High efficiency and high power density have always been the continuous needs of switching power supply, and, with the development and application of wide band gap (WBG) power devices such as silicon carbide (SiC) devices, they are expected to become substitutes for traditional silicon (Si) devices in many application fields. This article will show you the application of SiC MOSFETs in the power converters of high frequency and high power density.
Higher efficiency and power density are possible with SiC devices
The excellent switching speed and low switching losses of SiC devices, as well as the low dependence of turn-on resistance (RDS_ON) on temperature enable higher efficiency, higher power density, and greater robustness and reliability.
Studies have shown that SiC MOSFETs perform extremely well in 6.6kW DC/DC converters at frequencies ranging from 500kHz to 1.5MHz. The main advantages of high-frequency operation are the smaller size of transformers and EMI filters, and the integrated resonant inductor in the transformer further reduces the size of the converter. Compared with traditional 100kHz – 200kHz DC/DC converters, the leakage inductance of an LLC transformer can be used as a resonant inductor at a higher switching frequency. The size and weight of the magnetic circuit components operating at 500kHz is reduced by 50%, the power loss of the magnetic components is decreased by 30%, and the peak efficiency of an LLC converter (400V/16A output) is close to 98.5%. The much less severe cross - talk generated by the zero voltage switch (ZVS) enables the SiC MOSFET to operate reliably even without negative bias drive voltage, thus reducing the cost of drive circuits. These converters are suitable for bus converters, electric vehicle chargers, server power, and energy systems.
Using LTspice to simulate the performance of SiC MOSFETs and the factors affecting the converter efficiency, it is found that the simulated total power loss of four primary switches is 80.24W (20.06W each) at a switching frequency of 500kHz and a magnetizing inductance of Lm = 30 mH. Moreover, the ZVS conduction of all the master switches and the use of the diode as the output rectifier result in a total efficiency of 98.11%.
Design trade-off of LLC transformers
As for the design of an LLC transformer, after calculating the maximum magnetizing inductance, it is necessary to carefully consider the magnetic core material, air gap, and wire size during high frequency operations, otherwise it will cause great power loss and lead to the accidental failure of the transformer as a result of overheating. Among magnetic core materials suitable for high frequencies, Acme's P61 is a choice because of its low power loss and easy access to core shapes for high-power applications, with switching frequencies ranging from 500kHz to 1MHz.
When it comes to PCB design, the PCB layout is of critical importance for EMI, signal integrity, and circuit efficiency and operation, especially for high-frequency LLC converters. A large area copper areas can be used in PCBs to reduce the power loss of the PCB circuit and eliminate the magnetic field in the current loop. However, the large area of overlap between different copper layers can produce large parasitic capacitance. Reducing the overlap between the copper wire and trace on the PCB can significantly reduce the generated parasitic capacitance, requiring a tradeoff between reduction of copper loss and parasitic effect.
SiC-based power devices have a higher efficiency than silicon-based devices
With respect to the relationship between measured efficiency and switching frequency at 400V/16A DC output, there is no significant decrease in efficiency at an optimal switching frequency range of 500kHz~650kHz. With the increase in switching frequency, the decrease in efficiency is mainly due to the increase in frequency-related copper loss and core loss in the LLC transformer, as well as PCB trace loss. As the frequency increases from 500kHz to 1MHz, the power consumption due to the gate drive increases by 2.2W, while the power consumption per MOSFET adds 3.5W (from 20.06W to 23.56W during simulation), achieving a peak efficiency of about 98.5% at half load (about 3kW).
In addition, comparison tests were carried out with the silicon-based power device (Infineon’s IPW60R70CFD7, 57mW/600V) on the primary side switch. Compared with a silicon-based MOSFET, the turn-on resistance of the SiC-based device C3M0060065D released by Wolfspeed increases much less with the increase of the junction temperature. At 150℃, the normalized turn-on resistance of SiC devices is 1.3, while that of silicon-based devices can reach 2.3. The turn-on resistance of a silicon-based MOSFET increases significantly with the rise of temperature, resulting in a large switching loss. At high power, the efficiency decreases by 1%, and the MOSFET enters the thermal runaway state under the same heat dissipation conditions.
It is generally recommended to use a negative gate drive voltage (-3V~-4V for C3M006065D) to turn off the MOSFET used in half-bridge or full-bridge circuit to prevent the crosstalk caused by high dv/dt from the false conduction of fast switching devices. However, in the LLC circuit, the conduction of all switches through the soft switching at zero voltage results in much lower dv/dt, without serious crosstalk. Therefore, the complexity and cost of the drive circuit can be reduced without the need for negative voltage of switching off.
SiC MOSFET enjoys great opportunities in high power density applications
Comprehensive testing of LLC resonant DC/DC converters using SiC MOSFET and integrated magnetic components in the 500kHz – 1.5MHz range shows that a well-designed PCB layout and transformer are the key to achieving high conversion efficiency. At a power density of 128W/in3, a peak efficiency of over 98% is obtained. The efficiency data tested and the captured waveforms demonstrate the superior performance of SiC MOSFET when operating at much higher frequencies than conventional silicon-based devices.
In addition, tests have shown that in the resonance LLC topology, the low crosstalk caused by ZVS enables SiC MOSFET to operate reliably, even without negative drive voltage for turn-off of power devices, thus reducing drive complexity and cost.
In a variety of applications, these WBG devices offer unprecedented opportunities for power conversion at high efficiency and high power density. In future studies, planar magnetic components will be combined with surface-mounted power devices to achieve a converter design of higher power density.
Conclusion
With the development of SiC-based power devices becoming more and more mature, SiC MOSFET has been used in more and more voltage transformation and power conversion applications, and its advantages of high conversion efficiency, low heat, and small size make it more and more accepted in the market. The superior power conversion performance of Wolfspeed's SiC MOSFET devices will make them one of the first choices for related power products and applications.