Featured Lecture: SiC Power Solutions for Auxiliary Power Drives of Industrial Equipment

Figure 1. Auxiliary power supply topology of a conventional flyback converter

There are also approaches to use more complex topologies (double-ended flyback converter method, low-voltage device series connection, etc.) instead of using 1500V MOSFETs. However, these approaches not only increase the design difficulty, but also increase the number of parts.

If a 1700V SiC-MOSFET with a specific on-resistance of only 1/2 of that of a 1500V Si-MOSFET (see Figure 2) is used, designers of auxiliary power supplies will be able to use a simple single-ended flyback converter topology to achieve a compact size and good performance. ROHM has fully encapsulated TO-3PFM package and surface mount package (TO-268-2L) technology and provides high-voltage SiC-MOSFETs for such applications. These products are characterized by ensuring creepage distances of 5mm and 5.45m respectively.

Figure 2. Comparison of Si and SiC MOSFET performance at specific on-resistance conditions.

Performance of auxiliary power supply using SiC-MOSFET

Figure 4. Evaluation board for auxiliary power supply unit using SiC-MOSFET

The switching waveforms of the SiC-MOSFET are shown in Figure 5. The waveforms for different output loads show how the resonant drain-source voltage changes when the SiC-MOSFET is turned on. Quasi-resonant operation is adopted to minimize switching losses and EMI. After the burst mode operation at light load (Pout = 5W, left figure) ends, it switches to quasi-resonant operation. The frequency is controlled by skipping many valleys. When the output load increases (Pout = 20W, middle figure), the number of valleys decreases and the frequency increases. When approaching the specified maximum output load (in this case Pout = 40W, right figure), there will be only one valley. At this time, the switching frequency reaches the maximum value of 120kHz.

In addition, in order to extend the switch on time on the primary side, the switching frequency can be slightly reduced and the output power requirement can be increased. In this way, the primary current peak increases and the energy transmitted also increases (when Pout = 40W). When the maximum output power is exceeded, the overcurrent protection function works and blocks the switching action to prevent the system from overheating.

Figure 5. SiC-MOSFET switching waveforms during quasi-resonant operation

Figure 6. Evaluation of auxiliary power supply unit using SiC-MOSFET

Given here are measured values ​​for various DC input voltages, the evaluation board can also be operated with a 400 / 480 V three-phase AC supply. The diode bridge required for rectification is mounted on the PCB.

SiC-MOSFET technology enables miniaturization and improves system efficiency, reliability and simplicity

Si-MOSFET is not suitable for simple and cost-effective single-ended flyback solutions for three-phase input with tens of watts and DC input voltage exceeding 400V. This is because the performance of high-voltage Si power MOSFET is low. In addition, it is very time-consuming and labor-intensive to design a complex auxiliary power supply using a double-ended flyback or stacked MOSFET. This part of the energy should be used in the design of the main power system.

By utilizing the superior performance of 1700V SiC-MOSFET and BD768xFJ control IC, it is possible to design not only simple auxiliary power supplies for three-phase systems or high DC input voltages, but also outstanding performance. Using SiC-MOSFET-based technology, designers can improve the efficiency, simplicity, reliability and miniaturization of their products. The performance advantages of 1700V SiC-MOSFET can rival the cost of solution systems using Si-MOSFET, such as reducing the cost of expensive components such as heat sinks and coils. The optimized control IC can safely drive the SiC-MOSFET, and is a groundbreaking solution that can reduce the design burden and minimize the cycle time of system products to market.