SiC MOSFETs improve efficiency in an industrial motor


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Industrial power applications often rely on powerful electric motors that are used in fans, pumps, servo motors, compressors, sewing machines, and continuously running refrigerators. The most common configuration in the industrial sector is represented by a three-phase electric motor, which is driven by suitable inverter-based motors. These motors can absorb up to 60% of the industry’s total power requirements, thus it is imperative to ensure that the motors provide high levels of efficiency. In industrial power applications, electronic designers can achieve tremendous benefits using silicon carbide-based transistors (SiC MOSFETs), which provide significant efficiency improvements, smaller heat dissipation size and lower cost than traditional silicon-based solutions such as IGBT (Insulated Gate Bipolar Transistor) transistors. . SiC technology allows very low RDSON per unit area, high switching frequencies, and minimal energy loss during the reverse recovery phase that occurs after the diode is switched off to the body. An inverter dependent drive circuit is shown in Figure 1. The most common three phase drive circuit dependent on an inverter. This architecture, widely used on the industrial scale, is based on two-level three-phase transformers, which mostly use discrete IGBTs or power units, depending on power requirements, in addition to free-wheeled diodes. The six power transistors are attached to three half-bridge legs, which generate three-phase alternating current to electrical machines or other loads. Each half bridge is forced to switch at a certain frequency on the ohmic inductive load (motor), so that it can control its speed, position and electromagnetic torque. IGBT transistors are minority-carrier devices that are characterized by high input resistance and large bipolar current carrying capacity. In motor control applications, due to the characteristics of inductive load, it is often necessary to add an anti-parallel or freewheel diode to obtain a fully functional switch, although in some special cases a free wheel diode is not necessary. The free-wheeled diodes are placed parallel to the power transistors, and they connect between the collector terminal and the emitter terminal to conduct the reverse current. These diodes are required, because during switching off, the inductive load current can generate high voltage peaks if a suitable path is not provided. And this, in turn, can destroy the power switch. Due to its special structure, the IGBT transistor does not have a parasitic diode as it does in MOSFETs. The free-wheel diodes can be monolithically combined or added as a separate external diode for IGBT package. Figure 1: Three-phase inverter-based two-level motor (source: ST) when the free-wheel drive diode in the lower side is in reverse recovery, the direction of current flow is the same as the upper side switch and vice versa; Therefore, bypass occurs when switching operation, resulting in additional power loss, which affects the overall efficiency. SiC MOSFETs, thanks to the much lower values ​​of reverse recovery current and reverse time, allow for a significant reduction in recovery loss, with a marked improvement in efficiency compared to the free-wheeled diodes co-packed with silicon-based IGBTs. Requirements for on / off switching In industrial motors, special attention should be paid to turning on and off switching speeds. As a matter of fact, SiC MOSFET dV / dt can reach much higher levels than IGBT. If not handled properly, high dv / dt switching increases voltage surges on long motor cables and may generate common and differential fault currents that, over time, lead to failures in the insulation windings and motor bearings. Although faster on / off improves efficiency, the typical dv / dt in industrial drives is often set at 5 to 10 V / ns for the previously mentioned reliability reasons. ST’s comparison of two similar 1.2 kV power transistors, SiC MOSFET and Si-based IGBT, proved that a SiC MOSFET can guarantee much less power loss for both on and off, compared to Si IGBT, even under conditions Imposed 5 V / ns. Static and dynamic properties Using the same types of transistors, ST analysis also made it possible to compare characteristic curves (or current voltage), in both static and dynamic operation. The static characteristic curve, shown in Fig.2, was obtained at the junction temperature TJ = 125 ° C. From the comparison of the two curves, the significant advantages that SiC provides over the full range of voltages and currents are shown, thanks primarily to the lower linear forward voltage. On the contrary, the IGBT transistor exhibits a nonlinear voltage drop (VCE (Sat)), which itself depends on collector current. The break-even point is reached at a current of about 40 amperes: below this value the conduction loss of SiC MOSFET is less than that of IGBT. This happens because SiC MOSFET takes advantage of the static loss due to its linear static properties. And even if SiC MOSFET requires VGS = 18V to achieve excellent RDS (ON), it can provide much better stable performance than silicon-based IGBT, which greatly reduces conduction loss. Figure 2: Comparison of SiC Curve and IGBT MOSFETs VI (Source: ST) The two devices were also analyzed from a dynamic point of view, using the double pulse test. The intent of this specific test was to provide a dynamic loss metric during start and stop conditions. The results obtained indicated that SiC MOSFET exhibits significantly lower on and off energies (about 50%) across the entire current range under analysis, compared to Si IGBT, even under the condition of 5 V / ns. At 50 V / s, SiC MOSFET reduces losses further. IGBT cannot reach that high switching speed. Electrothermal Simulation for Comparison of SiC MOSFET and Si IGBT When operating in a typical industrial motor application, thermoelectric simulation is the best option. Regarding ST analysis, this simulation was performed using their software tool PowerStudio. This software provides comprehensive energy and heat analysis able to predict device performance, shorten solution design, and save time and resources. Moreover, the tool assists in selecting the appropriate suitable device for the application job profile. ST PowerStudio is based on a very accurate combined electrical and thermal model for each device, and thanks to iterative calculation, taking into account the effects of self-heating, it provides a highly accurate estimate of power losses, junction temperatures, and condition. ST electrothermal simulation conducted with PowerStudio demonstrated that much better energy efficiency can be achieved with SiC MOSFETs, thus reducing the thermal requirements of any heat exchangers with the benefits of weight, space and costs. SiC MOSFET solution provides much less total energy loss compared to Si IGBT for static and dynamic conditions, and for switch and diode. Please visit Power Electronics News for more information.


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