Chapter 10
Synopsis
Power 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the bipolar power transistor and the thyristor were the first technologies developed for this purpose, they have been replaced by power MOSFET and IGBT structures in modern applications due to the resulting simplification of the control circuit and elimination of snubbers.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 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 applications, 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 Topology
The control of motors using PWM circuits is typically performed by using the H-bridge configuration shown in Fig. 10.1. In this figure, the circuit has been implemented using four IGBT devices as the switches and four P–i–N 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 applications 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 are turned on while
B.J. Baliga, Fundamentals of Power Semiconductor Devices, doi: 10.1007/978-0-387-47314-7_10, © Springer Science + Business Media, LLC 2008
1028 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES
maintaining IGBT-2 and IGBT-3 in their blocking mode, the current in the motor will flow from 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 devices in pairs. This method allows synthesis of a sinusoidal waveform across the motor windings with a variable frequency that is dictated by the PWM circuit.1 DC POWER BUS FLY-BACK FLY-BACK DIODEDIODE
MOTOR IGBT-1IGBT-3 WINDING FLY-BACK FLY-BACK DIODEDIODEIGBT-2IGBT-4 Fig. 10.1 Typical H-bridge topology for motor control
IPT
IGBTVDCVDC
IM
VON,T
t
IMIM
Rectifier
VON,DVON,D
t
IPR
VDC
t1t2t3t4t5t6
Fig. 10.2 Typical waveforms during PWM operation Synopsis 1029
The 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 been 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 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 P–i–N 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 discussed in Chap. 5. To achieve this, the P–i–N 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 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 therefore 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 before 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 chapters. 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 diode 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 especially 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 Analysis
The total power loss incurred in the power transistor can be obtained by summing four components:
PL,T(total)=PL,T(on)+PL,T(off)+PL,T(turn-on)+PL,T(turn-off). (10.1)
1030 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES
The power loss incurred in the transistor during the on-state duration from time t3 to t4 is given by
PL,T(on)=
t4−t3
IMVON,T. T
(10.2)
The power loss incurred in the transistor during the off-state duration beyond time t6 until the next turn-on event is given by
PL,T(off)=
T−t6
IL,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 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 given by
PL,T−1(turn-on)=
1t2−t1
IPTVDC,
2T
(10.4)
where the peak transistor current is dependent on the peak reverse recovery current of the P–i–N 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 P–i–N rectifier and is independent of the operating frequency. The power loss incurred during the second segment is given by
PL,T−2(turn-on)=
1t3−t2⎛IPT+IM
⎜
2T⎝2
⎞
⎟VDC. ⎠
(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 P–i–N rectifier and is independent of the operating frequency. The power loss incurred in the transistor 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 during the first segment is given by
PL,T−1(turn-off)=
1t5−t4
IMVDC.
2T
(10.7)
The time interval (t5 − t4) is determined by the time taken for the transistor voltage to rise to the DC power supply voltage. This time duration was analyzed for each
Synopsis 1031
transistor in the previous chapters. The power loss incurred during the second segment is given by
PL,T−2(turn-off)=
1t6−t5
IMVDC.
2T
(10.8)
The time interval (t6 − t5) is determined by the time taken for the transistor current to 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 incurred in the power rectifier during the on-state duration from time t6 to the end of the period is given by
PL,R(on)=
T−t6
IMVON,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) is given by
PL,R(off)=
t4−t3
IL,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 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 loss incurred during the second segment is given by
PL,R−2(turn-on)=
1t3−t2
IPRVDC.
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 during 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 by
1032 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES
PL,R−2(turn-off)=
1t6−t5
IMVON,D. (10.13)
2T
This power loss is also small due to the low on-state voltage drop of power rectifiers.
