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TECHNICAL REPORTS R-Series HVIPMs Authors: Hitoshi Uemura* and Ryosuke Nakagawa** As environmental issues have attracted increasing worldwide attention in recent years, electric railways with low CO2 emissions are playing a key role. The railway industry expects the High Voltage Insulated Gate Bipolar Transistor (HVIGBT) to exhibit not only high reliability but also low power loss, high rated current, and high operating temperature. Mitsubishi Electric pioneered the development of a High Voltage Intelligent Power Module (HVIPM), which consists of HVIGBTs with a built-in drive circuit and protective functions. The HVIPM has a large power capacity and can be used in power conversion equipment for railway vehicles. Since the HVIPM was first used for traction control in 1997, Mitsubishi Electric has been awarded many projects. We have therefore now developed the R-Series HVIPMs having a dielectric strength of 3.3 kV/ 6.5 kV and 25% higher rated current than conventional products. This paper presents the features and evaluation results of the R-Series HVIPM which satisfies the performance requirements, focusing on the electrical characteristics of the 3.3-kV model. 1. Features of 3.3-kV R-Series HVIPM The R-Series HVIPM has been developed on the basis of the following design concepts: (1) Reduction of power loss: New IGBT and diode chips have been developed to reduce the power losses. (2) Expanded operating temperature range: New structures of IGBT and diode chips have been adopted and the packaging material has been modified to achieve high operating temperatures up to 150°C as well as the minimum operating temperature of −50°C. (3) Increased rated current: The rated current has been increased by 25% compared to the conventional HVIPM. (4) Improved protective functions: A function to detect current and temperature has been built into the IGBT chip, which, in combination with the ACIC mounted on the control board, ensures high-accuracy protection. (5) High reliability: Since electric railways require high reliability, sufficient Safe Operating Area (SOA) capability has been secured. In addition, the optimized package structure offers improved reliability. 2. R-Series IGBT Chip As shown in Fig. 1, the structure of the IGBT chip has been modified from the conventional Punch Through (PT) to the Light Punch Through (LPT) type, thus reducing the turn-on voltage (VCEsat) by optimizing the cell structure. In addition, as shown in Fig. 2, the output current exhibits negative feedback characteristics, that is, “as the temperature rises, the current decreases.” This maintains a stable current balance be- 3000 Collector Cullent [A] 2500 Tj =25°C 2000 1500 Tj =125°C 1000 Tj =150°C 500 0 0 PT LPT Fig. 1 Comparison of conventional PT and LPT IGBT chips *Power Device Works **Itami Works 1 2 3 4 5 6 Collector-Emitter Saturation Voltage [V] Fig. 2 Ic-VCE characteristics of Rseries HVIPM 8 TECHNICAL REPORTS tween IGBT chips, making it easy to operate multiple IGBTs connected in parallel. Figure 3 shows the trade-off curve between VCEsat and Eoff, which indicates that the R-Series HVIPM achieves a 25% lower VCEsat at the same Eoff compared to the conventional HVIPM. wiring resistance by 50% compared with the conventional model. Fig. 4 Output current of inverter Fig. 3 Comparison of Eoff-VCEsat characteristics of conventional HVIPM and Rseries HVIPM 3. Expanded Operating Temperature Range The operating temperature of the conventional HVIPM is restricted to −40°C to +125°C due to the limitations of the constituent materials and the thermal breakdown caused by increased power loss in the IGBT and diode chips. High operating temperatures are now made possible thanks to the new IGBT and diode structures, thus reducing the power loss. Meanwhile, one of the constituent materials that limit the temperature range is the filling material (silicone gel) used to ensure insulation, so in cooperation with the material manufacturer, Mitsubishi Electric has developed a new filling material with greatly improved thermal characteristics. With these improvements, the operating temperature range has been expanded to −50°C to +150°C, enabling a higher system output and greater freedom for designing the cooling system. 4. Increased Rated Current The maximum rated current of the R-Series HVIPM with a dielectric strength of 3.3 kV has been increased to 1500 A, a 25% improvement from the conventional 1200 A. As an example, the inverter output current is calculated with respect to the carrier frequency fc as shown in Fig. 4. A comparison made at Tjmax = 125°C indicates that the inverter’s output current can be increased by 250 A (about 25%) at fc = 500 Hz; and at Tj = 150°C, it can be increased by about 550 A (about 55%). To cope with the heat generated by the main electrode resulting from the increased current, the design of internal wiring was optimized to reduce the 5. High-Accuracy Protective Function Mitsubishi Electric’s HVIPM is equipped with protective functions (Under Supply Voltage (UV), Over-Current (OC), and Over-Temperature (OT)), and by improving the accuracy of these protective functions and reducing inter-component variance, an excessively wide margin in the design of power conversion equipment is no longer needed, and thus an economic and high-performance system design is possible. Electric railways involve frequent acceleration/deceleration and a wide range of load capacity, train occupancy and changes in gradient. Therefore, instantaneous over-current and abrupt temperature changes need to be considered, and in case of abnormality, the protective function must operate immediately. The R-Series HVIPM has a current detection sensor built into the IGBT chip, which is used in combination with the dedicated IC to realize a more accurate over-current protection circuit than the previous model. Similar to the current sensor, a temperature sensor is also built into the IGBT chip. Figure 5 shows an in-plane temperature distribution of the R-Series HVIPM and the accuracy of temperature measurement by the temperature sensor. A comparison between the surface temperatures measured by an IR camera and those estimated by the temperature sensor shows that, over any temperature range, the temperature of the chip is accurately detected. In addition, by using this function in combination with the dedicated IC mounted on the control board, high-accuracy over-temperature protection with improved responsiveness has been realized. 6. Higher SOA Capability The HVIPM requires excellent reliability such as a sufficiently high SOA capability. Figure 6 shows the type test results of the R-Series HVIPM for the turn-off and reverse recovery operations at the maximum junction temperature of 150°C. The turn-off waveforms indicate Mitsubishi Electric ADVANCE June 2011 9 TECHNICAL REPORTS no abnormalities and stable operation is confirmed even under test conditions exceeding the limit of SOA. The new diode chip was designed to perform soft recovery operation; and the reverse recovery waveforms indicate that the new design successfully suppresses oscillations during the recovery period, resulting in improved SOA capability. The marginal test confirmed that the peak loss (Prr), which is an indicator of the SOA capability, has a margin of three times the specification. Fig. 5 Chip temperature measurement result and the detection precision of the thermal sensor 7. Package Compatibility Figure 7 shows an external view of the R-Series HVIPM; the conventional external dimensions, electrode arrangement and signal interface are the same but the internal structure is improved, allowing easy upgrading from the existing system. We will continue to develop products that reflect diverse customer needs, and provide high performance, high reliability products. Fig. 6 Turn-off and reverse waveforms of the type test result a. 3.3kV R series HVIPM b. 6.5kV R series HVIPM Fig. 7 The R series HVIPMs 10 TECHNICAL REPORTS References (1) S. Iura et al.: “Development of New Generation 3.3 kV IGBT module,” PCIM Europe 2006 (2006). (2) T. Kobayashi et al.: “Trend of Power Device for Railway Vehicles,” Mitsubishi Giho, 83, No. 11, 657–659 (2009). (3) T. Kobayashi et al.: “Trend of Power Device for Railway Vehicles,” Proceedings of the 46th Railway Cybernetics Symposium, 513 (2009). Mitsubishi Electric ADVANCE June 2011 11