Benjamin Wrzecionko

SiC power semiconductors in HEVs: Influence of junction temperature on power density, chip utilization and efficiency

With SiC, junction temperatures of power semiconductors of more than 700?C are theoretically possible due to the low intrinsic charge carrier concentration of SiC. Hence, a lot of research on package configurations for power semiconductor operation above 175?C is currently carried out, especially within the automotive industry due to the possible high ambient temperatures occurring in hybrid electric vehicles (HEVs). This paper shows, that a higher junction temperature though does not necessarily guarantee a higher utilization of the SiC chips with respect to the current that the device can conduct without overheating. The reason is, that for most power devices the power losses start to increase very rapidly at high junction temperatures while the power that can be dissipated always increases linearly with the junction temperature. The junction temperature, where the device current starts to decrease at, is derived for different SiC chips using measured onstate conduction and switching losses in this paper. This paper furthermore analyzes in detail, how the junction temperature on the one hand is influenced by boundary conditions and on the other hand influences itself the core parameters of a converter such as efficiency, the required chip area (i. e. cost) as well as the volumetric power density and thus forms an additional degree of freedom in the design of a power electronic converter. While calculating the optimum junction temperature and analyzing its impact on the system performance, it is demonstrated, how these results can help to find the best suited power semiconductor device for the particular application. The performance of the calculations is shown on a design applied to a drive inverter for hybrid electric vehicles with normally-off SiC JFETs. Operated close to the optimum junction temperature of the SiC JFETs, it reaches a power density of 51 kW/l for the power modules and the air-cooling system, which is shown to be doubled by increasing chip size and using an advanced power semiconductor package with a lower thermal resistance from junction to ambient than the for this case assumed 1 K/W.

A 120 °C Ambient Temperature Forced Air-Cooled Normally-off SiC JFET Automotive Inverter System

The degree of integration of power electronic converters in current hybrid electric vehicles can be increased by mitigation of special requirements of these converters, especially those regarding ambient air and cooling fluid temperature levels. Today, converters have their own cooling circuit or are placed far away from hot spots caused by the internal combustion engine and its peripheral components. In this paper, it is shown, how the use of SiC power semiconductors and active control electronics cooling employing a Peltier cooler can help to build an air-cooled inverter system for 120 °C ambient temperature. First, a detailed analysis shows, how the optimum junction of this high-temperature system can be calculated. Then, the operating temperature ranges of power semiconductors, thermal interface materials, capacitors, and control electronics are investigated, leading to a comprehensive analysis of mechanical concepts for the inverter system in order to show new ways to solve electrical and thermal tradeoffs. In particular, the operation of the signal electronics and the gate driver for power semiconductors with a junction temperature of 250 °C within the specified operating temperature range is ensured by appropriate placement and cooling methods, while taking the electrical requirements for limits on the wiring inductances and symmetry requirements into account. The analysis includes an accurate thermal model of the converter and an optimized active cooling of the signal electronics using a Peltier cooler. Finally, a hardware prototype with discrete power semiconductor devices and thus with a junction temperature limit of 175 °C driving high-speed electrical machines is shown to validate the theoretical considerations in a custom-designed high-temperature test environment.

Novel AC-Coupled Gate Driver for Ultrafast Switching of Normally Off SiC JFETs

Over the last years, more and more SiC power semiconductor switches have become available in order to prove their superior behavior. A very promising device is the 1200 V 30 A JFET manufactured by SemiSouth. It features a very low on-resistance per die area (2.8 mΩ-cm2), switching within 20 ns, normally off characteristic, high-temperature operation and has already been commercialized in contrast to many other SiC switches. To fully exploit the potential of the SiC normally off JFET, conventional gate drivers for unipolar devices must be adapted to this device due to its special requirements. During on-state, the gate voltage must not exceed 3 V, while a current of around 300 mA (depending on the desired on-resistance) must be fed into the gate; during switching operation, the transient gate-source voltage should be around ±15 V and the low threshold voltage of less than 0.7 V requires a high noise immunity which is a severe challenge as the device has a comparably low gate-source but high gate-drain capacitance. To meet these requirements, several concepts have been published recently. They deal with the challenges mentioned, but they still show certain limitations (e.g., frequency and duty cycle limitations or need for additional cooling due to high gate driver losses). In this paper, a novel gate driver consisting of only one standard gate driver IC, resistors, capacitors, and diodes is designed and experimentally validated. It supplies enough gate current for minimum on-resistance, allows fast switching operation, features a high noise immunity, and can be used for any duty cycle and typical switching frequencies without significant self-heating.

A 120°C ambient temperature forced air-cooled normally-off SiC JFET automotive inverter system

The degree of integration of power electronic converters in current hybrid electric vehicles can be increased by mitigation of special requirements of these converters, especially those regarding ambient air and cooling fluid temperature levels. Today, converters have their own cooling circuit or are placed far away from hot spots caused by the internal combustion engine and its peripheral components. In this paper, it is shown, how the use of SiC power semiconductors operated at a junction temperature of 250 °C and active control electronics cooling employing a Peltier element can help to build an air-cooled inverter system for 120 °C ambient temperature. First, a detailed analysis of the operating temperature ranges of power semiconductors, thermal interface materials, capacitors and control electronics is conducted. Then, concepts for deriving a converter design that takes the electrical, mechanical and thermal requirements of the components and their interaction into account are shown. The inverter and the active Peltier cooling of the control electronics are dimensioned. Finally, a hardware prototype with discrete power semiconductor devices and thus with a junction temperature limit of 175 °C is shown to validate the theoretical considerations.

