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Power Semiconductors

Wolfspeed’s New C3MTM 1200 V SiC MOSFET Increases Power Density and Efficiency in Solar Boost Converters

22 May 2018

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There is a growing need for high-efficiency power conversion (DC/DC) in a variety of industrial applications, including solar power generation. In solar power generation, the photovoltaic (PV) cells utilize the sun’s energy to generate DC voltage in the range of 400 to 600 V DC. This DC needs to be boosted to approximately 850 V DC so that an inverter (DC/AC) can be utilized to generate 480 V AC to feed into the power grid (Figure 1).

Increasing the efficiency of the power conversion process enables designers to build smaller, lighter and less expensive power converters. Wolfspeed has successfully demonstrated the benefit of silicon carbide (SiC) devices in this application. In 2014, Wolfspeed proved that a 50 kW boost converter using C2M0080120D could achieve greater than 99 percent peak efficiency. Wolfspeed’s new generation of C3MM 1200 V SiC metal-oxide-semiconductor field-effect-transistor (MOSFET) family can push the efficiency even higher.

Figure 1. Functional block diagram of solar converter system. Source: WolfspeedFigure 1. Functional block diagram of solar converter system. Source: Wolfspeed

WolfspeedTM C2M vs. C3M SiC MOSFETs

Wolfspeed’s second generation of SiC planar MOSFETs (C2MTM technology) was commercialized in 2013, with voltage ratings of 1200 V and 1700 V, and a current rating up to 50 A. When compared to insulated-gate bipolar transistors (IGBTs), the major performance advantages of SiC MOSFETs are: a forward characteristic with no knee, which results in higher efficiencies when operating at a low fraction of full power; five to 10 times lower switching losses with no current tail during turn-off; and an internal body diode with low reverse recovery. These advantages have enabled SiC MOSFETs to gain widespread adoption in applications like PV boost converters by enabling higher efficiencies, lower overall system costs or both. The cell structures of C2M and C3M MOSFETs are shown in Figure 2.

Figure 2. Cell structure of C2M and C3M MOSFETs. Source: WolfspeedFigure 2. Cell structure of C2M and C3M MOSFETs. Source: Wolfspeed

C3M SiC planar MOSFET technology achieves significant reductions in on-resistance, which further reduces the die size and cost. An important improvement of the new C3M 1200 V SiC MOSFETs over previous C2M designs is the low temperature coefficient of on-resistance. In the 1200 V C3M MOSFET, on-resistance only increases by 1.33 times between 25° and 150° Celsius as shown in Figure 3. The main reason for this phenomenon is that the MOS channel mobility, which limits on-resistance in SiC MOSFETs, increases at higher temperatures. This effect partially compensates for the increase in drift resistance at high temperatures, thereby reducing the overall temperature coefficient of the device. The C3M generation of SiC MOSFETs also has improved transconductance, which is achieved by engineering the MOSFET top-cell structure. The device achieves full turn-on when the voltage gate to source equals 15-plus V at 25° Celsius due to the improvement in channel mobility, Transconductance is further improved so that the device fully turns on even at 12-plus V. Table 1 lists the difference in some key dynamic parameters.

Figure 3. C2M and C3M MOSFET RDSon vs. temperature. Source: WolfspeedFigure 3. C2M and C3M MOSFET RDSon vs. temperature. Source: Wolfspeed

Table 1. Comparing key parameters of C2M and C3M MOSFETs. Source: WolfspeedTable 1. Comparing key parameters of C2M and C3M MOSFETs. Source: Wolfspeed

System Comparison of C2M Boost vs C3M based Boost Converters

The 60 kW boost converter (Figure 4) consists of four interleaved 15 kW boost converters. The boost converter demonstrates the new low inductance TO-247-4L (Figure 5) package, which includes both the new Wolfspeed C3M 1200V SiC MOSFET (C3M0075120K) and a Kelvin source. The reduced inductance in the gate and Kelvin source path reduce ringing, which in turn allows the use of less gate resistance, resulting in less switching losses. The MOSFETs in this boost converter are all driven by the new Wolfspeed CGD15SG00D2 (Figure 6) isolated discrete gate driver, which is tailored to the drive requirements of the new Wolfspeed C3M MOSFETs.

