Semiconductor Devices
2.7 ADVANCED POWER DEVICES
Despite the technology saturation in what concerns conversion circuits and convert-ers, the power semiconductor sector is still dynamic. There is continuous develop-ment along existing devices like MOSFET and IGBT. New generations of IGBTs and MOSFETs are introduced each year to the market and their performance is continu-ously improving, especially through the design rule improvement (the pitch resolu-tion in defining the shape of each semiconductor region).
However, the most exciting news about completely new devices is their ability to change performance patterns through disruptive innovation. These disruptive devices can be classified in three categories:
• New devices solving certain issues with conventional devices and hence dedicated to certain peculiar applications (IGCT, IGBT-RC, IGBT-RB, and so on).
• High-frequency, high-voltage devices aiming at increasing the operation frequency with a degree of magnitude, and therefore requiring a complete overhaul of the inverter assembly.
• Devices using emerging new substrate materials and aiming at efficiency improvements through a new class of devices. Their novelty may lead to changes in the design of the gate driver and/or protection circuitry and hence may not be useable as a drop-in substitute for the existing components.
2.7.1 speCialty DeviCes
2.7.1.1 IGCT
A good example of device especially designed for medium and high-power applica-tions is the integrated gate commutated thyristor (IGCT). Its architecture is com-bining the best features of an IGBT and a GTO. The new solid-state switch is for medium-voltage applications from 2 to 6.9 kV, with maximum ratings of 4000 A, which builds upon the drawbacks of IGBTs that have high conduction losses and GTOs that are slow and require additional circuitry.
Through this new architecture, the IGCT makes possible designs that have not been feasible in the past. Thus, engineers need not design around IGBT and GTO
trade-offs, which often impose limits on starting torque and regeneration ability of motor drives.
2.7.1.2 IGBT-RC
The Reverse Conducting IGBT is a very new device proposed to the market by Infineon at the end of 2009 [43]. This new device addresses the motor drives market, and it is especially successful within the BLDC motor drives controlled with the 120° program. Other applications include the soft-switching converters used within the induction heating and induction cooking products. This device incorporates the diode monolithically along the IGBT, providing low conduction losses of both IGBT and the integrated diode. This helps reducing the overall losses.
2.7.1.3 IGBT-RB
The increasing use of power electronic converters in energy processing has brought into attention topologies like Current Source Inverter, Matrix Converter, or the Neutral Point Clamped Inverter that require AC switches. All of these topologies promise a better power density for a given application. The conventional solution for preventing reverse conduction consists in connecting a diode in series with the usual IGBT. This is increasing the voltage drop during conduction state.
An alternative solution was proposed in early 2000 s with the IGBT-RB device.
This device has the capacity to block voltage of both polarities without adding any supplemental series diode. Figure 2.33 is based on data compiled from [44,45], and it shows the improvement of the trade-off between the turn-off energy and the voltage drop during the conduction state.
The dependency shown in Figure 2.33 is used for comparing different power semiconductor technologies. Each curve is typical for a certain technology, and
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 Devices rated at 600 V, 100 Amp
IGBT-RB 10
9 8 7 6 5 4 3 2 1
VCE (ON) @ 125° C EOFF (mJ) @ 125° C
0
FIGURE 2.33 Performance improvement with the use of an IGBT-RB device instead of a conventional IGBT-Diode pair.
different products are at certain points of the technology performance curve. For instance, an IGBT for motor drives can aim at a low voltage drop in the conduc-tion state as it is switched at low switching frequencies and the efficiency can be improved with reducing the voltage drop. Such a device would sit in the top-left corner of the technology curve. By contrary a different IGBT product made with the same technology can be aimed at use within UPS and power supplies applications where switching at a high switching frequency would reduce the filter requirements.
Such an IGBT device would sit in the bottom-right corner of the technology curve.
Figure 2.34 illustrates further this figure of merit by showing Powerex devices and their technologies [46]. To avoid a direct comparison between products of different manufacturers (which is not the goal here), Powerex products and technologies of the year 1999 have been compared in Figure 2.34, and Fuji products and technologies for the year 2010 are shown in Figure 2.33. Both figures are given for illustration of this performance figure of merit.
