The efficiency and robustness of switch mode power supplies (SMPS) make them particularly suitable for applications such as electric vehicle (EV) charging stations, solar inverters, and industrial motor drives. However, due to the need for higher operating voltage and current, lower conduction and heat loss, and a more compact appearance, designers must adopt advanced silicon carbide (SiC) MOSFET technology. This technology must be carefully combined with MOS gated thyristors and fast recovery bridge rectifiers to create the best power conversion system.
This article takes electric vehicle charging stations as an example to outline the requirements of SMPS. Then, the SiC MOSFETs of IXYS/Litelfuse were introduced, their performance was examined, and how different device technologies (each optimized for specific circuit functions) were combined to create a more efficient and compact power conversion system was demonstrated.
Overview of modern SMPS using fast public electric vehicle charging stations as an example
Efficiency is a signature feature of SMPS, but modern high-power applications are pushing these designs to new extremes. Consider the requirements of public direct current (DC) fast charging stations, such as a 3-level system with a power of up to 350 kW. A 1% efficiency loss is equivalent to wasting 3.5 kilowatts of power, greatly increasing operating costs and thermal loads.
High performance SiC MOSFET is the core to achieve higher efficiency. They can perform high-frequency switching while maintaining low on resistance, allowing for the use of smaller passive components and reducing conversion losses. Unfortunately, these factors also make SiC MOSFETs susceptible to transient voltage surges. Therefore, efficient design often requires more advanced protection schemes.
Furthermore, SiC MOSFET is not the optimal solution for every part of a 3-level charging station. For example, public charging stations require an auxiliary power system for coolant pumps, network communication, and other system functions. Even if the main charging path is interrupted, these systems must remain operational. In this case, high reliability silicon (Si) diode devices may be a better choice.
It is necessary to understand the requirements of each part of the DC fast charging station and carefully choose the appropriate equipment technology.
Using low resistance SiC MOSFET to achieve high-power DC-DC conversion
The DC-DC conversion stage of the 3-level fast charging station demonstrates the challenges faced by modern SMPS design. Due to the high output voltage of up to 1 kilovolt (kV), this stage traditionally requires the use of high-voltage silicon insulated gate bipolar transistors (IGBTs) or high-voltage silicon carbide MOSFETs. Both methods result in efficiency losses: IGBT has high switching losses, while some early SiC MOSFETs have relatively high conduction losses. For example, the on resistance (RDS (ON)) of some early high-voltage SiC MOSFETs was about 100 m Ω.
The Littelfuse IXSJxxN120R1 SiC MOSFET series provides a convincing solution to this problem. This series of products has a blocking voltage as high as 1200 volts and an RDS (ON) as low as 18 m Ω. This low resistance characteristic can minimize conduction losses and achieve excellent thermal performance.
These devices are packaged in isolated ceramic with an isolation voltage capability of 2500 VAC (1 minute). This design reduces the thermal resistance to the heat sink and minimizes electromagnetic interference (EMI) by minimizing the stray capacitance of the heat sink. At the same time, it adopts the familiar TO-247-3L package, which facilitates integration.
IXSJ43N120R1 is a typical example (Figure 1). The rated continuous drain current ID of the device at+25 ° C is 45 A, and the RDS (ON) is 36 m Ω (typical value). It also has a low gate charge of 79 nC and an input capacitance of 2453 pF, making it suitable for designs with smaller magnets.
Littelfuse IXSJ43N120R1 1200 V SiC MOSFET Image
Figure 1: The IXSJ43N120R1 1200 V SiC MOSFET adopts an isolated TO-247-3L package, with a rated continuous drain current ID of 45 A and RDS (ON) of 36 m Ω (typical value) at+25 ° C. (Image source: Littelfuse)
The IXSJxxN120R1 series reduces conduction losses while maintaining high voltage blocking capability, enabling designers to simplify converter topology, reduce thermal overhead, and maximize overall system efficiency.
Minimize switch losses in active front-end performance
In other parts of the DC fast charging station, switch losses may be more important than on resistance. The active front-end converts AC power to DC power and shapes the current waveform to meet the requirements of power factor correction (PFC) and harmonic distortion. Due to the reliance on higher switching frequencies in this stage to minimize the size of inductors and filters, switching losses play an important role in overall efficiency.
Littelfuse's LSIC1MO120E SiC MOSFET series has been optimized for these high-frequency applications. These devices combine 1200 volt blocking capability and low dynamic losses, making them highly suitable for PFC boost converters in DC fast charging stations and other grid connected systems.
For example, the rated continuous drain current (II) of LSIC1MO120E0080 (Figure 2) at+25 ° C is 39 A, R (DSON) is 80 m Ω (typical value), and the switching energy per cycle is 252 µ J. The extended junction temperature range is from -55 ° C to+175 ° C, providing additional design margin for outdoor installations with large environmental conditions.