3. Pulse Gate Triggering 24
- 25. DC Gate Triggering A DC voltage of proper polarity is applied between gate and cathode ( Gate terminal is positive with respect to Cathode). When applied voltage is sufficient to produce the required gate Current, the device starts conducting. One drawback of this scheme is that both power and control circuits are DC and there is no isolation between the two. Another disadvantages is that a continuous DC signal has to be applied. So gate power loss is high. 25
- 26. AC Gate Triggering AC gate triggering is most commonly used as it provides proper isolation. Firing angle control is obtained by changing the phase angleconveniently. Here AC source is used for gatesignals. This scheme provides proper isolation between power and controlcircuit. Drawback of this scheme is that a separate transformer is required to step down AC supply. 26
- 27. Pulse Gate Triggering Pulse triggering is the most popular. Pulse transformer is used for isolation. No need to apply continuous signal, therefore lessloss. Sequence of high frequency pulse called “carrier frequency gating” isapplied. In this method the gate drive consists of a single pulse appearing periodically (or) a sequence of high frequency pulses. This is known as carrier frequency gating. A pulse transformer is used forisolation. The main advantage is that there is no need of applying continuous signals, so the gate losses are reduced. 27
- 29. Turn on Switching Characteristics Turn – On 29 t d = delay time; t r = rise time; t p = peak time
- 30. Turn off Switching Characteristics Turn Off 30 t rr = reverse recovery time; t gr = gate recovery time
- 31. Firing Angle Firing Angle 𝜶: It is the angle after which the thyristor fires of conducts. It is normally done by either analog, digital or microprocessor controller circuits which sends the desired pulse to the thyristor gate drive circuit that will produce the actual gate drive pulse. Varying this angle changes the effective RMS values of voltage and current and hence power. 31
- 32. Thyristor Protection For reliable and satisfactory operation, the specified ratings of SCR should not exceed due to overload, voltage transients and other abnormalities. Due to reverse process in SCR during turn OFF, the voltage overshoot occurs. In case of short circuit, a large current flows through SCR which may damage the device. 32
- 33. Snubber Circuit – dv/dt protection A snubber circuit comprises a series combination of capacitor and resistor connected across the SCR. It sometimes also consists of an inductor in series with SCR to prevent high di/dt. The value of the resistor is few hundred ohms. With the switch closed, the voltage that appears across the SCR is bypassed to the RC network as the capacitor acts as a short circuit, thus reducing the voltage tozero. With increment of time, the capacitor gets charged up at a slow rate which is significantly small to be able to turn on the SCR. Thus the dv/dt rating is always way lesser than the maximum dv/dt ratings. 33
- 34. Over Voltage Protection The major reason for SCR failure is overvoltage as this transient overvoltage leads to unscheduled turning ON of the SCR and sometimes the reverse transient voltage exceeds the reverse breakdown voltage. The reasons of this overvoltage may be commutation, chopping or lightening. Internal Overvoltage: During turn OFF, a reverse current continues to flow through the SCR after the anode current is decreased to zero. As this decaying current flows at a faster rate, due to inductance of the circuit, the high di/dt produces a high voltage which if crosses the SCR ratings will damage the SCR permanently. 34
- 35. Merits & Demerits of Thyristors Merits of SCR: 1. SCRs with high voltage and current ratings are available. 2. On state losses in SCRs are reduced. 3. Very small amount of gate drive is required since SCR is a regenerative device. Demerits of SCR: 1. Gate has no control after the SCR is turned ON. 2. External circuits are required to turn OFF the SCR. 3. Operating frequencies are very low. 4. Snubber circuits are required for dv/dt protection. 35
- 36. Applications of Thyristor: High Current High VoltageApplications. Controlling alternating currents, where the change of polarity of the current causes the device to switch off automatically; referred to as Zero Cross operation. Phase angle triggered controllers, also known as phase firedcontrollers. “Circuit breaker" or “Crowbar" to prevent a failure in the power supply from damaging downstream components, by shorting the power supply output toground. Load voltage regulated by thyristor phase control. Red trace: load voltage Blue trace: trigger signal. 36
- 37. Power Transistor Types Bipolar Junction Transistor (BJT) Metal Oxide Semiconductor Field Effect Transistor (MOSFET) Insulated Gate Bipolar Transistor (IGBT) 37
- 38. Transistors 38
- 39. Bipolar Junction Transistor 39 Three terminal, three layer, two junction semiconductor device. Emitter, Base and collector form the three terminals.
