Performance advantages of silicon carbide devices in switching power converters

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Performance advantages of silicon carbide devices in switching power converters [Copy link]

Introduction

Over the past few decades, the semiconductor industry has taken many steps to improve silicon-based MOSFETs (parasitic parameters) to meet the needs of switching converter (switching power supply) designers. The dual effect of industry efficiency standards and market demand for efficiency technology has led to a huge demand for semiconductor products that can be used to build more efficient and compact power solutions. This demand has led to the emergence of wide bandgap (WBG) technology devices, such as silicon carbide field effect transistors (SiC MOSFET). They are able to provide the lower parasitic parameters required by designers to meet the design requirements of switching power supplies (SMPS). After the introduction of 650V SiC field effect transistor devices, they can complement the previous design requirements of 1200V SiC field effect devices. Silicon carbide field effect transistors (SiC MOSFETs) have become more attractive because they can realize applications that silicon field effect transistors (Si MOSFETs) have never considered before.

Silicon carbide MOSFETs are increasingly being used in kilowatt-level power applications, including power supplies, server power supplies, and the rapidly growing electric vehicle battery charger market. The reason why silicon carbide MOSFETs are so attractive is that they have superior reliability than silicon devices. In continuous conduction mode (CCM) power factor correction (PFC) designs that continuously use the internal body diode, such as the hard switching topology of the totem power factor corrector, silicon carbide MOSFETs can be fully utilized. In addition, silicon carbide MOSFETs can also be used at higher switching frequencies, thus enabling smaller and more compact power converter designs.

There is no free lunch

Of course, there is no free lunch in the world. In terms of internal body diode and parasitic parameters, SiC MOSFET has more advantages than Si MOSFET, but the price is that SiC MOSFET has poor performance in some aspects. This requires designers to spend time to fully understand the characteristics and functions of SiC MOSFET and consider how to transition to new topology architectures. One thing is very obvious: SiC MOSFET is not a simple replacement for Si MOSFET. If SiC MOSFET is used in this way, it may lead to a decrease in efficiency rather than an increase.

For example, the body diode forward voltage (VF) of a silicon carbide CoolSiC device is four times that of a silicon CoolMOS device. If the circuit is not adjusted accordingly, there is a high chance that the efficiency may drop by as much as 0.5% at light load on a resonant LLC converter. Designers should also note that if the highest peak efficiency is to be achieved in a CCM totem PFC design, the boost must be performed by turning on the SiC MOSFET channel rather than just through the body diode.

Another factor to consider is the device junction-to-case thermal resistance, in which CoolMOS has a slight advantage. Due to the smaller size of the CoolSiC chip, in the same package, the CoolSiC thermal resistance is 1.0K/W (IMW65R048M1H), while that of CoolMOS is 0.8K/W (IPW60R070CFD7). However, practice has shown that the difference in these thermal resistances can be ignored in actual design.

On-resistance comparison with silicon devices over the operating temperature range

From the device parameters, designers can quickly understand one of the benefits of SiC MOSFET, which is the on-resistance RDS(on). At a chip temperature of 100°C, CoolSiC has a lower multiplication factor (K), about 1.13, while CoolMOS is 1.67, which means that at an operating temperature of 100°C chip temperature, an 84mΩ CoolSiC device has the same RDS(on) as a 57mΩ CoolMOS device. This also clearly shows that simply comparing the RDS(on) of silicon MOSFET and silicon carbide MOSFET in the data sheet does not reflect the actual conduction loss problem. In the low chip temperature range, CoolSiC has a higher breakdown voltage V(BR)DSS due to its lower slope multiplication factor and low dependence on temperature, so it has a greater advantage than silicon devices, which is very helpful for those devices located outdoors or need to start in low temperature environments.

Figure 1: At a chip temperature of 25°C, the on-resistance of the two devices is basically the same. The effect of temperature on CoolSiC RDS(on) is lower than that of CoolMOS

As in the CoolMOS driver design, the CoolSiC MOSFET can also use the EiceDRIVER driver IC. However, it should be noted that due to the difference in transfer characteristics (ID vs. VGS), the gate voltage (VGS) of this CoolSiC device should be driven at 18V instead of the typical value of 12V used by CoolMOS. This will provide the RDS(on) defined in the CoolSiC datasheet, such as 18% higher on-resistance when the CoolSiC voltage is limited to 15V. If the choice of a new driver IC is allowed when designing a CoolSiC circuit, it is worth considering a driver IC with a higher undervoltage lockout (about 13V) to ensure that the CoolSiC and the system can operate safely under any abnormal operating conditions. Another advantage of SiC MOSFETs is that the change in transfer characteristics is very limited between 25°C and 150°C.

Figure 2: Transfer characteristic curves at 25°C (left) and 150°C (right) show that SiC MOSFETs are significantly less affected than Si MOSFETs.

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