# Research on Composite Control Strategy of Quasi-Z-Source DC-DC Converter for Fuel Cell Vehicles
## 1 Introduction  
The development of the new energy automobile industry brings hope for solving the increasingly prominent environmental problems. Fuel cell vehicles have become a promising research direction in the new energy vehicle industry due to their clean, reliable and efficient advantages [1; 2; 3]. However, fuel cells also have some defects. The fuel cell output characteristic is \"soft\"--the output voltage drops too fast, as the output current increases. The dynamic response speed is slow and cannot follow the rapid variation of electric vehicle demand power. The fuel cell output voltage is low and cannot directly drive the car motor, which requires a high DC bus voltage (e.g., 400 V) [4]. Therefore, in order to provide a stable output voltage to the load, the DC-DC converter with high gain, wide voltage input range and fast dynamic response speed should be used in fuel cell vehicles [5; 6; 7; 8]. The isolated DC-DC converter can achieve high gain by changing the turns ratio of the transformer coil. However, due to the leakage inductance of the transformer, a spike voltage absorbing circuit is required and it is also easy to generate additional loss and electromagnetic interference problems [9; 10]. Considering the size and cost requirements, the non-isolated DC-DC converter is more suitable for fuel cell vehicles [11]. While the traditional boost converter is simple, low-gain and high voltage stress are not conducive to the components selection. High gain can be achieved by using the cascaded boost converter, but two or more cascaded boost circuits can cause problems such as circuit complexity, high cost and low efficiency [12; 13]. The non-isolated step-up DC-DC converters withswitched inductor and switched capacitor are proposed in [14; 15] and can achieve high gain. However, the voltage stresses across components are still high. The non-isolated converters proposed in [16; 17] can achieve high gain and low voltage stress. However, there is a diode voltage drop between the input and output due to the no-common-ground topology structure. The high frequency pulsating voltage appears between the input and output and easily causes safety problems and electromagnetic interference problems. The converter proposed in [18] has low voltage stress, but the topology has a flying capacitor, which increases the difficulty of control. The multi-level converters are proposed in [19; 20; 21] to achieve high gain. However, the complex structure, large size and multiple-switch drive circuits limit the application in fuel cell vehicles. In the research on DC-DC converter feedforward compensation, the traditional boost and buck circuits with the feedforward compensation strategy are proposed in [22; 23; 24] and good control effects are obtained. However, these topologies are simple, which are not suitable for fuel cell vehicles. In [25], a quasi-Z-source DC-DC converter, which has wide voltage input range, high gain, and low voltage stress, is proposed. The theoretical boost ratio can reach 2/(1-2D), where D is the duty cycle. While achieving high boost ratio, the duty cycle is always less than 0.5. However, the disturbance resisting ability and dynamic response performance of a DC-DC converter were not deeply explored in [25]. In the process of fuel cell vehicle driving, the output voltage of the fuel cell always fluctuates with the demand power variation. The output voltage decreases obviously with the increase of output current. The \"soft\" output characteristic of the fuel cell presents a great challenge to the input disturbance resisting ability of the DC-DC converter. Therefore, it is necessary to improve the input disturbance resisting ability and dynamic response performance of the DC-DC converter for fuel cell vehicles. The composite controller combining feedforward compensation and feedback control is applied to the quasi-Z-source network DC-DC converter proposed in [25]. The simulation model and 400 W experiment prototype are built to validate the controller by considering the input and output disturbances.