10.3 Low DC Bus Voltage Applications
In 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 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 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 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
Characteristics
MOSFET IGBT MOSFET On-State
Voltage 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 rating
The characteristics for the transistors that are pertinent to the analysis 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 mΩ cm2 and an on-state current density of 100 A cm−2. This specific on-resistance is typical for U-MOSFET devices, as well as the planar-gate SSCFET devices, described in Chap. 6. For comparison purposes, the silicon IGBT device
Synopsis 1033
and 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 by 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 mΩ cm2) and the channel (0.4 mΩ cm2) contributions because the drift region contribution is extremely small (see Fig. 6.162 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 cm−2.
Silicon 4H-SiC Silicon Characteristics
P-i-N Schottky Schottky
On-State
Voltage Drop 0.5 0.9 1.0
(V)
Turn-On Time (t2 – t1) 0.01 0.10 0.01 (µs) Turn-On Time
(t3 – t2) 0.01 0.10 0.01
(µs)
Peak Reverse
Recovery
0 5 0
Current
(Α)
Fig. 10.4 Characteristics of rectifiers with 30-V blocking voltage rating
The 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 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 cm−2 (see Fig. 4.7). For comparison purposes, the silicon P–i–N rectifier (or the integral diode within the MOSFET structure) and the silicon carbide Schottky rectifier are included in the power analysis. The on-state voltage drop (0.90 V) for the silicon P–i–N 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 rectifier structure, the on-state voltage drop is limited by the barrier height for the metal–semiconductor contact (see Fig. 4.8). These devices are also assumed to be operated at an on-state current density of 100 A cm−2. The reverse recovery current for the Schottky diodes is negligible because of unipolar operation in the on-state. For the P–i–N 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 operating frequency by using the equations provided in Sect. 10.2 and the numerical values in the above figures.
1034 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES
Silicon Power MOSFET with Silicon Schottky Rectifier
3.03.0
2.5 2.5 Total Power LossRectifier Power Loss 2.02.0
1.51.5
1.01.0
Transistor Power Loss 0.50.5 0.0002468101214161820 048121620Frequency (kHz) Operating Frequency (kHz)
Fig. 10.5 Power losses during motor control with 20-V DC bus: silicon power MOSFET
with silicon Schottky rectifier
As 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 (also 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 slight 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 by a P–i–N 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 P–i–N rectifier. However, the power loss also increases with frequency due to the contribution from the switching losses. The switching loss is associated with the reverse recovery of the P–i–N rectifier. It is worth pointing out that the reverse recovery transient for the power rectifier also produces increased switching losses in the transistor during its turn-on event. The total power loss is increased to about 5 W when 200 W of power is delivered to the load.
Power Loss (W)Power Loss (W) Synopsis 1035
6.06.0
Silicon Power MOSFET with Silicon P-i-N Rectifier5.05.0Power Loss (W)4.04.0Power Loss (W)Total Power LossRectifier Power Loss3.03.02.02.01.01.0Transistor Power Loss0.000024468810Frequency (kHz)1212141616182020Operating Frequency (kHz)Fig. 10.6 Power losses during motor control with 20-V DC bus: silicon power MOSFET with silicon P-i-N rectifier Silicon IGBT with Silicon P-i-N Rectifier12.012.010.010.0Power Loss (W)Power Loss (W)8.08.0Total Power Loss6.06.0Transistor Power Loss4.04.0Rectifier Power Loss2.02.00.000024468810Frequency (kHz)1212141616182020Operating Frequency (kHz)Fig. 10.7 Power losses during motor control with 20-V DC bus: silicon IGBT with silicon P-i-N rectifier 1036 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES
When the silicon power MOSFET is replaced by an IGBT and the silicon Schottky rectifier is replaced by a P–i–N rectifier, the power 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 P–i–N rectifier and the IGBT structure. Moreover, the power loss increases with frequency due to the contribution from the switching losses. The switching loss is associated with the reverse recovery of the P–i–N 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 the silicon power MOSFET structure is more suitable for applications operating from low DC bus voltages than the IGBT structure. 4H-SiC Power MOSFET with 4H-SiC Schottky Rectifier6.0 6.0 5.05.0 Total Power Loss Rectifier Power Loss4.04.0 3.0 3.0 2.02.0 Transistor Power Loss1.01.0 0.00 02468101214161820048121620Frequency (kHz) Operating Frequency (kHz) Fig. 10.8 Power losses during motor control with 20-V DC bus: silicon carbide power MOSFET with silicon carbide Schottky rectifier There is considerable interest in replacing silicon devices with those based 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 rectifier 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 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.
