SiC MOSFETs
How SiC boosts efficiency in power conversion The benefits silicon-carbide MOSFETs become evident from comparisons with their silicon counterparts. Rene Mente • Infineon Technologies Sizing up applications
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650-V SiC MOSFETs make it practical to hit power conversion efficiency levels exceeding 97% in switched-mode power supply (SMPS) designs. Simply replacing silicon MOSFETs with SiC versions in typical resonant designs will improve efficiency, but the most significant gains come from rethinking designs completely. There are several factors to consider when determining when to opt for a new design approach. SiC configurations can bring higher power density (and thus smaller footprints, reduced thermal management requirements) that silicon devices can’t provide. The result ultimately can be a lower total cost of ownership (TCO). For an example, consider server/telecom power supplies. Here, higher switching frequencies with SiC MOSFETs bring greater efficiency, a lighter and smaller power supply, higher reliability because of simpler overall topology, and a more economical BOM. Consider a full-bridge totem pole PFC that utilizes two SiC and two low-ohmic superjunction (SJ) MOSFETs in four-phase operation over each ac cycle. In both the positive and negative phases of the cycle, one of the SJ MOSFETs continuously conducts while the corresponding SiC MOSFETs alternate between conducting (on) and off states. At each of the on/off states there is a short period in which one SiC MOSFET body diode conducts and the second MOSFET is actively turned on. Because of this hard commutation on a conducting body diode, the low reverserecovery charge and output charge (Qrr and Qoss) characteristics of the SiC devices are a major factor in system efficiency. eeworldonline.com
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designworldonline.com
Here’s how applications for SiC MOSFETs compare with those for silicon MOSFETs in terms of output power and switching frequency.
SiC MOSFETs in action
Full bridge Totem Pole PFC
LLC stage Synchronous rectifier
An example illustrating the benefits of 650-V SiC MOSFETs: A continuous conduction mode (CCM) totem pole power-factor control (PFC) stage and a resonant LLC converter utilizing 650 V SiC MOSFETs. A final synchronous rectification stage, operating at a lower voltage, employs ordinary silicon MOSFETs.
10 • 2020
DESIGN WORLD — EE NETWORK
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