- 40. Structure - BJT 40 The n – layer is added in power BJT which is known as driftregion. There are alternating P – N – P – Nlayers. The characteristics is determined by the doping level in each layer and the thickness of the layers. The thickness of the drift region determines the breakdown voltage of the Power transistor.
- 41. V – I Curve - BJT 41 The major differences between signal and power BJT are Quasi saturation region & secondary breakdown region. The Quasi saturation region is available only in Power transistor characteristic not in signal transistors. It is because of the lightly doped collector drift region present in PowerBJT. The primary breakdown is similar to the signal transistor's avalanche breakdown. Operation of device at primary and secondary breakdown regions should be avoided as it will lead to the catastrophic failure of the device.
- 42. V – I Curve - BJT 42
- 43. Rating - BJT 43 Ratings: Voltage: VCE <1000, Current: IC < 400A. Switching frequency up to 5kHz. Low on-state voltage: V CE(sat): 2-3V Low current gain (𝜷 <10). Need high base current to obtain reasonable IC .
- 44. BJT: NPN & PNP 44
- 45. BJT Darlington pair 𝑰𝑪 𝑰𝑩𝟏 𝜷 = = 𝑰𝑪𝟏+𝑰𝑪𝟐 𝑰𝑩𝟏 𝑰𝑩𝟏 𝑰𝑩𝟏 = 𝑰𝑪𝟏 + 𝑰𝑪𝟐 = 𝜷 𝟏 + 𝑰𝑪𝟐 𝑰𝑩𝟐 𝑰𝑩𝟐 𝑰𝑩𝟏 = 𝜷𝟏 + 𝜷𝟐 𝑰𝑩𝟏+𝑰𝑪𝟏 𝑰𝑩𝟏 𝟏 = 𝜷𝟏 + 𝜷𝟐. 𝟏 + 𝜷𝟏 = 𝜷 + 𝜷𝟐 +𝜷𝟏𝜷𝟐 Normally used when higher current gain is required. 45
- 46. Metal Oxide Silicon Field Effect Transistor (MOSFET) Three terminal (Gate, Drain, Source), four layer unipolar device. Voltage Driven Device, Simple Driver Circuit Majority carrier device, Fast Switching Speed Gate is electrically isolated from source giving MOSFET good input impedance and good capacitance. Drain to source resistance has positive temperature coefficient hence self protective. Very low On resistance and no junction voltage drop when forward boased. 46
- 47. MOSFET – Types 47
- 48. Difference between Depletion MOSFET vs Enhancement MOSFET Figure show the construction of depletion type MOSFET. Due to its construction it offers very high input resistance (about 1010 to 1015). Significant current flows for given VDS at VGS of 0 volt. Figure show the construction of enhancement type MOSFET.Here continuous channel does not exist from source to drain. Hence no current flows at zero gate voltage. When positive voltage is applied to the gate, it will induce a channel between Drain and source terminal. 48
- 49. MOSFET – Ratings Ratings: Voltage VDS < 500 V, Current IDS < 300A. Devices (few hundred watts) may go up to MHz range. Frequency f >100 KHz for some low power. Turning on and off is very simple. – To turn on: VGS = +15V – To turn off: VGS = 0 V and 0V to turn off. Gate drive circuit is simple.
- 50. MOSFET Features Basically low voltage device . High voltage device are available up to 600V but with limitedcurrent. Can be paralleled quite easily for higher current capability. Internal (dynamic) resistance between drain and source during on state, R DS(ON), , Limits the power handling capability of MOSFET. High losses especially for high voltage device due to R DS(ON) . Dominant in high frequency application (>100kHz). Biggest application is in switched-mode power supplies. 50
- 51. MOSFET Characteristics The current equation for MOSFET is givenas 𝐷 𝐼 = 𝑢 𝐶 𝑊 𝑉 1 𝑛 𝑜𝑥 2 𝑉𝐺𝑆 − 𝑉𝑇𝐻 𝐷𝑆 − 2 𝑉𝐷𝑆 2 un = Mobility of the electrons; Cox = Capacitance of the oxide layer; W = Width of the gate area; L = Length of the channel; VGS = Gate to Source voltage; VTH = Threshold voltage; VDS = Drain to Source voltage.