Novel AC coupled gate driver for ultra fast switching of normally-off SiC JFETs

Over the last years, more and more SiC power semiconductor switches became available in low production volumes in order to prove their superior behavior with respect to fast switching speed, low on-resistance per chip area, high voltage range and high temperature operation. A very promising device among those introduced in numerous publications over the last years is the 1200 V 30 A JFET introduced by SemiSouth. It features a very low on-resistance (2.8m Ω cm2), switching operation within 20 ns, a normally-off characteristic and has already been commercialized in contrast to many other SiC switches. To fully exploit the potential of the SiC normally-off JFET, conventional gate drivers for unipolar devices must be adapted to this device due to its special requirements: During on-state the gate voltage must not exceed 3 V, while a current of around 300 mA must be fed into the gate, during switching operation the transient gate voltage should be around ±15 V and the low threshold voltage of 0.7 V requires a high noise immunity which is a severe challenge as the device has a comparably low gate-source but high gate-drain capacitance. To meet these requirements, several concepts have been published recently. They deal with the challenges mentioned, but they also note certain limitations (e. g. frequency and duty cycle limitations or need for additional cooling). In this paper, a novel gate driver consisting only of one standard gate driver IC, resistors, capacitors and diodes is designed and experimentally validated. It supplies enough gate current for minimum on-resistance, allows fast switching operation, features a high noise immunity and can be used for any duty cycle and usual switching frequencies without significant self-heating.

SiC vs. Si - Evaluation of Potentials for Performance Improvement of Power Electronics Converter Systems by SiC Power Semiconductors

Switching devices based on wide band gap materials as SiC oer a signicant perfor- mance improvement on the switch level compared to Si devices. A well known example are SiC diodes employed e.g. in PFC converters. In this paper, the impact on the system level perfor- mance, i.e. eciency/power density, of a PFC and of a DC-DC converter resulting with the new SiC devices is evaluated based on analytical optimisation procedures and prototype systems. There, normally-on JFETs by SiCED and normally-off JFETs by SemiSouth are considered.

High-Bandwidth High-Temperature (250 °C/500 °F) Isolated DC and AC Current Measurement: Bidirectionally Saturated Current Transformer

In an increasing number of application areas and industry sectors, such as the automotive, aerospace, military or oil and gas industry, a trend towards higher ambient temperature rating from 85 °C upwards for electrical machines and power electronic converters can be observed. To reduce the impact of high ambient temperatures on the power density, the interest in power electronic converters with SiC power semiconductors operated up to a junction temperature of 250 °C rises. The control of power electronic converters typically requires a precise, fast, and robust current measurement. However, analyzing current measurement concepts from the literature reveals that there is a lack of measurement systems, that are galvanically isolated and able to measure dc and higher frequency ac currents fast at high ambient temperatures of 250 °C. In this paper, a current measurement concept of a bidirectionally saturated current transformer is presented, that is able to measure dc and sinusoidal ac up to 1 kHz and 50 A at an ambient temperature of 250 °C with an relative error of 2.6% and less than 0.5% error after initial calibration. Furthermore, a prototype is designed, built, and used for the experimental verification of the concept.

High-temperature (250 °C / 500 °F) 19′000 rpm BLDC fan for forced air-cooling of advanced automotive power electronics

In an increasing number of application areas and industry sectors, such as the automotive, aerospace, military or oil and gas industry, a trend towards higher ambient temperature rating from 120 °C upward for electrical machines and power electronic converters can be observed. Forced air-cooling of power electronic converters offers reduced complexity of the cooling circuit compared to water-cooling. For high ambient temperature rated converters fans are required that withstand these temperatures and still feature performance comparable to standard conditions in order to still enable a high converter power density and efficiency. Commercially available fans for power electronics cooling are typically rated up to 75 °C, very rarely fans are specified up to 105 °C. In this paper, the electrical and mechanical design of a 40mm × 40mm × 28mm fan is presented in detail that offers an operational temperature range up to 250 °C at the rated speed of 19′000 rpm and similar fluid dynamical performance in terms of static pressure and volume flow at 120 °C as commercial high performance fans at 20 °C. The 3-phase BLDC machine driving the fan is integrated into its hub and has got an input power of 15W. The fan can be driven using a 3-phase inverter supplied from 12V DC voltage with an inverter switching frequency of less than 1.3 kHz.

Fast high-temperature (250 °C / 500 °F) isolated dc and ac current measurement: Bidirectionally saturated current transformer

In an increasing number of application areas and industry sectors, such as the automotive, aerospace, military or oil and gas industry, a trend towards higher ambient temperature rating from 85 °C upward for electrical machines and power electronic converters can be observed. To reduce the impact of high ambient temperatures on the power density, the interest in power electronic converters with SiC power semiconductors operated up to a junction temperature of 250 °C rises. The control of power electronic converters typically requires a precise, fast and robust current measurement. Analyzing current measurement concepts from the literature reveals that there is a lack of a current measurement systems, that are galvanically isolated and are able to measure dc and ac fast at high ambient temperatures of 250 °C. In this paper, a current measurement concept of a bidirectionally saturated current transformer is presented, that is able to measure dc and sinusoidal ac up to 1 kHz and 50A at an ambient temperature of 250 °C with an relative error of 2.6% and less than 0.5% after initial calibration. Furthermore, a prototype is designed, built and used for experimental verification of the concept.