功率半導(dǎo)體器件基本原理第10章

1、Chapter 10SynopsisPower devices are required for systems that operate over a broad spectrum of power levels and frequencies as discussed in Chap. 1. A variety of power rectifier and transistor structures were discussed in previous chapters for serving these applications. Although the bipolar power t

2、ransistor and the thyristor were the first technologies developed for this purpose, they have been replaced by power MOSFET and IGBT structures in modern applications due to the resulting simplification of the control circuit and elimination of snubbers. The choice of the optimum device suitable for

3、 an application depends upon the device voltage rating and the circuit switching frequency. From the point of view of presenting a unified treatment, it is convenient to analyze a typical pulse-width-modulated (PWM) motor control circuit as an example because it is utilized for both low-voltage appl

4、ications, such as disk drives in computers, and high-voltage applications, such as the drive train in hybrid electric vehicles and electric locomotives.10.1 Typical H-Bridge TopologyThe control of motors using PWM circuits is typically performed by using the H-bridge configuration shown in Fig. 10.1

5、. In this figure, the circuit has been implemented using four IGBT devices as the switches and four PiN rectifiers as the fly-back diodes. This is the commonly used topology for medium and high power motor drives where the DC bus voltage exceeds 200 V. When the H-bridge topology is used for applicat

6、ions that operate from a low DC bus voltage, it is typically implemented using four power MOSFET devices as the switches and four Schottky rectifiers as the fly-back diodes.The direction of the current flow in the motor winding can be controlled with the H-bridge configuration. If IGBT-1 and IGBT-4

7、are turned on whileB.J. Baliga, Fundamentals of Power Semiconductor Devices, doi: 10.1007/978-0-387-47314-7_10, © Springer Science + Business Media, LLC 20081028 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICESmaintaining IGBT-2 and IGBT-3 in their blocking mode, the current in the motor will flow f

8、rom the left side to the right side in the figure. The direction of the current flow can be reversed if IGBT-3 and IGBT-2 are turned on while maintaining IGBT-1 and IGBT-4 in their blocking mode. Alternately, the magnitude of the current flow can be increased or decreased by turning on the IGBT devi

9、ces in pairs. This method allows synthesis of a sinusoidal waveform across the motor1Synopsis 1029The typical waveforms for the current and the voltage across the power transistor and the fly-back diode are illustrated in Fig. 10.2 during just one cycle of the PWM operation. These waveforms have bee

10、n linearized for simplification of the analysis.2 The cycle begins at time t1 when the transistor is turned on by its gate drive voltage. Prior to this time, the transistor is supporting the DC supply voltage and the fly-back diode is assumed to be carrying the motor current. As the transistor turns

11、 on, the motor current is transferred from the diode to the transistor during the time interval from t1 to t2. In the case of high DC bus voltages, where PiN rectifiers are utilized, the fly-back diode will not be able to support voltage until the stored charge in its drift region is removed as disc

12、ussed in Chap. 5. To achieve this, the PiN rectifier must undergo its reverse recovery process. During reverse recovery, substantial reverse current flows through the rectifier with a peak value IPR reached at time t2. The large reverse recovery current produces significant power dissipation in the

13、diode. Moreover, the current in the IGBT at time t2 is the sum of the motor winding current IM and the peak reverse recovery current IPR. This produces substantial power dissipation in the transistor during the turn-on transient. The power dissipation in both the transistor and the diode is therefor

14、e governed by the reverse recovery characteristics of the power rectifier.The power transistor is turned off at time t4 allowing the motor current to transfer from the transistor to the diode. In the case of an inductive load, such as motor windings, the voltage across the transistor increases befor

15、e the current is reduced as illustrated in Fig. 10.2 during the time interval from t4 to t5. Subsequently, the current in the transistor reduces to zero during the time interval from t5 to t6. The turn-off durations are governed by the physics of the transistor structure as discussed in previous cha

16、pters. Consequently, the power dissipation in both the transistor and the diode during the turn-off event is determined by the transistor switching characteristics.In addition to the power losses associated with the two basic switching events within each cycle, power loss is incurred within the diod