Power Loss (W)Power Loss (W) Synopsis 1037
10.4 Medium DC Bus Voltage Applications
In this section, the 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 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 P–i–N rectifier as the fly-back diode. Silicon Silicon 4H-SiC
Characteristics
MOSFET IGBT MOSFET
On-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 rating
The 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 silicon power MOSFET structure, the specific on-resistance is 0.1 Ω cm2 for a device capable of blocking 600 V as shown in Fig. 6.50. This results in an on-state voltage drop of 10 V for the silicon MOSFET device based upon using an on-state current density of 100 A cm−2. The on-state voltage drop (1.80 V) and switching times for the silicon IGBT device with 600-V blocking voltage rating are based upon scaling the characteristics for the 1,200-V structure modeled in Chap. 9. The on-state voltage drop for the asymmetric IGBT structure shown in Fig. 9.57 and the turn-off waveforms shown in Fig. 9.114 for the 1,200-V structure were used as the basic for the scaling. In the case of the 4H-SiC power MOSFET structure, the drift region contribution increases to 0.03 mΩ cm2 (see Fig. 3.6), which is still much smaller than the specific on-resistance contributed by the N+ substrate (0.4 mΩ cm2) and the channel (0.4 mΩ cm2). Consequently, a specific on-resistance of 0.8 mΩ cm2 has been used for the silicon carbide MOSFET structure. These devices are also assumed to be operated at an on-state current density of 100 A cm−2.
The characteristics for the power rectifiers that are pertinent to the analysis
of the power loss are provided in Fig. 10.10. The on-state voltage drop of 5.4 V for the silicon Schottky diode is based upon an on-state current density of 100 A cm−2 (see Fig. 4.7). The high on-state voltage drop for the silicon Schottky diode precludes
1038 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES
its use in this application despite its excellent switching behavior. For comparison purposes, the silicon P–i–N rectifier and the silicon carbide Schottky rectifier are included in the power analysis. The on-state voltage drop (2.0 V) for the silicon P–i–N rectifier is typical for fast switching rectifiers. In the case of the 4H-SiC Schottky rectifier structure, the on-state voltage drop is limited by the barrier height for the metal–semiconductor contact due to the low specific on-resistance for the drift region (see Fig. 4.8). These devices are also assumed to be operated at an on-state current density of 100 A cm−2. The reverse recovery current for the Schottky diodes is negligible because of unipolar operation in the on-state. For the P–i–N rectifier, the peak reverse recovery current is assumed to be equal to half the on-state current. Based upon the information provided in Fig. 10.10, the silicon Schottky rectifier is not a viable alternative for power loss analysis in medium voltage applications due to its high on-state voltage drop.
