Three‐level GaN inverter with SiC diodes for a possible three‐phase high power solution

Abstract

GaN device is a potential alternative to SiC as a wide band gap device. At present, there are almost no high-voltage GaN devices above 650 V, which makes an inverter design difficult for three-phase input using the standard two-level (2L) inverters. Therefore, at present, a three-level (3L) inverter is an obvious choice for the GaN inverter for three-phase 400/480 V input applications. Moreover, a 2L inverter suffers from drawbacks like increased filtering efforts, high dv/dt and limited switching frequency due to the effect of power loss on semiconductors. Therefore, at the medium-to-high-power level, a hard switched GaN inverter with a 2L structure is still questionable. To address some of the challenges, this study brings in a 700 V dc-link-based three-phase, 3L inverter with GaN and SiC diodes. This study discusses multiple aspects of the design such as (a) advantages over the 2L design at a higher power, (b) filters designs, (c) power losses in the devices and (d) design challenges of the inverter through comprehensive simulation models and experimental investigations. The study claims that the GaN inverter for the medium-to-high-power level makes more sense with a 3L design. 11Introduction A pulse-width modulation (PWM) inverter with Si technology is standard within the industry at present. The Si technology is fairly mature and has been in operation for decades. A two-level (2L) inverter structure is a pretty much standard for motor drives or standalone/off-grid applications within industry. Due to some inherent limitations with the Si technology, a significant research started over the last couple of years deploying wide band gap (WBG) devices in power converters [1-4]. SiC and GaN devices have been considered to be the candidates for this technology. SiC technology has been around for almost a decade now and there is an interest with SiC in industry as well. However, the main issue with the SiC technology is the cost and parasitic component around the device where the GaN technology is expected to bring advantages. This affects the inverter cost and limits the switching frequency. As a result of this, the practical SiC inverter cannot switch >50 kHz (possibly less than this) without additional soft-switching techniques [5, 6] due to its loss implications. Soft-switching is a good technique in DC-DC applications, but for DC-AC or AC-AC applications, the accuracy without additional components is poor and also impractical, especially at higher power. Therefore, GaN devices are getting attention at the same time as SiC. GaN technology has much less on-state resistance, requires much less switching energy and possesses reduced parasitic components. However, a pure GaN or vertical GaN device could be an expensive solution; therefore, a 'GaN-on-Si' device is considered to be the reasonable choice to enhance the switching performance, especially at lower voltages. These features make it more suitable than the SiC device in hard-switched applications. Practical GaN devices are available at low voltage predominantly at 100 and 650 V, which is not sufficient to feed from a full three-phase 400 or 480 V input. Therefore, a multilevel or series-connected device structure is a necessity. Moreover, the current levels can be in the range of 60-100 A, e.g. in GS66516 or GS61008. Therefore, at the medium-to-high-power range, a three-level (3L) or multilevel structure is a more practical approach. Apart from this argument, the filtering effort is significant in a 2L inverter, especially in high-frequency applications where the output filter is mandatory. There have been some efforts in the academia to design a 3L SiC inverter but predominantly with the T-type structure, as shown in [7]. All SiC is a practical approach, but it does not eliminate the inherent drawback of SiC technology at higher frequencies. The 3L structure for GaN has also been discussed but mainly in grid applications such in PV [8] and in 3L T-type GaN topology along with SiC as in [9]. This is an impractical approach because two new switching devices within the same inverter structure makes it complicated to realise in terms of drivers and controlling dv/dt, and thermal management also becomes non-uniform. For these reasons, this paper studies all GaN devices in a 3L neutral point clamped (NPC) structure with SiC diodes, as shown in Fig. 1. The proposed inverter uses a small LC filter at the output of the inverter to deliver a sine-wave output instead of PWM output. This paper studies the losses within the inverter and filters and brings in some of the practical challenges of the GaN inverter. At last, some simulation and experimental results were added to support the claim of this paper. 22GaN devices at present GaN technology used today can mainly be categorised into two types: (a) cascade type as shown in Fig. 2 with normally ON-type device and (b) enhancement mode type or enhancement-mode high electron mobility transistor (E-HEMT) as shown in Fig. 3 with normally OFF-type device. The former relies on a low-voltage MOSFET to control the switching of HEMT. The latter fully depends on the HEMT itself. In terms of switching performance, E-HEMT is better than the cascade HEMT. However, there is no high-voltage (>650 V) HEMT present. Therefore, the 3L structure is deemed an appropriate choice, especially at a higher power level. The predominant advantages of the GaN technology are follows: i. low R ds(on) to reduce the conduction loss, ii. high-speed switching speed (>100 V/ns) to reduce the switching loss, iii. no intrinsic body diode to avoid recovery loss, iv. SMD housing/package to reduce the parasitic components.