17、e and the transistor during their respective on-state operation due to a finite on-state voltage drop. It is common practice to trade off a larger on-state voltage drop to obtain a smaller switching loss in the bipolar power devices. Consequently, the on-state power loss cannot be neglected especial

18、ly if the operating frequency is low. The leakage current for the devices is usually sufficiently small, so that the power loss in the blocking mode can be neglected.10.2 Power Loss AnalysisThe total power loss incurred in the power transistor can be obtained by summing four components:PL,T(total)=P

19、L,T(on)+PL,T(off)+PL,T(turn-on)+PL,T(turn-off). (10.1)1030 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICESThe power loss incurred in the transistor during the on-state duration from time t3 to t4 is given byPL,T(on)=t4t3IMVON,T. T (10.2)The power loss incurred in the transistor during the off-state dura

20、tion beyond time t6 until the next turn-on event is given byPL,T(off)=Tt6IL,TVDC. T (10.3)The leakage current (IL,T) for the transistors is usually very small allowing this term to be neglected during the power dissipation analysis.The power loss incurred in the transistor during the turn-on event f

21、rom time t1 to t3 can be obtained by analysis of the segments between the time intervals t1 to t2 and t2 to t3. The power loss incurred during the first segment is given byPL,T1(turn-on)=1t2t1IPTVDC, 2T (10.4)where the peak transistor current is dependent on the peak reverse recovery current of the

22、PiN rectifier:IPT=IM+IPR. (10.5)In the power loss analysis, it will be assumed that the time duration (t2 t1) is determined by the reverse recovery behavior of the PiN rectifier and is independent of the operating frequency. The power loss incurred during the second segment is given byPL,T2(turn-on)

23、=1t3t2IPT+IM2T2VDC. (10.6)In the power loss analysis, it will be assumed that the time duration (t3 t2) is also determined by the reverse recovery behavior of the PiN rectifier and is independent of the operating frequency.The power loss incurred in the transistor during the turn-off event from time

24、 t4 to t6 can be obtained by analysis of the segments between the time intervals t4 to t5 and t5 to t6. The power loss incurred during the first segment is given byPL,T1(turn-off)=1t5t4IMVDC. 2T (10.7)The time interval (t5 t4) is determined by the time taken for the transistor voltage to rise to the

25、 DC power supply voltage. This time duration was analyzed for eachSynopsis 1031transistor in the previous chapters. The power loss incurred during the second segment is given byPL,T2(turn-off)=1t6t5IMVDC. 2T (10.8)The time interval (t6 t5) is determined by the time taken for the transistor current t

26、o decay to zero. This time duration was analyzed for each transistor in the previous chapters.In a similar manner, the total power loss incurred in the power rectifier can be obtained by summing four components:PL,R(total)=PL,R(on)+PL,R(off)+PL,R(turn-on)+PL,R(turn-off). (10.9) The power loss incurr

27、ed in the power rectifier during the on-state duration from time t6 to the end of the period is given byPL,R(on)=Tt6IMVON,R. T (10.10)In writing this expression, it is assumed that the cycle begins at time t1. The power loss incurred in the power rectifier during the off-state time duration (t4 t3)

28、is given byPL,R(off)=t4t3IL,RVDC. T (10.11)The leakage current (IL,R) for the power rectifier will be assumed to be very small (even for the silicon Schottky rectifier) allowing this term to be neglected during the power dissipation analysis.The power loss incurred in the power rectifier during the

29、turn-on event from time t1 to t3 can be obtained by analysis of the segments between the time intervals t1 to t2 and t2 to t3. The power loss incurred during the first segment is much smaller than during the second segment due to the small on-state voltage drop for the power rectifiers. The power lo

30、ss incurred during the second segment is given byPL,R2(turn-on)=1t3t2IPRVDC. 2T (10.12)The power loss incurred in the power rectifier during the turn-off event from time t4 to t6 can be obtained by analysis of the segments between the time intervals t4 to t5 and t5 to t6. The power loss incurred dur