Silicon 4H-SiC Silicon Characteristics
P-i-N Schottky Schottky
On-State
Voltage Drop 5.4 2.0 1.0
(V)
Turn-On Time (t2 – t1) 0.01 0.20 0.01 (µs) Turn-On Time
(t3 – t2) 0.01 0.20 0.01
(µs)
Peak Reverse
Recovery
0 10 0
Current
(Α)
Fig. 10.10 Characteristics of rectifiers with 600-V blocking voltage rating
The power losses in the case of the silicon power MOSFET as the power switch with a silicon P–i–N rectifier as the diode are provided in Fig. 10.11 for frequencies ranging to 20 kHz. The on-state power losses in the transistor are dominant in this case. The power loss in the transistor increases with increasing frequency due to the increasing turn-on losses associated with the reverse recovery of the P–i–N rectifier. The power loss in the transistor is much larger than in the rectifier due to its large on-state voltage drop. The total power loss is 185 W at 20 kHz when 8,000 W of power is delivered to the load. When the silicon power MOSFET is replaced by the IGBT structure, the power losses are considerably reduced as shown in Fig. 10.12. A major portion of the reduction in the power loss occurs due to the smaller on-state voltage drop for the IGBT structure. The switching power loss for the IGBT structure increases and becomes much larger than the on-state power loss at 20 kHz. The switching power
Synopsis 1039
200200
Silicon Power MOSFET with Silicon P-i-N RectifierTotal Power Loss150150Power Loss (W)Power Loss (W)100100Transistor Power Loss5050Rectifier Power Loss000024468810Frequency (kHz)1212141616182020Operating Frequency (kHz)Fig. 10.11 Power losses during motor control with 400-V DC bus: silicon power MOSFET with silicon P-i-N rectifier Silicon IGBT with Silicon P-i-N Rectifier120120100100Power Loss (W)8080Total Power LossPower Loss (W)60604040Transistor Power LossRectifier Power Loss2020000024468810Frequency (kHz)1212141616182020Operating Frequency (kHz) Fig. 10.12 Power losses during motor control with 400-V DC bus: silicon IGBT with silicon P-i-N rectifier 1040 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES
loss in the IGBT during the turn-on event is about twice as large as during the turn-off event. The total power loss is reduced to 110 W when 8,000 W of power is delivered to the load. Due to the greater efficiency in the motor control application, the IGBT-based drives are commonly used for hybrid electric vehicles and other consumer applications, such as air conditioning and ventilation, operated from medium DC bus voltages. Silicon IGBT with Silicon Carbide Schottky Rectifier6060
5050 Total Power Loss
4040
3030
Transistor Power Loss
2020
Rectifier Power Loss 1010
00 02468101214161820048121620Frequency (kHz)
Operating Frequency (kHz)
Fig. 10.13 Power losses during motor control with 400-V DC bus: silicon IGBT with silicon carbide Schottky rectifier
Ever since the first demonstration of the low on-state voltage drop and outstanding switching characteristics for the silicon carbide Schottky rectifiers,5 there has been a considerable interest in replacing the silicon P–i–N rectifiers with this device for improving the performance of motor drives. The reduction in the power losses that can be achieved with this approach is shown in Fig. 10.13. A significant decrease in power loss is achieved due to the smaller on-state voltage drop of the 4H-SiC Schottky rectifier. Moreover, the power loss in the IGBT is reduced due to the elimination of the reverse recovery of the P–i–N rectifier. The total power loss is reduced to 53 W at 20 kHz when 8,000 W of power is delivered to the load. This example represents the earliest potential adoption of silicon carbide-based power devices in commercial applications.
