POWER: SIC
for cost and size.
Figure 3 illustrates a 25kW air compressor inverter switching at 8kHz and the efficiency improvements seen with a 30A SiC 1200 V 6-pack power module. This power module is smaller in current rating and physical dimensions then the 1200V IGBT modules it is compared to (50 and 100A rated). The graph on the right shows the 30A rated SiC and the 50A rated Si IGBT reaching similar junction temperatures at peak loads, despite lower current rated SiC MOSFET with a 77% smaller heat sink. Efficiency improvement would be greater if similar sized heat-sinks were used in systems not optimised for size or cost. The inverter efficiency improvement with SiC MOSFETs at partial loads is higher than at peak loads due to linear transfer characteristics of SiC vs. non-linear characteristics of Si IGBT with a knee voltage, leading to significantly higher conduction losses at partial loads (Figure 4). SiC also has lower switching losses than IGBT, making them ideal to reduce heat pump audible noise in fast switching air compressor inverters. Figure 5 demonstrates how Si IGBTs reach thermal overdrive at partial loads in the same 25kW inverter switched at 16kHz instead of 8kHz, while the SiC based inverter exhibited
higher thermal stability and efficiency improvement with a 40% smaller heat sink. This could result in a quieter, more optimised and efficient heat pump. Energy efficiency and system lifetime payback in warm vs cool climates The correlation between climate and efficiency/energy savings is also seen in system lifetime payback calculations. A thermally stable residential SiC powered 25kW inverter switching at 16kHz with a mix of peak and partial load conditions in our cold Norway climate could realise an estimated ¢4,586 savings over the systems’ lifetime when compared to a Si IGBT powered inverter switching at 8kHz. Additional savings could also be realised if the SiC powered inverter were switching at higher frequencies. This is clearly highlighted (Figure 6) in the variable cost difference analysis of the two systems and including the active front end,
Figure 3. Illustrating the efficiency improvements with SiC at partial and peak loads despite a smaller heat-sink, in a 25KW peak load 3-phase air compressor inverter switching at 8kHz
21 ELECTRONICSPECIFIER.COM
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