Components, Electronics & Scientific Guide

SiC MOSFETs for higher efficiency and lower system costs

Many industrial applications require grid-connected power conversion, whether AC / DC or DC / AC. Reduce switching losses, save weight and reduce component costs are the most important requirements of the user to the converter. The paper describes a cost-effective, efficient alternative to industrial power factor correction (PFC) based on SiC power MOSFETs and suitable for mass production.

Areas of application for grid-connected power converters in the field of renewable energies are the interfaces for solar and wind power plants as well as accumulators. Especially the increasing demand for charging stations for cars and trucks, but also alternatively driven industrial plants, from the elevator to the turning shop, set the direction.
Under current regulations, these systems are designed for a source/sink slope of the AC with a THD (Total Harmonic Distortion) of less than five percent. Electric vehicle fast-charging stations are a practical example of a three-phase, grid-connected AC / DC application. A functional diagram of a three-phase fast charger for electric vehicles. With the bidirectional alignment of the converter, power can also be fed back from the car to the grid. Bidirectional charging is also crucial for loading stagnant vehicles among themselves. This basic idea is becoming more and more important even for larger battery-based industrial plants.
For bidirectional systems, many manufacturers still often use two-tier topologies with 1200V IGBTs. These are simple in construction, first, keep the semiconductor costs low and reach outputs above 20 kW. However, the switching frequency is limited to less than 20 kHz, resulting in systems with low power density and efficiency, which require more costly inductors. Three-stage topologies with super junction FETs or fast 650 V IGBTs can also be used. Such multi-stage topologies, such as the NPC rectifier (Neutral Point Clamped), offer higher power density and efficiency as well as lower switching losses, but add extra cost to the high circuit complexity. The two-stage IGBT and three-stage NPC rectifier topologies.

Challenges in design

The two-stage rectifier based on SiC MOSFETs is much less complicated in circuit design and allows higher switching frequencies. Wolf speed

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: Specifications for a PFC system. To achieve these requirements, an exact dimensioning of the semiconductor components must take place. Wolf speed

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.

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