Using 650V silicon carbide in switched power converters

December 09, 2020 //By René Mente, Senior Staff Engineer

Much has been undertaken by the semiconductor industry over the past decades to improve on the parasitic components of silicon MOSFETs to meet the needs of switching converter designers. Through a combination of regulation and market demand for green technologies, these have resulted in a demand for products that can be used to build ever more efficient and compact power solutions. In the meantime, wide bandgap (WBG) technologies such as silicon carbide (SiC) have emerged that provide the improved parasitics switch-mode power supply (SMPS) designers are requesting. With the introduction of 650 V SiC MOSFETs to complement existing 1200 V discrete power devices, SiC becomes more attractive for applications for which they had not been previously considered.

As a result, SiC MOSFETs are increasingly being turned to in applications reaching into the kilowatt range, covering everything from power supplies for telecom and servers, to battery charging for the growing electric vehicle market. Their allure is linked to their superior robustness compared to their silicon counterparts, coupled with the ability to use them in hard-switched topologies of Continuous Conduction Mode (CCM) Power Factor Correction (PFC) designs which continuously uses the internal body diode. Furthermore, their support for high switching frequencies supports the industry’s drive towards smaller, more compact power converters.

Of course, as is often the way, the benefits that SiC provides over Si devices are not provided ‘for free’, and there are some areas where SiC performs less well. This requires designers to take the time to fully understand the characteristics and capabilities of these novel new devices, as well as considering a move to new topologies. One thing should be clear: these devices are not drop-in replacements and using them as such may result in a loss in efficiency rather than a gain.

For example, the body diode of a CoolSiC device has a forward voltage (VF) that is some four times greater than that of a silicon CoolMOS device. Without adapting the circuit accordingly, a resonant LLC converter might actually see a drop in efficiency of up to 0.5% at light loads. Designers should also note that it is mandatory to boost through the channel and not through the body diode if the highest possible peak efficiency in CCM Totem Pole PFC designs are to be attained.

Another consideration is the thermal resistance, junction to case. Here CoolMOS provides a slight advantage with a value of 0.8 K/W (IPW60R070CFD7) against the 1.0 K/W of CoolSiC (IMW65R048M1H) on package level due to the smaller chip size of CoolSiC, although this thermal drawback proves to be negligible in actual designs.

Wide bandgap (WBG) technologies such as silicon carbide (SiC) have emerged that provide the improved parasitics switch-mode power supply (SMPS) designers are requesting. With the introduction of 650 V SiC MOSFETs to complement existing 1200 V discrete power devices, SiC becomes more attractive for applications for which they had not been previously considered.

SiC

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