How Moving From IGBT To SiC MOSFET Affects PFC Efficiency

The ongoing global effort to reduce carbon emissions has fueled an increase in interest in electric vehicles (EV). As a result, there is an increasing demand for refueling infrastructure to recharge them. Millions of charging points capable of delivering between 3.6 kW and 22 kW will be installed over the next few years, with fast-charging solutions capable of delivering up to 150 kW.

Not only must this massive increase in load be supplied and managed, but the charging solutions installed must also play a role by utilizing cutting-edge technology to achieve the highest possible efficiencies and power factor unity (PF). Since the European Union established harmonic current limits, power factor correction (PFC) stages have been standard in electrical equipment. EN61000-3-2 applies to power supplies with an input power of 75 W or greater.

Furthermore, market demands are putting a strain on design volume and heat dissipation, with a general trend toward passive cooling where possible. This increases the pressure on designers to develop more efficient approaches to power converter design. 

PFCs were previously the domain of silicon IGBTs, which took advantage of their high VCES, current handling capability, and robustness. The introduction of wide-bandgap power devices, on the other hand, is changing how PFCs are implemented.

Silicon carbide MOSFETs for PFCs

Wide-bandgap technologies, such as silicon carbide (SiC), provide power converter designers with a plethora of new possibilities. SiC provides significant reductions in turn-on and turn-off losses, as well as improvements in conduction and diode losses, when compared to existing IGBT devices. A careful examination of their switching characteristics reveals that silicon carbide mosfets fully turn on almost instantly, whereas IGBTs exhibit a significant slope. As a result, Eon energy losses are reduced significantly.

Under the same laboratory conditions, a fast-switching IGBT and a Toshiba TW070J120B SiC MOSFET showed that switching losses in the SiC MOSFET were 0.6 mJ. This was roughly a quarter of the IGBT’s measured 2.5 mJ.

Each case was tested at 800 V, 10 A drain/source current, 150 °C ambient temperature, and optimal gate-emitter threshold voltages.

Figure 1. When compared to latest-generation IGBTs, the TW070J120B SiC MOSFET has significantly faster switching speeds, resulting in higher efficiencies in power converters. Image courtesy of Bodo’s Power Systems magazine.

The silicon carbide mosfet outperforms the IGBT in a 3-phase, 400 V PFC simulation. When all switching losses, resistance-related conduction losses, and forward voltage loss of the internal diode are considered, a SiC MOSFET-based design saves approximately 66% in losses over a comparable IGBT-based design.

This efficiency improvement allows designers to reduce the volume of their PFC designs while delivering the same power, or to increase power in a design of the same volume.

The  silicon carbide mosfet outperforms the IGBT in a 3-phase, 400 V PFC simulation. When all switching losses, resistance-related conduction losses, and forward voltage loss of the internal diode are considered, a SiC MOSFET-based design saves approximately 66 percent in losses over a comparable IGBT-based design.

This efficiency improvement allows designers to reduce the volume of their PFC designs while delivering the same power, or to increase power in a design of the same volume.

Figure 2. When used in a 3-phase PFC, the SiC MOSFET reduces power loss by 66 percent when compared to an IGBT-based design. The image was provided by Bodo’s Power Systems magazine.

The TW070J120B integrated diode has an excellent forward voltage (VDSF) of only -1.35 V (typical) and is very resistant to current surges, handling current pulses of up to 72 A (TC = 25 °C). The -10 V to 25 V gate-source (VGSS) range is wider than competing products, making design easier, and the high gate threshold (Vth) range of 4.2 to 5.8 V protects against unwanted switching caused by gate voltage fluctuations and noise.

Accelerating Sic-Based Pfc Design

While the transition from silicon IGBT to  silicon carbide  MOSFET in power converters is relatively simple in comparison to other Wide Band Gap (WBG) technologies, access to reference designs is universally acknowledged as the quickest way to master new technologies. The Toshiba 3-phase, 400 V AC input PFC reference design was created with EV charging applications in mind (Figure 3).

Figure 3. In Toshiba’s 3-phase, 400 V PFC reference design, the TW070J120B SiC MOSFETs are linked with the gate drive optocoupler. Bodo’s Power Systems magazine provided the image.

Figure 4. Toshiba’s 3-phase, 400 V PFC reference design is ideal for battery charging applications requiring bidirectional DC-DC converters. Bodo’s Power Systems magazine provided the image.

It achieves a conversion efficiency of 97% and a power factor of 0.99 or better while producing a 750 V DC link output. It employs a bridgeless 3-phase totem pole design, switching each phase directly from a 50 or 60 Hz line of between 312 V AC and 528 V AC, and is suitable for use with bidirectional DC-DC converters to implement EV battery chargers, for which a further reference design is being developed.

To achieve optimal performance, the reference design combines 1200 V TW070J120B SiC MOSFETs with gate drive optocoupler. This enables switching frequencies of 50 kHz to be used, which is higher than those permitted by IGBTs, resulting in a reduction in the size of both the inductors and the power converter. Higher switching frequencies can make meeting EMI requirements more difficult. The switching speed of the gate drive circuit, on the other hand, is easily adjustable, albeit at the expense of overall efficiency.

The application of the correct gate signal is critical to the optimal control of SiC MOSFETs, as is adhering to the application of gate voltages as defined in the datasheet. The reference design ensures that this is always between -10 V and 25 V, with the turn-on voltage set to be between 18 V and 20 V and the turn-off voltage set to be between 0 V and -5 V.

The gate requires 70 nC at turn-on, so the gate driver must be capable of supplying this energy at the selected switching frequency. The gate drive optocoupler can sink and source up to 4 A, which is enough to drive and discharge the TW070J120B gate. It also has overcurrent and undervoltage lock-out protection to ensure that system abnormalities are handled effectively.