An even superior technical solution can be produced by using the silicon
carbide power MOSFET as the switch and the silicon carbide Schottky rectifier as the fly-back diode as illustrated by the power loss provided in Fig. 10.14. In this case, the power loss in the transistor is greatly reduced due to the low on-state voltage drop for the silicon carbide power MOSFET. The largest component of the
Power Loss (W)Power Loss (W) Synopsis 1041
total power loss is associated with the on-state voltage drop for the silicon carbide Schottky rectifier. The power loss does not increase as rapidly with frequency in this approach because of its implementation with only unipolar devices. The total power loss is reduced to only 14 W at 20 kHz when 8,000 W of power is delivered to the load. This example demonstrates the full potential for improving the efficiency of motor drives by using silicon carbide-based power devices. Silicon Carbide MOSFET with Silicon Carbide Schottky Rectifier15 15 Total Power Loss
1010 Rectifier Power Loss 55
Transistor Power Loss
00 02468101214161820048121620 Frequency (kHz)Operating Frequency (kHz) Fig. 10.14 Power losses during motor control with 400-V DC bus: silicon carbide MOSFET
Power Loss (W)Power Loss (W)with silicon carbide Schottky rectifier
10.5 High DC Bus Voltage Applications
In this section, the above power loss analysis is applied to a motor control application using a high DC bus voltage with a duty cycle of 50%. The DC bus voltage (VDC) will be assumed to be 3,000 V as pertains to the power source for electric locomotive drives such as the Shinkansen bullet train. In this case, the device blocking voltage rating is typically 4,500 V. The current being delivered to the motor winding (IM) will be assumed to be 1,000 A. Due to the larger blocking voltage required for this application, the motor drive is commonly implemented using silicon bipolar devices as the power switch and the silicon P–i–N rectifier as the fly-back diode. Until the turn of the century, the power switch that was utilized by the traction industry was the gate turn-off thyristor. Since then, it is commonplace to implement the motor drive using IGBT devices.
1042 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES
Characteristics
On-State Voltage Drop
(V)
Turn-Off Time
(t5 – t4) (µs)
Turn-Off Time
(t6 – t5) (µs)
Silicon MOSFET
Very High
Silicon IGBT
3.0
4H-SiC MOSFET
0.38
- 2.0 0.1
- 1.0 0.1
Fig. 10.15 Characteristics of transistors with 4,500-V blocking voltage rating
Characteristics
On-State Voltage Drop
(V)
Turn-On Time
(t2 – t1) (µs)
Turn-On Time
(t3 – t2) (µs)
Peak Reverse Recovery Current (Α)
Silicon Schottky
Very High
Silicon P-i-N
2.5
4H-SiC Schottky
1.0
- 2.0 0.1
- 2.0 0.1
- 1000 0
Fig. 10.16 Characteristics of rectifiers with 4,500-V blocking voltage rating
The characteristics for the transistors that are pertinent to the analysis of
the power loss are provided in Fig. 10.15. In the case of the silicon power MOSFET structure, the specific on-resistance increases to a prohibitively large magnitude (10 Ω cm2) for a device capable of blocking 4,500 V as shown in Fig. 3.6. The silicon MOSFET device is therefore not viable for such high-voltage applications. The on-state voltage drop (3 V) and switching times for the silicon IGBT device with 4,500-V blocking voltage rating are based upon scaling the characteristics for the 1,200-V structure modeled in Chap. 9 as well as the values reported in the literature.6,7 In the case of the 4H-SiC power MOSFET structure, the drift region contribution increases to 3 mΩ cm2 (see Fig. 3.6), which is comparable to the specific on-resistance contributed by the N+ substrate (0.4 mΩ cm2) and the channel (0.4 mΩ cm2). Consequently, a specific on-resistance of 3.8 mΩ cm2 has been used for the silicon carbide MOSFET structure. These devices are also assumed to be operated at an on-state current density of 100 A cm−2.
Synopsis 1043
The characteristics for the power rectifiers that are pertinent to the analysis
of the power loss are provided in Fig. 10.16. The on-state voltage drop of the silicon Schottky diode is extremely large due to the very high resistance of the drift region (see Fig. 4.7). For comparison purposes, the silicon P–i–N rectifier and the silicon carbide Schottky rectifier are included in the power analysis. The on-state voltage drop (2.5 V) for the silicon P–i–N rectifier and its reverse recovery current are typical for such high-voltage structures.7 In the case of the 4H-SiC Schottky rectifier structure, the on-state voltage drop is mostly incurred in the drift region due to its high specific on-resistance (see Fig. 4.8). These devices are assumed to be operated at an on-state current density of 100 A cm−2. From the information provided in Fig. 10.16, it can be concluded that the silicon Schottky rectifier is not a viable alternative for such high-voltage applications.
