Table of Contents
Challenges in design
With SiC MOSFETs, a much more efficient circuit can be built. Their use significantly reduces the switching losses compared to 1200 V IGBTs and considerably expands the usable switching frequency range of the two-stage 6-switch PFC rectifier. Also, at the same time, a higher full load and partial load efficiency can be achieved. Another advantage of using SiC MOSFETs is that the body diode can be used as an antiparallel diode, reducing circuit complexity and cost. An example of a two-stage SiC-MOSFET rectifier.
SiC and Si in comparison
Table 1 shows the specifications for a PFC system. To achieve these properties, accurate sizing of the semiconductor devices of the two-stage 6-switch SiC system must be performed. Standard rating equations can be used for three-phase, two-level voltage source inverters. For this power stage, a 1000 V / 65 mΩ SiC MOSFET is installed, which offers shallow switching losses thanks to the four-pole TO-247 housing with a dedicated Kelvin source terminal. The MOSFET’s optimized 1000V body blocking diode capability minimizes chip costs while supporting link operation of up to 800V DC. The switching loss behavior of the component depending on the drain current. This PN diode has a much smaller Q or(Reverse Recovery Charge) as Si parasitic PN diodes. The RDS (on)behavior of the device about temperature.
Compared to a similar 1200V / 50A Si-IGBT (IGW25N120H3), switching losses using a SiC MOSFET is seven times lower based on these specifications. This is primarily due to the high turn-off losses of the IGBT. Taking into account the static and dynamic properties of the system, the system losses can be calculated by simulation. The losses resulting from a constant junction temperature of 110 ° C. The simulation was carried out initially for both the SiC-MOSFET and also for the Si-IGBT at a switching frequency of 48 kHz, in which the losses of the IGBT were about 900 W.
For a reasonable range of application with acceptable losses, the switching frequency for the Si-IGBT has been reduced to 16 kHz. To produce a DC voltage with similar residual ripple with a switching frequency of 16 kHz, a 1.2 mH inductor must be used. When using an amorphous core, AMCC-50 is required at 48 kHz frequency and AMCC-200 at 16 kHz. A cost comparison for both configurations.
Volume and costs are drastically reduced
Wolfspeed’s SiC MOSFETs allow a switching frequency of 48kHz, which, dramatically reduces the size and cost of the inductor. Specifically, the size of the volume decreases to one third and the component costs to 40 percent compared to the Si-IGBT. A further advantage of the two-stage 6-switch SiC system presented here is the simplified cooling, since it takes up a minimum power of 230 W, while the system with Si-IGBTs is at 360 W. Based on a typical aluminum heatsink, such as the Aavid Thermalloy 82160 with 400 LFM air flow, the volume of the SiC-MOSFET system is 1.7 dm³, while the volume for the system based on Si-IGBTs is 3.9 dm³. This not only results in significant savings in size and weight,
Hardware system with SiC MOSFETs
A two-stage 20kW SiC hardware system with its power stage having two parallel 1000V / 65mΩ SiC MOSFETs per switch position. The devices are mounted on discrete TO-247-4L devices with a dedicated Kelvin source terminal. They offer a cost-optimized solution, as it no longer requires anti-parallel Schottky diodes. This also simplifies the layout of the power PCB and the heat sink assembly.
Input voltage and input current for phase A under full load conditions with a switching frequency of 48 kHz. The measurement results underscore the clean arrangement of voltage and current characteristic lines with minimal distortion. The converter thus achieves exactly the desired correction of the power factor. System efficiency and THDI were determined with the aid of a power analyzer, with particular emphasis on the high efficiency and low switching losses of SiC MOSFETs.