Comparison of Si and SiC MOSFETs
Maurizio Di Paolo Emilio
Content Editor & Technical Writer | Ph.D in Physics | Power Electronics, Renewable Energy, Embedded Systems, Quantum Computing
Comparison of Si and SiC MOSFETs in Half-Bridge LLC Resonant Converters
By Saumitra Jagdale
High efficiency is one of the significant parameters in power electronics, but also the most challenging one to achieve. Wideband switches can help accomplish this goal, but they add additional costs to the system. That is where LLC?resonant converters?come into the picture; this type of converter has more advantages compared to others as it provides high electrical isolation, high power density, low EMI, and high efficiency. Further, LLC resonant converters can be used in various applications like consumer electronics, as well as in renewable energy applications such as PV, wind, hydro and geothermal, etc. This article provides a detailed comparison of the Si and SiC MOSFETs modeled in a 3KW half-bridge LLC converter with a wide range of input voltage.
The results show more efficiency in SiC MOSFETs than Si MOSFETs for high-frequency power applications. Si MOSFETs are still preferred in low-voltage and low-power applications due to their low-cost advantage.
The above diagram shows the circuit of a half-bridge LLC resonant converter. It has two MOSFETs Q1 and Q2 that are switched on and off 180o out-of-phase at the frequency fs. The ontime of each MOSFET is the same and slightly shorter than 50% of the switching period Ts = 1/fs. In fact, a small dead-time Td is also inserted between the turn-off of either MOSFET and the turn-on of the other one. This dead-time is essential for the operation of the converter, it makes sure that Q1 and Q2 do not cross-conduct and enables soft-switching (i.e. zero-voltage switching ZVS at turn-on) for both the MOSFETs.
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The resonant tank includes three reactive elements (Cr, Ls and Lp) and thus has two resonance frequencies. One is linked to secondary winding conduction: only Ls is active, while Lp disappears because there is a constant voltage across it, reflected back from the secondary side; its value is:
The other resonant frequency corresponds to the condition of the secondary winding(s) being open. The tank circuit turns from LLC to LC because Ls and Lp are effectively in series:
The frequency fR1?is greater than fR2. The separation between fR1 and fR2 depends on the Lp to Ls ratio: the larger it is, the further apart the two frequencies are, and vice versa.