Lecture Note on Switches Marc T. Thompson, 2003 Revised 2007 Use with gratefulness for ECE 3503 B term 2018 WPI Tan Zhang Lecture note on switches_tan_thompsonpage 1 of 21
1. DEVICES OVERVIEW... 4 1.1. DIODES... 4 1.1.1. Bipolar Diode... 4 Deviations from the Ideal... 4 Example: Half-wave rectifier... 5 1.1.2. Reading a Diode Datasheet... 6 1.1.3. Schottky Diode... 7 1.1.4. Zener Diode... 8 Example: Zener diode regulator... 9 1.2. BIPOLAR JUNCTION TRANSISTOR... 9 1.2.1. Reading a Transistor Datasheet... 10 1.2.2. Power Transistor... 14 Safe Operating Area (SOA)... 14 1.2.2.1. Darlington Transistor... 14 1.3. MOSFET... 15 1.3.1. Safe Operating Area (SOA)... 16 1.3.2. Example: Flyback Converter]... 17 1.4. IGBT... 18 1.4.1. IGBT Conduction Characteristics... 19 1.5. OVERALL COMPARISON OF FULLY-CONTROLLABLE POWER SWITCHES... 20 1.6. SCRS/THYRISTORS... 20 2. REFERENCES... 21 Lecture note on switches_tan_thompsonpage 2 of 21
List of Figures Figure 1. Ideal diode characteristics... 4 Figure 2. Typical diode characteristics... 5 Figure 3. Half-wave rectifier... 5 Figure 4. 1N4148 maximum ratings... 6 Figure 5. 1N4148 electrical characteristics... 7 Figure 6. 1N4148 forward and reverse characteristics vs. temperature... 7 Figure 7. Schottky diode V/I curve... 8 Figure 8. Zener regulator... 9 Figure 9. NPN transistor... 9 Figure 10. NPN Transistor V/I Curve... 10 Figure 11. 2N3904 package styles... 10 Figure 12. 2N3904 electrical characteristics... 11 Figure 13. 2N3904 electrical characteristics... 12 Figure 14. 2N3904 thermal characteristics... 13 Figure 15. Thermal Model... 13 Figure 16. Darlington... 14 Figure 17. MOSFET cross-section and symbol... 15 Figure 18. MOSFET V/I curve... 16 Figure 19. MOSFET safe-operating area (SOA)... 17 Figure 20. Flyback converter... 18 Figure 21. IGBT cross section... 18 Figure 22. Comparison of IGBT and MOSFET conduction loss... 19 Figure 23. Comparison of power switches... 20 Figure 24. SCR... 20 Lecture note on switches_tan_thompsonpage 3 of 21
1. Devices Overview Following is a brief overview of semiconductor devices which are commonly used in amplifiers and also as switches in power electronics systems. 1.1. Diodes Diodes are uncontrolled switches, used in many power and signal processing circuits, including: Power electronics circuits o Switching power converters o Motor drives Signal processing circuits o Absolute value circuits o Rectifiers The ideal diode has the characteristics in Figure 1. The diode current is given by: ID = 0 for VD 0 ID is whatever the circuit forces it to be for VD > 0 I D V D Figure 1. Ideal diode characteristics 1.1.1. Bipolar Diode From PN junction theory, the V/I characteristic of a bipolar diode is: qvd [1.] kt I = 1 D I R e There are many deviations in a real-world diode from the ideal expression contained in this equation. This equation predicts a diode which turns ON with VD = 0.6 0.8V or so, and which has a small reverse current when VD < 0. Deviations from the Ideal A real diode has many differences from an ideal diode (Figure 2): Maximum forward current: The diode has a maximum forward current that it can carry without being damaged. Maximum reverse voltage: The diode has a reverse voltage rating that cannot be exceeded without damaging the diode. Lecture note on switches_tan_thompsonpage 4 of 21
Diode ON resistance: The diode has an Ohmic resistance which is not accounted for in the ideal diode equation. Diode capacitance: The diode has a capacitance which can significantly affect operation of switching and power electronics systems. Reverse recovery time: When a diode is switched OFF, reverse current flows in the diode for a period of time, called the reverse recovery time. Reverse leakage current. In the reverse direction there is a small leakage current up until the reverse breakdown voltage is reached. This leakage is undesirable, obviously the lower the better, and is specified at a voltage less the than breakdown. Maximum current rating. The current rating of a diode is determined primarily by the size of the diode chip, and both the material and configuration of the package, Average Current is used, not RMS current. A larger chip and package of high thermal conductivity are both conducive to a higher current rating. Figure 2. Typical diode characteristics Example: Half-wave rectifier 1:10 V out 120 VAC Figure 3. Half-wave rectifier Lecture note on switches_tan_thompsonpage 5 of 21