Figure 4. Block diagram of 60 kW C3M SiC Boost Converter. Source: WolfspeedFigure 4. Block diagram of 60 kW C3M SiC Boost Converter. Source: Wolfspeed

Table 2. Boost Converter Electrical Specifications. Source: WolfspeedTable 2. Boost Converter Electrical Specifications. Source: Wolfspeed

Figure 5. C3M0075120K TO-247-4L with Kelvin source.Figure 5. C3M0075120K TO-247-4L with Kelvin source. Figure 6. CGD15SG00D2 Isolated Gate Driver. Source: WolfspeedFigure 6. CGD15SG00D2 Isolated Gate Driver. Source: Wolfspeed

By comparison, the previous boost converter design is a 50 kW, four-channel, interleaved design consisting of four 12.5 kW channels. Each 12.5 kW channel utilizes two Wolfspeed C2M 1200 V/80 mΩ devices (C2M0080120D).

The C3M-based boost converter delivers greater efficiency as a result of combined faster switching speeds and lower RDSon rise in high temperature. The overall volume of the C3M-based boost converter is 470 in.3 versus 561 in.3 for the C2M boost converter shown in Figure 7. The new generation boost converter’s power density is improved by 43 percent (Table 3).

Figure 7. Left: C2M boost converter; right: C3M boost converter. Source: WolfspeedFigure 7. Left: C2M boost converter; right: C3M boost converter. Source: Wolfspeed

Table 3. Power density. Source: WolfspeedTable 3. Power density. Source: Wolfspeed

Figure 8 shows the results of testing both generations of converters under identical conditions. Both the C3M and C2M boost converters were tested using the same magnetics (inductors and common mode chokes), were switched at the same frequency, and used the same rectifier (C4D10120D) to be certain any differences in efficiency would be completely due to MOSFET losses. The result: at an input of 600 V, the C3M boost converter increased the peak efficiency from 99.11 percent to 99.5 percent.

Figure 8. Efficiency comparison with Vin = 600 V. Source: WolfspeedFigure 8. Efficiency comparison with Vin = 600 V. Source: Wolfspeed

Figure 9 shows the total power loss comparison. The new C3M 60 kW boost converter has 250 W less power dissipation at 50 kW output.

Figure 9. C3M and C2M total loss with Vin = 600 V. Source: WolfspeedFigure 9. C3M and C2M total loss with Vin = 600 V. Source: Wolfspeed

Figure 10. Operating waveforms at 50 percent load. VDS = Yellow, VGS = Green, IL = Blue. Source: WolfspeedFigure 10. Operating waveforms at 50 percent load. VDS = Yellow, VGS = Green, IL = Blue. Source: Wolfspeed

Figure 11 shows thermal scans of one MOSFET while the C3M boost converter was operating at full load (60 kW). The condition is Vin = 600 V, ambient temperature = 25° Celsius. The case temperature of the MOSFET is 76.6° Celsius.

Figure 11. MOSFET case temperature image. Source: WolfspeedFigure 11. MOSFET case temperature image. Source: Wolfspeed

This article compares the results of SiC-based boost converters using Wolfspeed’s latest C3M SiC MOSFET and diode technology versus the second generation C2M SiC MOSFET technology. The new generation C3M0075120K MOSFET’s RDSon is 22 percent lower than the C2M0080120D at a junction temperature of 150 degrees Celsius. This translates to a 22 percent reduction in conduction losses for the new boost converter design. The C3M MOSFET has roughly half the rise and fall times of C2M (C2M0080120D) and only 72.5 percent output capacitance (COSS). The C3M0075120K’s TO-247-4L package with the Kelvin source has lower source inductance than traditional TO-247-3L packages. This new package option and proper use of the Kelvin source pin offer reduction in switching losses for hard switching applications. Combining this switching performance with the C3M SiC MOSFETs’ low conduction losses allow the new boost converter to not only improve its efficiency, but also to increase its rating from 50 kW to 60 kW utilizing the same heatsink. This increase in the boost converter’s output power rating combined with a reduction in its footprint leads to a 43 percent increase in power density.

www.wolfspeed.com

Wolfspeed, C2M, and C3M are trademarks of Cree Inc.



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