31、ing the first segment is negligible due to the low leakage current for the power rectifier. The power loss incurred during the second segment is given by1032 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICESPL,R2(turn-off)=1t6t5IMVON,D. (10.13) 2TThis power loss is also small due to the low on-state volta

32、ge drop of power rectifiers.10.3 Low DC Bus Voltage ApplicationsIn this section, the above power loss analysis is applied to a motor control application using a low DC bus voltage with a duty cycle of 50%. The DC bus voltage (VDC) will be assumed to be 20 V as pertains to the backplane power source

33、in desktop computers. In this case, the device blocking voltage rating is typically 30 V. The current being delivered to the motor winding (IM) will be assumed to be 10 A. Due to the low blocking voltage required for this application, it is commonly implemented using silicon unipolar devices, namely

34、 the power MOSFET as the power switch. To reduce the cost and packaging complexity, it is attractive to use the integral body diode within the power MOSFET structure instead of a separate antiparallel fly-back diode. The reverse recovery characteristics of the integral body diode can be optimized by

35、 using electron irradiation.3 Alternately, a Schottky diode has been integrated in the power MOSFET cell for the JBSFET structure4 allowing suppression of the reverse recovery phenomenon without the packaging complexity of using an external Schottky fly-back diode.Silicon Silicon 4H-SiC Characterist

36、ics MOSFET IGBT MOSFETOn-StateVoltage Drop 0.05 0.90 0.08 (V) Turn-Off Time (t5 t4) 0.01 0.1 0.01 (µs) Turn-Off Time (t6 t5) 0.01 0.1 0.01 (µs) Fig. 10.3 Characteristics of transistors with 30-V blocking voltage ratingThe characteristics for the transistors that are pertinent to the analys

37、is of the power loss are provided in Fig. 10.3. The on-state voltage drop of 0.05 V for the silicon MOSFET device is based upon using a specific on-resistance of 0.5 m cm2 and an on-state current density of 100 A cm2. This specific on-resistance is typical for U-MOSFET devices, as well as the planar

38、-gate SSCFET devices, described in Chap. 6. For comparison purposes, the silicon IGBT deviceSynopsis 1033and the silicon carbide power MOSFET device are included in the power analysis. The on-state voltage drop (0.90 V) for the silicon IGBT device with such a low blocking voltage rating is limited b

39、y the voltage drop across the P+ collector/N-base junction. In the case of the 4H-SiC power MOSFET structure, the specific on-resistance becomes limited by the N+ substrate (0.4 m cm2) and the channel (0.4 m cm2) contributions because the drift region contribution is extremely small (see Fig. 6.162

40、for the shielded trench-gate 4H-SiC MOSFET structure with 1-µm channel length). These devices are also assumed to be operated at an on-state current density of 100 A cm2.Silicon 4H-SiC Silicon CharacteristicsP-i-N Schottky Schottky On-State Voltage Drop 0.5 0.9 1.0 (V) Turn-On Time (t2 t1) 0.01

41、 0.10 0.01 (µs) Turn-On Time(µs) Peak Reverse Recovery 0 5 0 Current ()Fig. 10.4 Characteristics of rectifiers with 30-V blocking voltage ratingThe characteristics for the power rectifiers that are pertinent to the analysis of the power loss are provided in Fig. 10.4. The on-state voltage

42、drop of 0.5 V for the silicon Schottky diode (or the integral diode within the JBSFET structure) is based upon an on-state current density of 100 A cm2 (see Fig. 4.7). For comparison purposes, the silicon PiN rectifier (or the integral diode within the MOSFET structure) and the silicon carbide Schot

43、tky rectifier are included in the power analysis. The on-state voltage drop (0.90 V) for the silicon PiN rectifier (or the integral diode within the MOSFET structure) with such a low blocking voltage rating is limited by the voltage drop across the P/N junction. In the case of the 4H-SiC Schottky re

44、ctifier structure, the on-state voltage drop is limited by the barrier height for the metalsemiconductor contact (see Fig. 4.8). These devices are also assumed to be operated at an on-state current density of 100 A cm2. The reverse recovery current for the Schottky diodes is negligible because of un