- 52. MOSFET Selection Factors To select a MOSFET for a particular application, following parameters have to be considered from the device datasheet 1. Maximum Drain to Source voltage (VDSS) 2. On-state drain to source resistance RDS(ON) 3. Drain Current ID 4. Gate to source VoltageVGS 5. Input Capacitance 𝐶𝑖𝑠𝑠 6. Power Dissipation PD
- 53. Comparison – BJT vs MOSFET BJT MOSFET It is a Bipolar Device It is a Unipolar Device Current control Device Voltage control Device. Output is controlled by controlling base current Output is controlled by controlling gate voltage Negative temperature coefficient Positive temperature coefficient So paralleling of BJT is difficult. So paralleling of this device is easy. Drive circuit is complex. It should provide constant current(Base current) Drive circuit is simple. It should provide constant voltage(gate voltage) Losses are low. Losses are higher than BJTs. So used in high power applications. Used in low power applications. BJTs have high voltage and current ratings. They have less voltage and current ratings. Switching frequency is lower than MOSFET. Switching frequency is high.
- 54. Insulated Gate Bipolar Transistor – IGBT IGBTs are preferred devices for voltages above 300V and below5kV. Three terminal device: Gate, Emitter, Collector. They are turned on and off by applying low voltage pulses to theirgate. Combination of BJT and MOSFET characteristics. Superior on – state characteristics, good switching speed and excellent safe operating area (SOA). Advantage: High current capability of BJTs and easy control of MOSFET.
- 55. Insulated Gate Bipolar Transistor IGBTs are preferred devices for voltages above 300V and below 5kV. They are turned on and off by applying low voltage pulses to theirgate. Combination of BJT and MOSFET characteristics. Gate behaviour similar to MOSFET - easy to turn on and off. Low losses like BJT due to low on-state Collector - Emitter voltage (2-3V). Ratings: Voltage: V CE < 3.3kV, Current,: I C < 1.2 kA currently available. Latest: HVIGBT 4.5 kV/1.2 kA. Switching frequency up to 100 KHz. Typical applications: 20-50 KHz.
- 58. IGBT – Merits & Demerits Merits 1. Voltage controlled device 2. Less On state loss 3. High switching frequency 4. Gate has full control over operation 5. Flat temperature coefficient. Demerits: 1. Static charge problem. 2. Costlier than BJT and MOSFET.
- 59. IGBT – Comparison Device Characteristic Power Bipolar Power MOSFET IGBT Voltage Rating High > 1kV High <1kV Very High >1kV Current Rating High > 500A Low <200A High >500A Input Drive Current, hFE 20-200 Voltage, VGS 3-10V Voltage, VGE 4-8V Input Impedance Low High High Output Impedance Low Medium Low Switching Speed Slow (uS) Fast (nS) Medium Cost Low Medium High
- 60. Gate turn-off Thyristor (GTO) Behave like normal thyristor, but can be turned off using gatesignal However turning off is difficult. Need very large reverse gate current (normally 1/5 of anode current). Gate drive design is very difficult due to very large reverse gate current at turnoff. Ratings: Highest power ratings switch: Voltage: V AK < 5 kV; Current: I A < 5 kA. Frequency < 5 KHz.
- 61. Gate turn-off Thyristor (GTO) These are capable not only to turn ON the main current with a gate drive circuit, but also to turn it OFF. A small positive gate current triggers the GTO into conduction mode and also by a negative pulse on the gate, it is capable of being turned off. The gate current required to turn off the GTO is relatively high. For example, a GTO rated with 4000V and 3000A may need -600A gate current to switch it off. Due to this large negative current, GTOs are used in low power applications. During the conduction state GTO behaves just like a thyristor with a small ON state voltage drop. The GTO has faster switching speed than the thyristor and has higher voltage and current ratings than the power transistors.
- 62. Construction - GTO Almost similar to SCR. In this, n + layer at the cathode end is highly doped to obtain high emitter efficiency. Due to this, breakdown voltage of the J3 junction is low (20 to 40V). The doping level of p type gate is highly graded as trade off between high emitter efficiency (low doping required) and good turn off properties (high dopingrequired).