1.1.2. Reading a Diode Datasheet Following are specifications for a common, inexpensive signal diode, the 1N4148 1 Figure 4. 1N4148 maximum ratings Some comments on maximum ratings: Reverse voltage: This diode has a rating of -75V DC, or -100V pulsed. Average current: The diode can carry 150 ma DC (if power dissipation limits allow it) and can carry 500 ma for a short period of time if you keep it cool. Power dissipation: You can dissipate 500 mw total in the diode, at an ambient temperature of 25C. At higher ambient temperatures, you have to derate vs. temperature. Thermal resistance: This figure tells you what temperature the junction of the diode will operate at. The junction temperature is found by: T j = T + < P > R [2.] a D θja The maximum junction temperature of the 1N4148 is 175C. If you want to keep Tj < 150C with TA = 75C, the maximum power dissipation is <PD>=214 mw. 1 http://www.gensemi.com/pdfs/1n4148.pdf Lecture note on switches_tan_thompsonpage 6 of 21
Figure 5. 1N4148 electrical characteristics For the electrical characteristics, there is often a wide range of values shown; this is called specmanship. Also, note that many of the diode parameters vary with temperature. For instance, the forward and reverse diode characteristics vs. temperature are shown in Figure 6. (a) (b) Figure 6. 1N4148 forward and reverse characteristics vs. temperature 1.1.3. Schottky Diode A Schottky diode is made with a metal-semiconductor junction, and have been used in power electronics and switching applications. Schottky advantages are a low forward voltage and low switching losses. However, in some applications Schottky use is limited Lecture note on switches_tan_thompsonpage 7 of 21
due to the high reverse leakage current and effects of temperature. The Schottky has many important differences from a bipolar diode: Forward voltage: The forward voltage for a Schottky will be a few tenths of a volt lower than a comparably sized bipolar diode. Reverse voltage rating: Schottky diodes have worse reverse current specifications than comparably sized diodes. For that reason, Schottky families have maximum reverse voltage ratings of a 100 V or so. Reverse recovery: Schottky diodes do not exhibit reverse recovery problems, and hence are used in many switching and power electronics systems. Figure 7. Schottky diode V/I curve 1.1.4. Zener Diode A Zener diode is a diode with a well-controlled reverse breakdown. Zeners are commonly used as shunt regulators and as transient voltage suppressors. A simple Zener regulator is shown in Figure 8. This circuit is very simple, but has the disadvantages of high power dissipation and poor regulation. Lecture note on switches_tan_thompsonpage 8 of 21
Example: Zener diode regulator V reg V in Figure 8. Zener regulator 1.2. Bipolar Junction Transistor Bipolar junction transistors (BJTs) are a ubiquitous device, used for many applications in amplification and power electronics, including: Amplifiers o Power amplifiers o Signal amplifiers Motor drives Solenoid drives Power electronics The transistor has 4 regions of operation: In cutoff, IB = IC = 0 and VBE << 0.7V. The transistor is essentially OFF. In the forward-active or linear region, IC = βfib and VBE = 0.7V or so. In saturation, VCE is small. In the reverse-active region, the roles of collector and emitter are interchanged. Essentially, the transistor is being used "backwards." qvbe kt I C = I S e 1 I B I C = β F [3.] C I B I C B + V BE - E I E Figure 9. NPN transistor In normal operation in transistor amplifiers, the transistor is used in the forwardactive region, where: Lecture note on switches_tan_thompsonpage 9 of 21
