Chapter 5: Power Electronics: Devices and Circuits potx

Chapter 5 Power Electronics: Devices and Circuits 5.1 Introduction Power electronics is an enabling technology for all electrical and electronic apparatus requiring electric power to drive. Over the past twenty years, the power electronics industry has grown tremendously. Its growth is a result from increasing demand of re- liable, efficient, compact and cost effective power supplies for telecommunication, com- puter, and motor drive industries as well as for medical equipments and military use. This growth is facilitated by the significant improvement in semiconductor technology in which smaller packaging and higher power handling devices have been marketed. In response to the advancement in semiconductor and magnetics technology, power elec- tronics researchers and engineers h ave strived to thoroughly employ these technologies through new circuit design and topologies, optimized control and packaging techniques, in order to meet the industry demands. Power electronics is all about using electronic devices and circuits with storage el- ement to control the level of voltage and current, either in the form of AC or DC. Power electronics circuits are switching converters with periodic switching actions to process the electrical energy to meet the design s pecification. Apart from semiconductors, in- ductor and transform er are the critical magnetic components in the power switching converter. Th eir functions such as storage element, power splitting, and safety isolation 83 5. Power Electronics: Devices and Circuits 84 will be explained in detail in Chapter 6. In order to control power, some form of control techniques are needed and will be discussed in Chapter 7. This ch apter we will begin with the semiconductor devices used for power con- verters. We th en analyze the basic DC/DC converters at the steady state. That is, the output voltage and current are at stable cond ition. Finally, the operation of gate driver, w hich drives the transistor, is presented. 5.2 Electrical Energy Conversion by Switching The characteristics of power conversion by power electronics converters are summarized as follows: 1. Electrical energy can be generated, transmitted and converted to a form that is suitable for the load we are interested in. 2. Power Electronics concerns conversion and processing of electrical energy by power semiconductor devices and storage elements. 3. Power Electronics technologies enable great efficiency enhancement, tremendous size and weight reduction of electrical equipment. 4. Power Electronics technologies are based on switching on and off the power source by power semiconductors. The electrical energy conversion process can be pre- cisely controlled in a manner far much better than electromech an ical devices. 5. Power Electronics applications include power supplies for computers, communi- cation equipment, machine dr ives, lighting, automobile and m any applications. 6. Electrical energy conversion can be classified into the following four categories : AC to AC, AC to DC, DC to DC, and DC to AC. 5. Power Electronics: Devices and Circuits 85 Figure 5.1: Four categories of electrical energy conversion. Figure 5.2: Diode: (a) Symbol, (b) I-V characteristic, (c) idealized characteristics. Sources: Mohan 1995 [2]. 5. Power Electronics: Devices and Circuits 86 Figure 5.3: Diode switching characteristics. Sources: Mohan 1995 [2]. 5. Power Electronics: Devices and Circuits 87 5.3 Power Semiconductor Devices as Switches 5.3.1 Diodes A diode performs as a switch. It is driven by the voltage applied across its two terminals: anode and catho de. Fig 5.2(a) shows the symbol of a diode. A is the anode, the positive terminal. K is the cathode, the negative terminal. When a diode is forward biased, v d is positive (i.e. potential at A is higher than K), th e arrow shows the direction of the diode current i D . When the diode conducts, a small forward voltage drop denoted as V F is established and the magnitude is usually around 1V. When the diode is reverse biased, it is blocked and the diode current becomes slightly negative. This is du e to the contribution of reverse saturation current. For example, 1N4004 has a reverse current of 50µA. And this reverse current is of temperature-dependent; when temperature is higher the reverse current is increased and vice versa. The reverse voltage applied on a diode has a limit. Beyond the limit the diode will breakdown and becomes a short circuit. This limit is usually called the peak inverse (or reverse) voltage. Another interesting fact of semiconductor is that it can handle repetitive pulse current which has a magnitude much higher than the continuous diode current. For example, 1N4004 has a maximum forward current at 1A but its allowable repetitive pulse current is at 10A. This property is of particular interest to power electronics circuit because of its switching nature. Owing to the intrinsic resistance, inductance and capacitance of a diode, it expe- riences voltage overshoot (especially in power