- 63. Construction - GTO Junction between p+ anode and N base is called anode junction. Heavily doped p+ anode region is required to obtain high efficiency anode junction so that a good turn ON properties is achieved . But turn Off property is affected. This problem is solved by introducing heavily doped n+ layers at regular intervals in p+ anode layer. So this N+ layer makes a direct contact with N layer at junction J1. This cause the electrons to travel from base N region directly to anode metal contact without causing hole injection from P+ anode. This is called anode shorted GTO structure. But reverse blocking capacity of GTO is reduced and speeds up turn Off mechanism.
- 64. Working - GTO The Gate turn off thyristor (GTO) is a four layer PNPN power semiconductor switching device that can be turned on by a short pulse of gate current and can be turned off by a reverse gate pulse. This reverse gate current amplitude is dependent on the anode current to be turnedoff. There is no need for an external commutation circuit to turn it off. So inverter circuits built by this device are compact andlow-cost. The device is turned on by a positive gate current and it is turned off by a negative gate cathode voltage.
- 66. GTO V – I Characteristics: It is similar to SCR during turn on. The 1st quadrant characteristics are similar to that ofSCR. Latching current and holding current are considerably higher thanSCR. The gate drive can be removed if anode current is more than the holding current. It is though not recommended as cathode is subdivided into small finger elements causing the anode current to go below the holding current hence destroying thedevice. GTO is turned off by applying reverse gate current in either ramp or step mode. The dashed line in the figure shows i-v trajectory during the turn OFF for an inductive load. dV/dt triggering is avoided by placing a rated resistor between gate and cathode or by means of a reverse bias voltage.
- 67. GTO V – I Characteristics: A symmetric GTO has higher reverse blocking capability than an asymmetric type. After a small reverse voltage (20 to 30 V) GTO starts conducting in the reverse direction due to anode short structure.
- 68. GTO Turn OFF Current Gain: The Turn Off Current Gain of a GTO is defined as the ratio of anode current prior to turn off to the negative gate current required for turnoff. It is typically very low (4 or 5). It means a 6000Arating GTO requires 1500Agate current pulse. However, the gate pulse duration and the power loss due to the gate pulse issmall. It can be supplied by low voltage power MOSFETs. This gate turn off capability is advantageous because it provides increased flexibility in circuit application. Now it becomes possible to control power in DC circuits without use of elaborated commutation circuitry.
- 69. Summary GTO Advantages of GTO: 1. High blocking voltage capabilities. 2. High over current capabilities during turn off. . 3. Exhibits low gate currents. 4. Fast and efficient turn off. 5. Better static and dynamic dv/dt capabilities. 6. Enhanced Safe Operating Area during turn off. Disadvantages: 1. Magnitude of latching, holding currents is several times more than thyristors. 2. On state voltage drop and the associated loss is more. 3. Due to multi cathode structure of GTO, triggering gate current is higher than that required for normal SCR. 4. Gate drive circuit losses are more. 5. Its reverse voltage blocking capability is less than the forward voltage blocking capability.
- 70. Summary GTO Applications of GTO: 1. Motor drives, 2. static VAR compensators (SVCs) 3. AC/DC power supplies with high power ratings. 4. DC circuit breakers 5. Induction heating 6. Low power applications. 7. DC choppers.
- 71. Insulated Gate-Commutated Thyristor (IGCT) Among the latest Power Switches. Conducts like normal thyristor (latching), but can be turned off using gate signal, similar to IGBT turn off; 20 V issufficient. Power switch is integrated with the gate-drive unit. Ratings: Voltage: V AK < 6.5 kV; Current: I A < 4kA. Frequency < 1 KHz. Currently 10kV device is beingdeveloped. Very low on state voltage: 2.7V for 4kAdevice.
- 72. Insulated Gate-Commutated Thyristor (IGCT) Specifications GTO IGCT Full Form Gate Turn-Off Thyristor Insulated Gate Commutated Thyristor Advantages • Controlled turn-off ability. •Relatively high overload capacity. • Series connection possibility. •Working frequency of hundreds of Hz. • Controlled turn-off ability. •Relatively high overload capacity. • Low on-state losses. • Working frequency of kHz. Disadvantages • Higher on-state losses. • High control power. Applications • High power drives • Static compensators • Continuous supply sources • Induction heating sources • High power drives •Supply inverter sources for DC transmissions • Big frequency converters