## 1 Introduction  
Brushless DC (BLDC) machines have the starting characteristics of series-excited DC machines and the speed regulation features of shunt-excited DC machines [1]. They are commonly used in aerospace, industrial control, robotics, electrical vehicles, office automation, and domestic appliances for their simple construction, high output torque, efficiency and reliability [2]. Towards the end of the 1970s, the ideas of \"all electric aircraft (AEA)\" and \"more electric aircraft (MEA)\" were put forward, promising the advantages of reduction of mass, increased reliability and easy maintenance [3]. Owing to the advances in high power-density machines and power devices, the electromechanical actuator (EMA) gradually became the key to MEA instead of the hydraulic-based actuator [4; 5]. The induction machine (IM), BLDC machine, permanent magnet synchronous machine (PMSM), switched reluctance machine (SRM) and synchronous reluctance permanent magnet machine (Sync-RM) are the main candidates of the EMA to drive the hydraulic oil ram through the mechanical gearbox [6; 7; 8]. In addition, the BLDC machine is a strong contender for conventional aerospace applications compared to the aforementioned machines [9]. The performance of an EMA system is determined by the characteristics of electrical drive, which should be designed to meet the relevant requirements [10]. Generally speaking, a BLDC machine system consists of three components: the power driving circuit, the position sensor and the machine. The power driving circuit, commonly called an inverter, has several types: half-bridge, full-bridge, C-dump, H-bridge and four-switch. The half-bridge and C-dump types are seldom used, especially in aerospace applications, due to their disadvantages of large torque ripples and the low utilization of the windings. The typical shortage of the H-bridge type is that each winding is controlled separately by an H-bridge inverter, and the control algorithm of the four-switch type is more complicated. Therefore, the full-bridge type is the most popular choice in common use due to its acceptable static performance. However, the system's dynamic characteristics under the conventional full-bridge mode may no longer meet the rigorous demands of aerospace applications. For instance, the response time of speed and current may be too long, the overshoots may increase, and the speed range may not be wide enough to match requirements under abrupt load torque or under-voltage fault. Therefore, research on the response enhancement and fault protection improvement of BLDC machine drive systems for certain circumstances is of considerable value. In recent years, several studies have focused on characteristics of the modified topology of BLDC machines [11; 12; 13; 14; 15; 16; 17; 18; 19]. In [11], an on-line fault diagnosis method for buck converter-BLDC motor combinations was proposed. The method can detect and identify open-circuit and short-circuit damage to single switches, but protective measures were not introduced. Research in [12] mainly centered around the design of a bridgeless Zeta (BL-Zeta) converter-fed BLDC machine to achieve a wide range of speed control performances. However, more elements and complex control techniques were involved without any protection improvement, which may introduce reliability penalties. A bidirectional DC-DC converter (BDC) for electromechanical energy storage systems as an interconnection between power and inverter was reported in [13]. However, the dynamic response and protections are out of consideration during the charge and discharge states. In [14; 15], a Z-source inverter (ZSI) that could adjust the output voltage according to the shoot-through zero state of a switching cycle was developed. Nevertheless, a relatively complicated control strategy was required, and the system was less attractive in terms of the fault protection characteristics.