Comparative Evaluation of Three-Phase Three-Level GaN and Seven-Level Si Flying Capacitor Inverters for Integrated Motor Drives Considering Overload Operation

Abstract

Integrated Motor Drives (IMDs) are gaining popularity in industrial Variable Speed Drive (VSD) applications, thanks to their more compact realization and simpler installation. However, mission profiles of, e.g., servo applications, demand overload torques of two to three times the nominal value during several seconds, which is thermally challenging for the power electronics. Accordingly, high efficiency and power density of the inverter are of paramount importance for motor integration. Multi-Level Flying Capacitor inverters (ML-FCis) benefit from a reduced output filter volume and improved switching and onstate performance of low-voltage devices for increasing number of levels N , whereas the PCB overhead and gate drive volume increases. In this paper, the most power dense solution between a seven-level (7L) FCi with Si semiconductors and a three-level (3L) FCi with GaN semiconductors is evaluated. Thereby, a straightforward design procedure allows to dimension both FCis for 99% efficiency at nominal operation, while providing a high short-term overload torque (three times the nominal torque) also for low inverter output frequencies. Due to its slightly higher power density and greatly reduced complexity, a phase module of the 3L GaN FCi is realized as an IMD hardware demonstrator. A transient thermal model is employed to specify the feasible overload operating range, considering the limited heat spreading in the baseplate and parameter variations, e.g., from manufacturing tolerances. The experimental analysis of the demonstrator verifies an efficiency of 98.94% and the practically required overload capability.INDEX TERMS Integrated motor drive, variable speed drive, multi-level inverter, flying capacitor, overload operation, WBG power semiconductors. 1) Integrated Motor Drives (IMDs)Accordingly, a trend has developed towards drive systems with motor-integrated inverters (so-called Integrated Motor Drives (IMDs)), which largely eliminate the requirement of complex shielded wiring and thus allow a simple and compact

Comparative Evaluation of Three-Phase Three-Level Flying Capacitor and Stacked Polyphase Bridge GaN Inverter Systems for Integrated Motor Drives

“…For this voltage range Wide-Bandgap (WBG) devices (e.g., SiC or GaN technology) offer superior on-state and switching characteristics, which allow high switching frequencies at low semiconductor losses. The impact of the overload capability on the selection of semiconductors available on the market has been discussed in detail in [5]. The large currents during the overload operation lead to dominating conduction losses, and thus, only semiconductors with low R dson values (i.e., large die areas) and good cooling performance (low R th and large cooling pads) are feasible.…”

“…This leads to an implementation of a single switch with two parallel top-cooled 650 V GaN HEMTs (GS66516-T, 25 mΩ, [36]) for both, the 3L-FCC and the 2L-SSC. The detailed switching loss data of these devices are also provided in [5].…”

“…for the worst-caste duty cycle of d = 0.5 [29] and a specified peak-to-peak FC voltage ripple ∆V FC,max in Table 1. Thereby, the short-term overload capability of three times the nominal current determines C FC since then i out = I out,OL [5]. Note that the Flying Capacitor volume, as well as the HF DC-link volume (for both the 3L-FCC and the 2L-SSC), could be reduced by increasing the switching frequency during overload as performed in [5].…”

“…Compared to standard two-level inverters, implementing the motor drive as a multilevel inverter with a higher effective switching frequency and lower voltage steps at the switch-node output benefits the motor with reduced output voltage harmonics and lower winding voltage stresses [3,4]. Typically, three-level topologies are employed, as more levels lead to significantly higher complexity [5]. Prominent three-level topologies are, first, T-type concepts that, however, employ two different types of transistors (i.e., with different voltage blocking capabilities) [3,6].…”

“…This facilitates a small inverter stage footprint and a high power conversion efficiency, that are both fundamentally important for the motor integration of the inverter. However, for these FCC topologies, as shown in [5], overload operation requirements, such as three times the rated torque for 3 s as expected from motor drives in, e.g., servo applications [9], can only be handled with relatively large Flying Capacitors (FCs) to keep the maximum FC voltage ripple amplitude and/or the losses per capacitor below the design limits. The latter can be avoided by increasing the switching frequency in overload periods, which, however, increases the implementation complexity.…”