I C I Se This simple model used so far leads us to believe that the transistor collector current doesn't change when the collector voltage changes. In other words, the simple model indicates infinite output impedance at a transistor's collector. If you look closely at transistor curves (for instance, on a datasheet or on a curve tracer) you see that there is a finite slope in the IC/VCE linear region of operation. This corresponds to the small-signal output resistance of the transistor when operated in the linear operating region. I C qvbe kt slope = 1/r o V CE Figure 10. NPN Transistor V/I Curve This effect is due to the widening/narrowing of the collector-base depletion region when VCB changes, which in turn changes the effective base width. 1.2.1. Reading a Transistor Datasheet Following are selected specification sheets for the 2N3904 transistor. 2 The device is available in a variety of packages (Figure 11) each with its own specification on maximum power dissipation. Figure 11. 2N3904 package styles Following are specifications for the device, taken from a manufacturer's website. 2 Datasheets taken from http://www.fairchildsemi.com/ds/2n/2n3904.pdf Lecture note on switches_tan_thompsonpage 10 of 21
Max. V CE Also called "BETA" Note the wide range of BETA Fundamental figure-of merit for transistor intrinsic speed Note that V CE,sat = 0.2-0.3 for signal transitors Maximum frequency at which this transistor may be useful as an amplifier Transistor junction capacitances SPICE models...use with caution! Figure 12. 2N3904 electrical characteristics Lecture note on switches_tan_thompsonpage 11 of 21
(a) Note the significant variation in BETA with temperature. Figure 13. 2N3904 electrical characteristics (a) Junction capacitances. (b) DC BETA vs. temperature. (c) Power dissipation derating vs. temperature. (d) Variation of VCE,sat with temperature. Lecture note on switches_tan_thompsonpage 12 of 21
Figure 14. 2N3904 thermal characteristics After calculating the power dissipated, you can calculate the temperature of the transistor junction by using the thermal model shown. The goal is to keep the junction temperature below a maximum value. The value of RTH,jc, thermal resistance from device junction to device case, can be found from the data sheet. T j (Junction temp.) R TH, jc P total (Watts) T c (Case temp.) R TH, cs T s (Heat sink temp.) R TH, sa Lecture note on switches_tan_thompsonpage 13 of 21 T A (Ambient temp.) Figure 15. Thermal Model Primary considerations when selecting a transistor are: (a) Voltage ratings of all three junctions (b) Power rating and thermal resistance (c) Current handling capability and the transistor case size (d) Leakage currents (e) Frequency response and /or switching times. (f) Current gain (hfe and hfe) (g) Temperature parameter variation.
(h) Saturation resistance (I) h-parameters for linear applications 1.2.2. Power Transistor Power transistors are optimized to carry high levels of current and to have a large OFF state voltage. Device trade-offs result in important differences from signal transistors: Lower βf. Unlike signal transistors where βf (or hfe) may be > 100 or more, a common value of BF for power transistors is 5-20 at rated current. Slower. Larger devices are necessarily slower. Secondary breakdown. At high currents and voltages, power transistors exhibit a phenomena known as secondary breakdown, which can result in catastrophic damage to the device. Safe Operating Area (SOA) Power transistors are limited in operation by the following parameters: Current Voltage Power Secondary breakdown 1.2.2.1.Darlington Transistor The Darlington connection (Figure 16a) results in a composite device with a much higher βf than an individual device. This connection results in a lower base current than a single power transistor, simplifying circuit design. The Darlington has several disadvantages. The first is that the ON-state voltage is higher than a comparable single power transistor. The ON-state voltage of a Darlington is between 2 and 5 Volts, depending on the device rating [Kassakian, pp. 418]. The second disadvantage is that the switching speed is slow, as there is no active way to turn off the second transistor in the pair. Monolithic Darlington manufacturers attempt to reduce this affect by adding external elements, as shown in Figure 16b. + V BE - (a) Figure 16. Darlington (b) Lecture note on switches_tan_thompsonpage 14 of 21
1.3. MOSFET The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) (Figure 17) is a voltage-controlled device which is finding many uses in power electronics. The MOSFET is faster than a comparably sized BJT and is easier to drive. Figure 17. MOSFET cross-section and symbol Ideal specifications for a MOSFET are: Zero DC input current For VGS < VT (threshold voltage), ID = 0 For VGS > VT and VDS > VDS,sat, then ID = gm(vgs - VT) where gm is the transconductance of the device (A/V) The MOSFET has a similar V/I curve as a transistor The RDSon of the MOSFET increases with voltage rating of the device as Rds,on VDSS 2.5 Lecture note on switches_tan_thompsonpage 15 of 21
