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Power Point | September 2017

Transformative Steps

<span style="font-weight: bold;">Pradeep Singh Bisht, Sr. Manager Application Engineering - Power Semiconductor Modules, Mitsubishi Electric India Pvt. Ltd talks about how SiC set to transform power conversion technology.</span> <p></p> <p> Converting clean energy from resources efficiently into a commercially usable power is where power semiconductor devices play a crucial role and they are almost indispensable. The mainstream power semiconductor device in today's power converters (Wind, Solar and others) are Insulated Gate Bipolar transistor (IGBT) based on Silicon (Si) wafer. </p> <p>A revolutionary new wafer material 'Silicon Carbide (SiC)', however, already knocked the door. This synthetically produced crystalline compound of Silicon (Si) and Carbon(C) is the ideal substitute for traditional semiconductors and have proved its potential to transform the conventional power conversion technologies used today, due to its excellent features:</p> <p><span style="font-style: italic;">Power loss reduction</span>: SiC has approximately 10 times the critical breakdown strength of Silicon. This allows reduction in drift layer, the main cause of electrical resistance in a switch, thickness. Thus on-state resistance is reduced which, in turn, reduces power loss.</p> <p><span style="font-style: italic;">High Speed Switching operation</span>: Owing to high dielectric breakdown strength of SiC, it is possible to realise a high-voltage MOSFET (Unipolar structure) Switch. A MOSFET due to negligible tail current feature, which is a dominant loss contributor in a Silicon IGBT chip structure, can drastically reduce switching losses. Similarly, a Schottky Barrier Diode (SBD) structure can be realized using SiC, which because of no accumulation carriers, eliminates recovery losses and thus enables high speed switching operation.</p> <p><span style="font-style: italic;">High-temperature operation &amp; heat dissipation</span>: When the temperature increases, electrons are excited to the conduction band and the leakage current increases. At times this results in abnormal operation of a switch. However, SiC has 3 times the band gap width of Silicon, preventing the flow of leakage current and thus enabling operation at high temperatures. Also SiC has three times the heat conductivity of Si, which further improves heat dissipation. </p> <p>Today mainly two types of SiC Power Module are available commercially. <br /> </p> <p><span style="font-style: italic;">Hybrid SiC Module:</span> This is a combination of a Si-IGBT (Switch) plus a SiC-SBD (Diode). A Hybrid SiC module can reduce power losses in a typical Pulse Width Modulation (PWM) inverter to the tune of 20-30% compared to conventional Si module. SiC-SBD is the key component to reduce power losses, due to negligible recovery loss feature, in a hybrid module. Hybrid SiC module is a reasonable choice for system designers to reduce power losses moderately, without incurring much on the chip cost, compared to a full SiC module. Further, gate drive circuit modification is unnecessary as Si-IGBT (Switch) can be controlled with an existing gate driver.</p> <p><span style="font-style: italic;">Full SiC Module: </span>As the name suggests, it is a combination of SiC-MOSFET (Switch) plus a SiC-SBD (Diode). It reduces power losses in a typical PWM inverter phenomenally to the tune of 70% plus compared to a conventional Si module. This improves the system efficiency dramatically. An effort to design a special gate drive circuit is also necessary respecting high di/dt switching and lower threshold turn-on characteristics of a SiC-MOSFET.</p> <p>One of the bottlenecks today is high cost of SiC chips. However, due to continuous research effort and improving wafer process technologies, SiC chip cost is expected to reduce in near future for sure. Although system designers are now trying to compensate the high cost of SiC Module by exploiting its low power loss and high switching capability features. It is possible to achieve a more efficient and more compact design by SiC modules uses and reduce total cost of a conventional system using Si module. </p> <p>The rising focus on the development of low carbon societies has increased the use of renewable sources, such as solar power and wind power, more efficiently. Thus renewable energy system designers have been striving to achieve higher system efficiency and compact designs. An energy efficient system can increase the amount of energy produced per year and result into a faster return on investment (ROI). On the other hand, by increasing switching frequency, the weight, size and cost of passive elements can be reduced. </p> <p>SiC Module's low power loss with fast switching capability, therefore, makes it a right candidate for renewable energy systems. Further, SiC's ability to operate at higher temperatures shall enable more compact designs in future. This shall not only reduce the cost of cooling systems, but shall help enhancing power density of systems, thus offering smaller foot print than today's silicon-based systems. This eventually shall be enhancing market acceptance of renewable energy sources for a better future.</p> <p>In conclusion, the unique properties of SiC devices enable substantial improvement of existing power conversion systems. SiC devices offer not only lower losses, which increase converter efficiency, but its high switching speed ability reduces weight and cost of conversion systems. Like Renewable, Automotive (EV/HEV) shall be another key application segment for SiC modules.</p>
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