diode) and reverse recovery transition. When the diode is forward biased, large amount of excess carriers are driven across the junction and the depletion region is reduced. This behaves like charging a capacitor plus the ohmic resistance and in ductance that cause the voltage overshoot. When the charging action is finished the forward current I F becomes steady and the effect of di/dt on inductance becomes zero, and thus the drop after the overshoot. When the diode is reversed biased, it will turn off and the current decreases. If it was an ideal diode, cur rent would have dropped to zero and remained zero afterwards. In practice, I F becomes negative for a while before it settles to zero. This period is known as reverse recovery period. The reverse recovery period is due to charge storage in the diode when it is forward biased. When the diode turns off the charge storage has to b e 5. Power Electronics: Devices and Circuits 88 Figure 5.4: N-Channel MOSFET: (a) Symbol, (b) I-V characteristic, (c) idealized characteristics. Sources: Mohan 1995 [2]. removed before the junction can become reverse biased again. The effects of reverse recovery of diode are not only increasing the power dissi- pation of diode itself but also increaseing the losses of other devices connected. For power electronics circuits switching at high frequency in the range of hu ndred kHz to few MHz, fast recovery time diodes are preferred. There are at least three different types of diode: • Line frequency diode or general purpose diode - on-state or forward voltage of theses diodes is made as low as possible but higher t rr , which is acceptable for line frequency applications (50 Hz or 60 Hz). • Fast recovery diode - small reverse recovery time, t rr less than a few microseconds, for high frequency switching circuits. • Schottky diode - these diodes have low forward voltage drop(typically 0.3V) and the dio de dissipation is reduced. However Schottky diodes are limited in their voltage blocking capabilities, typically less than 250V. 5.3.2 MOSFET The MOSFET has three terminals: Gate (G), Drain (D) and Source (S). It is a voltage- controlled device which needs a voltage across gate-to-source (V GS ) be greater than a threshold voltage V th to drive the transistor on. When the transistor is on, the drain-to- source becomes a channel for electric current to pass through at both directions. This 5. Power Electronics: Devices and Circuits 89 channel has an internal resistance called on-state r esistance R ON which is voltage- and temperature-dependent. In general, the higher the voltage and temperature, the higher the on-state resistance. R ON is at the range from a few milli-ohms to a few ohms. Besides, due to the formation structure of MOSFET, there is a body diode across the source-to-drain term inals. The MOSFET has intrinsic capacitances across all its terminals. Of particular concern is the capacitances across G-S and G-D. These capacitances will cause delay in the turning on or off the MOSFET. T his leads to switching losses. In order to minimize the losses, the gate driver circuit (i.e. to provide V GS ) has to be of high current and fast switching response. We will discuss that in more detail in the last Section of this chapter. The gate-to-source voltage needs to stay above the threshold voltage to maintain the transistor on. One important point to stress is that, for a practical MOSFET, V GS and drain current I D are inter-related. For example, in Fig. 5.5, the drain current I D only reaches 6.5A maximum when V GS is at 4.5V. I D increases when V GS increases. When the V GS decreases to zero, the transistor is off and I D decreases to zero. Current MOSFET can sustain a reverse voltage up to 1kV. 5.4 Basic Power Converter Topologies 5.4.1 Buck The buck converter with MOSFET is shown in Fig. 5.6. The buck converter performs voltage step-down function. That is, V o is less than V S . By the switching actions of MOSFET, the buck converter can be described by two basic operation stages as s hown in Fig. 5.7. The switching waveforms of the buck converter is shown in Fig. 5.8. In this operation mode, the inductor current d oes not reach zero. Th is mode is called continuous (inductor) conduction mode (CCM). Stage 1 Prior to this stage, the switch Q1 is turned off. But there is current flowing in the inductor L. When Q1 is turned on at the beginning of this stage (t = 0), the voltage 5. Power Electronics: Devices and Circuits 90 Figure 5.5: N-Channel MOSFET IRF540N I-V characteristic. Sources: International Rectifier. ❄ Q1 v P ulse1 L C R L V S i D1 ✻ + −V L (t) + − V C (t) ❄ I c (t) I a V o (t), V a D f w ✲ ✲ ✲ i s (t) i L (t) I o + − Figure 5.6: Circuit diagram of a buck converter. 5. Power Electronics: Devices and Circuits 91 L C R L V S i D1 ✻ + −V L (t) + − V C (t) ❄ I c (t) I a V o (t), V a D fw ✲ ✲ ✲ i s (t) i L (t) I o + − (a) Stage 1 (0 − DT ) Q1 L C R L V S i D1 ✻ + −V L (t) + − V C (t) ❄ I c (t) I a V o (t), V a D fw ✲ ✲ ✲ i s (t) i L (t) I o + − (b) Stage 2 (DT − T ) Figure 5.7: Equivalent circuits for the buck converter of the two operation stages. 