Abstract
Fast switching characteristic of wide bandgap devices enables high switching frequency of power devices and thereby, can facilitate high fundamental frequency operation of electrical machines. However, with the switching transition times in orders of tens of nanoseconds, the high dv/dt is observed across the switching device. The high dv/dt experienced by the switches, and consequently by the machine, can degrade winding insulations or bearings over a period of time. Therefore, it is imperative to maintain the dv/dt below recommended values depending on the machine insulation. The dv/dt across the devices can be adjusted by varying the gate resistance. A high value of gate resistance, however, introduces additional switching losses on the device. Using different dv/dt filtering techniques can also help to control the dv/dt on the machine terminals. These techniques do not increase the switching losses on the device. However, it introduces additional losses in the filter resistors and also increases the cost of the system. In this paper, an analysis based on the impact of gate resistance on the dv/dt across the machine, and the corresponding losses is carried out. An analytical dv/dt filter design strategy is proposed to limit the dv/dt to a particular value. With the proposed design scheme, the value of each filter component can be easily obtained, and filter losses can be estimated accurately. Lastly, a comparison is performed on the basis of efficiency between these two techniques.
Highlights
High-speed machines (HSMs) with rated operational speeds over 10,000 r/min have been widely used in traction, compressor, turbocharger, and pump applications [1]
This paper presents the effect of variation in gate resistance on the dv/dt of a silicon carbide (SiC)-MOSFET and proposes a method for choosing the gate resistance for achieving a desired dv/dt
Based on the loss data obtained from experimental results, periodically averaged switching losses with different two external gate resistance values are presented in Fig. 17-(a) and -(b)
Summary
High-speed machines (HSMs) with rated operational speeds over 10,000 r/min have been widely used in traction, compressor, turbocharger, and pump applications [1]. Choosing proper gate resistances can provide effective control over the dv/dt of the WBG devices This method does not require any lossy, costly, and bulky filter components but introduces additional switching losses. It is noted that the surge voltage can reach two times the dc bus voltage on the machine terminals in case of SiC-MOSFET based inverters with 25 ns rise time with a power cable of just 1.3 meters. Even if the recommended rise time of two NEMA standards is satisfied, it could have the same overvoltage issue depending on the length of power cable In this manner, overvoltage induced from the wave reflection phenomenon should be considered carefully in inverter or filter design steps. The device’s operating temperature should be carefully considered before selecting the external gate resistance to achieve dv/dt limit
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