Figure 18. MOSFET V/I curve 1.3.1. Safe Operating Area (SOA) Note that in the MOSFET there is no secondary-breakdown region. Also, the device can withstand short duration high power as shown by the curves. Lecture note on switches_tan_thompsonpage 16 of 21
(a) (b) Figure 19. MOSFET safe-operating area (SOA) (a) Comparison with BJT (b) Pulsed specification 1.3.2. Example: Flyback Converter The flyback converter (Figure 20) provides voltage step-up with static transfer function Vo = VCCND/(1-D) when operated in the continuous conduction mode. Lecture note on switches_tan_thompsonpage 17 of 21
V cc N1 :N2 V o C R D Figure 20. Flyback converter 1.4. IGBT The IGBT (Insulated Gate Bipolar Transistor) is a relatively new device, with input characteristics similar to that of a MOSFET, and output characteristics similar to a BJT. From the cross section figure (Figure 21), note the following features: Silicon cross-section of an IGBT is virtually identical to that of a power MOSFET except for P substrate. Both devices share a similar polysilicon gate structure and P wells with N+ source contacts In both devices the N- type material under the P wells is sized in thickness and resistivity to sustain the full voltage rating of the device. IGBT is a minority-carrier device, similar to BJT (note output PNP) Figure 21. IGBT cross section Lecture note on switches_tan_thompsonpage 18 of 21
1.4.1. IGBT Conduction Characteristics The output voltage of IGBT never less than a diode drop. However, at high currents and/or high voltage, IGBTs may win over high voltage MOSFETS since Rds,on VDSS 2.5 and the output voltage drop of a BJT increases more slowly with voltage rating (Figure 22). (a) (b) Figure 22. Comparison of IGBT and MOSFET conduction loss (a) vs. temperature. (b) For different voltage ratings Lecture note on switches_tan_thompsonpage 19 of 21
1.5. Overall Comparison of Fully-Controllable Power Switches Figure 23. Comparison of power switches 1.6. SCRs/Thyristors The Silicon Controlled Rectifier (SCR) is a conventional rectifier controlled by a gate signal. Current devices have ratings of > 5000V and > 1000 A, so these devices are used in very high voltage systems. The device is not fully controllable; you turn it on but the external circuitry turns it OFF. The main circuit is a rectifier, however the application of a forward voltage is not enough for conduction. A gate signal controls the rectifier conduction. The schematic representation is: A G i G k Figure 24. SCR The device is triggered on if ig>0 while vak > 0. The rectifier circuit (anode-cathode) has a low forward resistance and a high reverse resistance. It is controlled from an off state (high resistance) to the on state (low resistance) by a signal applied to the third terminal, the gate. Once it is turned on it remains on even after removal of the gate signal, as long as a minimum current, the holding current, Ih, is maintained in the main or rectifier circuit. To turn off an SCR the anode-cathode current must be reduced to less than the holding current, Ih. Major considerations when selecting a SCR are: (a) Peak forward and reverse breakdown voltages (b) Maximum forward current (c) Gate trigger voltage and current (d) Minimum holding current, Ih Lecture note on switches_tan_thompsonpage 20 of 21
(e) Power dissipation (f) Maximum dv/dt A special kind of SCR, called the gate turn off thyristor (GTO) can be turned off. 2. References [1] Paul E. Gray and Campbell L. Searle, Electronic Principles Physics, Models and Circuits, John Wiley, 1969 [2] Paul R. Gray and Robert G. Meyer, Analysis and Design of Analog Integrated Circuits, 2d edition, John Wiley 1984 [3] John G. Kassakian, Martin F. Schlecht, and George C. Verghese, Principles of Power Electronics, Addison-Wesley, 1991 [4] Ned Mohan, Tore M. Undeland and William P. Robbins, Power Electronics Converters, Applications, and Design, Second Edition, John Wiley, 1995 [5] Richard S. Muller and Theodore I. Kamins, Device Electronics for Integrated Circuits, 2d edition, John Wiley, 1986 Lecture note on switches_tan_thompsonpage 21 of 21