45、ipolar operation in the on-state. For the PiN rectifier (or the integral diode within the MOSFET structure), the peak reverse recovery current is assumed to be equal to half the on-state current. The power loss incurred in the transistor and the fly-back diode can be computed as a function of the op

46、erating frequency by using the equations provided in Sect. 10.2 and the numerical values in the above figures.1034 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICESAs an example, the power losses in the case of the silicon power MOSFET as the power switch with a silicon Schottky rectifier as the diode (al

47、so representative of the JBSFET structure) are provided in Fig. 10.5 for frequencies ranging to 20 kHz. The power losses during the on-state are dominant in both the transistor and the diode in this case because of the fast switching speeds for the unipolar devices. Consequently, there is only a sli

48、ght increase in power losses as the frequency increases. The power loss in the rectifier is much larger than that in the transistor due to its much larger on-state voltage drop. The total power loss is only 2.75 W when 200 W of power is delivered to the load.When the Schottky rectifier is replaced b

49、y a PiN rectifier representative of the integral body diode in the power MOSFET structure, the power losses increase considerably as shown in Fig. 10.6. A major part of the increase in power loss is due to the larger on-state voltage drop of the PiN rectifier. However, the power loss also increases

50、with frequency due to the contribution from the switching losses. The switching loss is associated with the reverse recovery of the PiN rectifier. It is worth pointing out that the reverse recovery transient for the power rectifier also produces increased switching losses in the transistor during it

51、s turn-on event. The total power loss is increased to about 5 W when 200 W of power is delivered to the load.Synopsis 10351036 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICESWhen the silicon power MOSFET is replaced by an IGBT and the silicon Schottky rectifier is replaced by a PiN rectifier, the power

52、losses increase by an even greater amount as shown in Fig. 10.7. A major part of the increase in power loss is due to the larger on-state voltage drop of the PiN rectifier and the IGBT structure. Moreover, the power loss increases with frequency due to the contribution from the switching losses. The

53、 switching loss is associated with the reverse recovery of the PiN rectifier producing increased switching losses in the transistor during its turn-on event. The total power loss is increased to about 10 W when 200 W of power is delivered to the load. The above example is provided to illustrate that

54、 the silicon power MOSFET structure is more suitable for applications operating from low DC bus voltages than the IGBT structure.upon wide band-gap semiconductors to improve system efficiency. The impact of using a silicon carbide MOSFET device as the transistor and a silicon carbide Schottky rectif

55、ier as the fly-back diode is provided in Fig. 10.8. When compared with the silicon unipolar devices, there is a considerable increase in the power dissipation especially due to the high on-state voltage drop for the 4H-SiC Schottky rectifier. The total power loss is increased to about 5.5 W when 200

56、 W of power is delivered to the load. This example is provided to illustrate that silicon unipolar devices are more suitable for applications operating from low DC bus voltages than devices based upon wide band-gap semiconductors.Synopsis 103710.4 Medium DC Bus Voltage ApplicationsIn this section, t

57、he above power loss analysis is applied to a motor control application using a medium DC bus voltage with a duty cycle of 50%. The DC bus voltage (VDC) will be assumed to be 400 V as pertains to the power source in a hybrid electric car. In this case, the device blocking voltage rating is typically

58、600 V. The current being delivered to the motor winding (IM) will be assumed to be 20 A. Due to the larger blocking voltage required for this application, it is commonly implemented using silicon bipolar devices, namely the IGBT as the power switch and the PiN rectifier as the fly-back diode.Silicon

59、 Silicon 4H-SiC CharacteristicsMOSFET IGBT MOSFETOn-State Voltage Drop 10 1.8 0.08 (V) Turn-Off Time (t5 t4) 0.01 0.1 0.01 (µs) Turn-Off Time (t6 t5) 0.01 0.2 0.01 (µs) Fig. 10.9 Characteristics of transistors with 600-V blocking voltage ratingThe characteristics for the transistors that are pertinent to the analysis of the power loss are provided in Fig. 10.9. In the case of the