Common Grounded H-Type Bidirectional DC-DC Converter with a Wide Voltage Conversion Ratio for a Hybrid Energy Storage System
Hybrid energy storage systems (HESS) play an important role in maintaining the power balance of a direct current (DC) micro-grid. A HESS is mainly composed of high power density super-capacitors (SCs) and high energy density batteries. According to the operational requirements of an SC, a bidirectional DC-DC converter with the characteristics of a good dynamic response and a wide voltage conversion ratio is needed to interface the SC and a high-voltage DC bus. In this paper, a novel common
... a novel common grounded H-type bidirectional converter characterized by a good dynamic response, a low inductor current ripple, and a wide voltage conversion ratio is proposed. In addition, it can avoid the narrow pulse of pulse width modulation (PWM) voltage waveforms when a high voltage conversion ratio is achieved. All of these features are beneficial to the operation of the SC connected to a DC bus. The operating principle and characteristics of the proposed converter are presented in this paper. A 320 W prototype with a wide voltage conversion ranging from 3.3 to 8 in step-up mode and 1/8 to 1/3 in step-down mode has been constructed to validate the feasibility and effectiveness of the proposed converter. Energies 2018, 11, 349 2 of 22 ratio bidirectional DC-DC converter is needed. Considering that the SC needs to compensate for transient power, the bidirectional DC-DC converter needs to have the feature of an outstanding dynamic response. Many isolated and non-isolated bidirectional converters present in the literature can achieve a wide voltage conversion ratio. Due to using a high-frequency transformer, the isolated bidirectional converters can obtain a wide voltage conversion ratio in the step-up and step-down modes by adjusting the turns ratio of the transformer. Because of the use of a transformer and a high number of switching devices, the cost and switching losses of these converters will be increased and the control schemes will be more complicated     . From the viewpoint of system cost-saving and improving system efficiency, a non-isolated DC-DC converter is more suitable for HESS applications [21, 22] . Research on non-isolated converters is focused on coupled-inductor, switched-capacitor, switchedinductor, voltage-multiplier, and multi-stage/-level techniques  . The coupled-inductor-based converter in  can have a higher voltage gain, and also reduce the reverse recovery losses of the diodes. However, the current ripple of the low-voltage side is relatively large, which may damage the low-voltage side source. With a simple structure, switched-capacitor converters are easy to expand. At the same time, the control methods of these converters are also simple. In different switching states, the energy of the capacitors in these converters can be delivered through different paths, making it easier for these converters to achieve a high voltage gain     . Switched-inductor converters can also achieve a wide voltage conversion ratio while avoiding extreme duty cycles. At the same time, the voltage stress of the power semiconductors in these converters can be reduced. However, more inductors affect the power density of these converters [29, 30] . In , a new non-isolated single capacitor bidirectional DC-DC converter with a simple structure and a wide voltage conversion ratio is presented. However, the voltage stress of the power semiconductor is relatively high. A common ground switched-quasi-Z-source bidirectional DC-DC converter is presented in  . The advantages of this converter are a wide voltage conversion ratio and a lower voltage stress across the power switches. However, with the low voltage level (240 V) and low rated power (300 W), the volume of the capacitors (470/520 uF) and inductors (434/600 uH) is high, which results in a lower power density for this converter. This paper presents a novel common grounded H-type bidirectional DC-DC converter, which not only achieves a wide voltage conversion ratio, but also has the advantages of a low inductor current ripple and a good dynamic response. In addition, a clamp capacitor is used in this converter to keep all of the power switches turned on and off only once during each switching period, aiming at reducing switching losses and avoiding narrow pulses of the pulse width modulation (PWM) voltage waveform at high voltage gain. These features are beneficial to improve the efficiency and reliability of this converter and its suitability for HESS applications. As shown in Figure 1 , a HESS contains a battery and an SC. The SC outputs are varying low dc voltages (25-60 V) which cannot be used directly, and must be regulated via the bidirectional converter. The proposed converter can fulfill the task well. The remainder of this paper is structured as follows. In Section 2, the novel H-type bidirectional DC-DC converter topology and operating principles are proposed. In Section 3, the characteristics of the proposed converter are analyzed and its small signal model is established. Experimental results for the proposed converter are provided in Section 4. The conclusion of this paper is introduced in Section 5. Energies 2018, 11, 349 4 of 22 When S 1 S 2 = 00: As shown in Figure 3b , both switches S 1 and S 2 are OFF. The anti-parallel diodes of Q 3 , Q 4 , and Q 5 are forward biased. Capacitors C 1 and C high are charged from L. The load is supplied by L. There is no energy exchange between capacitors C 1 and C high . When S 1 S 2 = 01: As shown in Figure 3c , where switch S 1 is OFF and S 2 is ON. The anti-parallel diode of Q 3 is forward biased, while the anti-parallel diodes of Q 4 and Q 5 are reverse biased. Inductor L is charged from the input source. Current i L rises linearly. The load is supplied by output capacitor C high . In this case, the voltage of capacitor C 1 remains steady.