5. Power Electronics: Devices and Circuits 92 ✲ ✲ ✲ ✲ ✲ ✲ t I a V a I c (t) V DW (t) i L (t) i S (t) v C (t) = V o (t) I o (t) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D T T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D T T V S I 1 I 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I L = I a I 1 I 2 ❄ ✻ ∆V C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ❄ ✻ ∆I L Figure 5.8: Key switching waveforms of a buck converter. [...]... in power electronics circuits that they process energy to be stored and transferred from inductive element such as inductor and transformer It is in this circuit modeled as Io Vd is the power source, vT and iT are the voltage and current through the transistor respectively 5 Power Electronics: Devices and Circuits Figure 5.11: Cause of switching losses in transistor Sources: Mohan 1995 [2] 98 5 Power. .. soft-switching technique can go to Chapter 3 of Ang’s book [1] Conduction Loss The conduction loss of the transistor is defined as the energy dissipation in the transistor during this on-state interval It can be approximated as Won = Von Io ton (5.25) The power dissipation of conduction loss is then approximated as Pon = Von Io ton T (5.26) 5 Power Electronics: Devices and Circuits 100 Totem-pole VDD ic... iG = vP W M − VBE − Vth β Ri (5.29) 5 Power Electronics: Devices and Circuits 101 Figure 5.13: Floating gate drive IR2111 Sources: International Rectifier Figure 5.14: Floating gate drive IR2111 functional block diagram Sources: International Rectifier The base-to-emitter voltage of Q2 is reverse biased and it is in off-state To switch off Q3, we need to turn off Q1 and switch on Q2 It is achieved by decreasing... and VB , capacitor, lower transistor then the return path to charge up this capacitor When the upper switch control signal is On, the capacitor will transfer its charge through the internal circuit of the chip to the intrinsic input capacitor of upper MOSFET Bibliography [1] S Ang, and A Oliva, Power- switching converters, CRC Press, 2nd Ed., 2005 [2] N Mohan, T M Undeland, and W P Robbins, Power electronics:. .. charging part as an example It can be written as IC = ∆IL 2 × 2T T 2 = ∆IL 8 (5.13) The capacitor ripple voltage ∆vC of each period is ∆vC = 1 C T 0 ∆IL T ∆IL dt = 8 8C (5.14) 5 Power Electronics: Devices and Circuits 95 Substitute (5.3) and (5.9) into (5.14), the capacitor ripple voltage is expressed as ∆vC = VS D(1 − D) 2 8fs LC (5.15) In fact, the capacitor ripple voltage ∆vC is the output ripple voltage... 0 - iL (t) - - Figure 5.10: Key switching waveforms of a buck converter at DCM 5 Power Electronics: Devices and Circuits 97 Discontinuous Condution Mode (DCM) In discontinuous conduction mode (DCM), there is an additional stage after Stage 2 in which the inductor current has already reached zero, as shown in Figs 5.9 and 5.10 The output voltage is sustained by the output capacitor C Since the average... gate-to-source of the MOSFET For example, the half-bridge 5 Power Electronics: Devices and Circuits 102 driver model IR2111 from International Rectifier in Fig 5.13 The functional block diagram is shown in Fig 5.14 The whole idea of this driver is to provide voltage to charge up the upper MOSFET vGS This voltage is from the small capacitor across VB and VS in Fig 5.13 How to charge up this capacitor? It...5 Power Electronics: Devices and Circuits 93 applied across the inductor is VL (t) = VS − Vo (t) (5.1) Since the resultant voltage is positive on VL , inductor L is charged up linearly with a rate equals VL (t) VS − Va diL = = dt L L (5.2) where we assume Vo t is constant and at a value equals Va As the inductor current rises from I1 to... to decrease, as indicated from (5.4) L = DT (VS − Va ) 0.417 12 − 5 = × = 0.146mH ∆I 100000 0.2 We can see that by doubling the switching frequency, the inductance is reduced by half 5 Power Electronics: Devices and Circuits Q1 is -(t) iL (t) - 6 iD1 Df w VS 96 L Io - Ia + VL (t) − + VC (t) − Ic ? (t) + C RL Vo (t), Va − Figure 5.9: Stage 3 of the buck converter at DCM VDW (t)  - D1 T... switching losses in transistor Sources: Mohan 1995 [2] 98 5 Power Electronics: Devices and Circuits 99 Switching Loss When the switch control signal is at On state, after certain delay td(on) the transistor is closed The current of the transistor increases while the voltage across it decreases The duration takes tc(on) = tri + tf v for iT to reach Io and vT to reach Von Von is non-zero in practical transistor . [2]. 5. Power Electronics: Devices and Circuits 86 Figure 5.3: Diode switching characteristics. Sources: Mohan 1995 [2]. 5. Power Electronics: Devices and Circuits 87 5.3 Power Semiconductor Devices. Chapter 5 Power Electronics: Devices and Circuits 5.1 Introduction Power electronics is an enabling technology for all electrical and electronic apparatus requiring electric power to. new circuit design and topologies, optimized control and packaging techniques, in order to meet the industry demands. Power electronics is all about using electronic devices and circuits with storage