2.7.2 HiGH-FrequenCy, HiGH-vOltaGe DeviCes
Another power semiconductor device that is picking up in the market is the CoolMOS, a MOSFET with a special structure, rated up to 600 V, and able to switch up to 50 A.
These devices change the entire way we think about power converters.
Design of the power stage is limited by the parasitic of the implementation (printed circuit or busbar). The idea of switching, say, a 400 V bus at 250 kHz, pushes the designer to be very careful while designing circuit details.
Given the requirements of a general overhaul of the power electronics equipment, the penetration of these devices as a substitute for conventional IGBTs was not very
1.0 1.5 2.0 2.5 3.0 3.5 4.0 U-series
A B
C H-series
T-series E D 30
25 20 15 10
5
VCE (ON) @ 125° C EOFF (mJ) @ 125° C
FIGURE 2.34 Evaluation of different Powerex IGBT technologies: A = conventional NPT, tN = 250 mm, Jc = 100 A/cm2; B = PT planar gate uniform lifetime control, Jc = 100 A/cm2; C = thin drift region NPT, tN= 150 mm, Jc= 100 A/cm2; D = PT planar gate, uniform life-time control, Jc= 75 A/cm2; E = PT trench gate, local lifetime control, Jc= 140 A/cm2.
spectacular. They remain attractive solutions for high-power density equipment like certain aviation systems.
2.7.3 usinG new substrate Materials (siC, Gan, anDsOOn)
A generic trend in emerging power semiconductor devices is the use of new substrate semiconductors [47].
The development of new device technology started from within academic lab-oratories and small-business companies, and hence their experimental character has recommended them onto lower power converter market. The first applications included industrial applications in the HEV automotive market, IT and consumer, grid connected low power converters, or certain appliances. According to [44], the most dynamic sector is represented by the power factor correction converters in sub-kW power range. The SiC device technology being a little more advanced was able to bring up devices for applications up to MW power range. The GaN technology started 10 years after the SiC technology, and is currently featuring mostly diodes.
The success of the new substrate material semiconductors is mostly seen in diodes replacing conventional recovery diodes. The first application was the power factor correction circuitry in early 2001. The use of SiC diodes in the PFC application leads to power conversion improvement of around 2.4% [47] and a more optimal packag-ing. The losses are reduced tremendously and there is virtually no cost of circuit adaptation. Moreover, different auxiliary components like small inductor snubbers can be removed from circuit. The remaining obstacle is yet the slight cost difference (to beat the $0.20/Amp barrier).
Another good example is the recent release of IGBT-Diode copack devices built of standard Si-based IGBT and emerging SiC diodes, and dedicated to motor drives as a drop-in replacement or for new designs.
Conversely, the application of the power semiconductor switch made on new sub-strate material is more limited. The new devices are also requiring special gate driv-ers for control and protection and this is complicating the replacement in existing designs. Another limitation is the lack of reliability information or qualifications that prohibit somehow the use of these devices in applications with a critical lifetime or ruggedness requirements.
The new substrate materials devices do not present a major improvement for medium voltage converters switched at low switching frequency. The operation of high voltage (multi kV-range) converters will benefit from these new devices.
Especially here, the plasma science or physics instrumentation equipment with oper-ation at 50–100 kV would see major improvement with the use of SiC diodes for a considerable voltage drop across the device.
What concerns the high switching frequency application, here the voltage level matters again. At very low voltage levels, the soft-switching operation can easily be achieved with resonant converters and the merits of the new substrate material devices fade. At medium voltage (100s V) the new devices have clear merits over the use of conventional semiconductors at high switching frequencies.
The major advantages of using a complete SiC based technology instead of con-ventional Si devices can be briefly stated as [48]
• Approximately half of the chip area is required for a SiC JFET in order to achieve the same efficiency as with a Si MOSFET.
• When operating SiC devices with a junction temperature limit above 165°C, the heat-sink temperature can be higher and the size of the cooling system decreases.
• Hence, the power density increases.
• The same power density could be achieved if the SiC devices have 30–40%
of the Si MOSFET chip area.