Читать книгу Soft-Switching Technology for Three-phase Power Electronics Converters - Rui Li - Страница 17

As we mentioned earlier, if the inverter operates at higher switching frequency, the inverter will have smaller filter size and smaller loss on the filter in UPS and smaller DC‐side film capacitor in EV power train. In addition, we can get better dynamics. In some applications of ultra‐high‐speed drives for the industry or aeronautics, inverters are required to operate at very high switching frequency to provide lower ripple fundamental frequency current to the motor/generators. It seems higher switching frequency operating has advantage. What is the maximum switching frequency the inverter can operate at?

Actually, the switching frequency is limited by loss of the power semiconductor devices in the inverter. Figure 1.6 shows loss of the power semiconductor switches of the inverter of 100 kVA UPS exampled in the last section. Typical Insulated Gate Bipolar Transistor (IGBT) devices (Si IGBT FF300R12KT) are used as the switches. The loss of the inverter power semiconductor devices is composed of conduction loss, turn‐on loss, turn‐off loss, and reverse recovery loss. Conduction loss is static loss, which does not change with the switching frequency shown as the black part of the bar at the bottom of the figure. The other three losses – turn‐on loss, turn‐off loss, and reverse recovery loss – are dynamic losses, which increase linearly with the switching frequency as shown in the figure. As a result the dynamic loss of the power device depends on the switching frequency. If we want to design the inverter with required efficiency, its maximum switching frequency should be restricted to constrain total power device loss to certain value. Dynamic loss has another commonly used name: switching loss. It means the loss is caused by the device switching actions, either turning on or turning off.

For the same inverter parameters, when SiC MOSFET (CAS300M12BM2) is used, total SiC MOSFET loss of the inverter vs. the switching frequency is shown in Figure 1.7. Since the recovery loss is smaller, it is ignored. The loss of the inverter power semiconductor devices is composed of conduction loss, turn‐on loss, and turn‐off loss. Similar to the IGBT inverter, the conduction loss is constant while the dynamic loss is proportional to the switching frequency. Although the power device loss of the SiC inverter loss is much smaller than that of the IGBT inverter, the dynamic loss is still the main factor to limit the upper switching frequency.

Figure 1.6 Power semiconductor loss of the inverter vs. switching frequency.

Figure 1.7 Total SiC MOSFET loss of the inverter vs. switching frequency.

As a result, conducting loss is almost constant while switching loss increases with switching frequency. Actually, the voltage and the current of the power device have an overlap time during the switching transient process, which causes switching losses. The average switching loss is proportional to the switching frequency. To get an expected conversion efficiency, the switching frequency of inverters needs to be restricted. This type of converter is called hard‐switching converter. Hard‐switching converters can operate only at lower switching frequency. Lower switching frequency operation in turn results in bulkier passive components and higher audible noise. The wall to prevent the switching frequency from increasing is due to the switching loss. Can we reduce the switch loss of the inverter so that it can operate at higher switching frequency? To realize this goal, a technology to shape voltage and current of the power device during switching transient process, known as soft‐switching, occurs. Before a power device in a converter changes its status, either from on‐state to off‐state or from off‐sate to on‐state, the voltage across it or the current through it is set to zero with the help of the resonance between inductance and capacitance in the circuit. Soft‐switching is able to reduce the switching loss of the power semiconductor devices so that the converter can operate at higher switching frequency.