Silicon IGBT with Silicon P-i-N Rectifier100000 0.10
800000.08
600000.06 Total Power Loss 400000.04Transistor Power Loss
20000 0.02 Rectifier Power Loss
00 012345012345Frequency (kHz)
Operating Frequency (kHz)
Fig. 10.17 Power losses during motor control with 3,000-V DC bus: silicon IGBT with silicon P-i-N rectifier
As an example, the power losses in the case of the silicon IGBT as the power switch with a silicon P–i–N rectifier as the fly-back diode are provided in Fig. 10.17 for frequencies ranging to 5 kHz. The power losses during switching are dominant in this case. The power loss in the transistor grows with increasing frequency due to the greater turn-on and turn-off losses. The turn-on losses in the silicon IGBT, associated with the reverse recovery of the P–i–N rectifier, are twice as large as the turn-off losses. The total power loss is 0.085 MW at 5 kHz when 3 MW of power is delivered to the load. There is considerable interest in replacing the silicon P–i–N rectifiers with the silicon carbide-based Schottky rectifier for improving the performance of
PowLoss ) (MW)Poewrer Loss (MW1044 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES
motor drives even in the case of high DC bus voltage applications. The reduction in the power losses that can be achieved with this approach is shown in Fig. 10.18. A significant decrease in power loss is achieved due to the smaller on-state voltage drop of the 4H-SiC Schottky rectifier. Moreover, the power loss in the IGBT is reduced due to the elimination of the reverse recovery of the P–i–N rectifier. The total power loss is reduced to 0.025 MW at 5 kHz when 3 MW of power is delivered to the load. This example represents another early potential adoption of silicon carbide-based power devices in commercial applications because epitaxial material suitable for delivering blocking voltage capability of over 5,000 V is already available. Silicon IGBT with Silicon Carbide Schottky Rectifier300000.030
250000.025 200000.020
Total Power Loss
150000.015
Transistor Power Loss
10000 0.010 50000.005 Rectifier Power Loss
00 012345012345Frequency (kHz) Operating Frequency (kHz)
Fig. 10.18 Power losses during motor control with 3,000-V DC bus: silicon IGBT with
silicon carbide Schottky rectifier
An even superior technical solution for applications using high DC bus voltages can be produced by using the silicon carbide power MOSFET as the switch and the silicon carbide Schottky rectifier as the fly-back diode as illustrated by the power loss provided in Fig. 10.19. In this case, the power loss in the transistor is greatly reduced due to the low on-state voltage drop for the silicon carbide power MOSFET structure. The turn-on and turn-off losses contribute equally to the increase in power loss with frequency in the silicon carbide power MOSFET structure. The power loss does not increase as rapidly with frequency in this approach because of its implementation with only unipolar devices. The total power loss is reduced to only 0.004 MW at 5 kHz when 3 MW of power is delivered to the load. This example demonstrates the full potential for improving the efficiency of motor drives by using silicon carbide-based power devices.
Power Loss (MW)Power Loss (W) Synopsis 1045
Silicon Carbide MOSFET with Silicon Carbide Schottky Rectifier
40000.004
30000.003Total Power Loss
20000.002 Transistor Power Loss
1000 0.001Rectifier Power Loss 00012345 012345Frequency (kHz) Operating Frequency (kHz)
Fig. 10.19 Power losses during motor control with 3,000-V DC bus: silicon carbide
MOSFET with silicon carbide Schottky rectifier
10.6 Summary
This chapter allows comparison of the benefits of utilizing various technologies for a broad range of applications. It can be concluded that silicon power MOSFETs and Schottky rectifiers provide the best performance in applications working from a low DC bus voltage. Silicon bipolar devices and those created from wide band-gap semiconductors will not displace this technology in the future. On the other hand, for applications working from a medium DC bus voltage, it is advantageous to utilize the silicon IGBT and P–i–N rectifier as a low cost technology. In these applications, silicon carbide-based Schottky rectifiers greatly reduce power losses when used in conjunction with the silicon IGBT devices. Even further gains in efficiency can be obtained by replacing the silicon IGBT with silicon carbide power MOSFET devices when the cost and manufacturing capability for the silicon carbide switches mature. The same benefits translate to the applications working from a high DC bus voltage as well.
Problems
10.1 Name the four basic components of power loss in transistors and rectifiers
used in PWM motor control circuits.
Power Loss (MW)Power Loss (W)1046 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES
10.2
10.3
Consider motor control performed from a 20-V DC bus. What is the best technology for the transistor and rectifier to minimize power losses? Calculate the power loss occurring at a PWM operating frequency of 10 kHz for the technology choice in Problem 10.2 if the motor current is 10 A. Use the on-state voltage drop and switching times provided in Figs. 10.3 and 10.4 for the analysis.
Consider motor control performed from a 400-V DC bus. What is the best silicon technology for the transistor and rectifier to minimize power losses?
Calculate the power loss occurring at a PWM operating frequency of 10 kHz for the technology choice in Problem 10.4 if the motor current is 20 A. Use the on-state voltage drop and switching times provided in Figs. 10.9 and 10.10 for the analysis.
Consider motor control performed from a 400-V DC bus. What is the best silicon carbide technology for the transistor and rectifier to minimize power losses?
Calculate the power loss occurring at a PWM operating frequency of 10 kHz for the technology choice in Problem 10.6 if the motor current is 20 A. Use the on-state voltage drop and switching times provided in Figs. 10.9 and 10.10 for the analysis.
Consider motor control performed from a 3,000-V DC bus. What is the best silicon technology for the transistor and rectifier to minimize power losses?
Calculate the power loss occurring at a PWM operating frequency of 3 kHz for the technology choice in Problem 10.8 if the motor current is 1,000 A. Use the on-state voltage drop and switching times provided in Figs. 10.15 and 10.16 for the analysis.
10.4
10.5
10.6
10.7
10.8
10.9
10.10 Consider motor control performed from a 3,000-V DC bus. What is the
best silicon carbide technology for the transistor and rectifier to minimize power losses?
10.11 Calculate the power loss occurring at a PWM operating frequency of
3 kHz for the technology choice in Problem 10.10 if the motor current is 1,000 A. Use the on-state voltage drop and switching times provided in Figs. 10.15 and 10.16 for the analysis.
Synopsis 1047
10.12 Based upon the power loss analyses done in the previous problems,
estimate the DC bus voltage above which silicon carbide technology will provide an improvement in efficiency for motor control applications.
References
B.K. Bose, Power Electronics and Variable Frequency Drives (IEEE, New York, 1997) 2
B.J. Baliga, Power semiconductor devices for variable-frequency drives,
1
Proceedings of the IEEE, 82, 1112–1122, 1994 3
B.J. Baliga and J.P. Walden, Improving the reverse recovery of power MOSFET integral diodes by electron irradiation, Solid-State Electronics, 26, 1133–1141, 1983 4
B.J. Baliga and D.A. Girdhar, Paradigm shift in power MOSFET technology, Power Electronics Technology Magazine, 24–32, 2003 5
M. Bhatnagar, P.K. McLarty, and B.J. Baliga, Silicon carbide high voltage (400 V) Schottky barrier diodes, IEEE Electron Device Letters, 13, 501–503, 1992 6
R. Hotz, F. Bauer, and W. Fichtner, On-state and short circuit behavior of high voltage trench gate IGBTs in comparison with planar IGBTs, IEEE International Symposium on Power Semiconductor Devices and ICs, 224–229, 1995 7
F. Bauer et al., 6.5 kV Modules using IGBTs with field stop technology, IEEE International Symposium on Power Semiconductor Devices and ICs, 121–124, 2001
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