CASCADED HIGH-VOLTAGE-GAIN BIDIRECTIONAL SWITCHED-CAPACITOR DC–DC CONVERTERS FOR DISTRIBUTED ENERGY RESOURCES APPLICATIONS
A family of bidirectional switched-capacitor (SC) converters with high gain ratio of any positive integer is proposed in this paper for distributed energy resources (DERs) applications. As compared with other existing SC converters achieving a same conversion gain, the main advantages of the proposed converters are that they require a relatively lower number of switches and capacitors, have a relatively lower switch’s and capacitor’s stress, and that their associated driver circuits are simpler to realize. Importantly, with the achievable conversion ratio being flexible and that the input and output of the proposed converters are of common ground, the proposed converters are widely suitable for many applications. Moreover, as the proposed converters do not possess magnetic component or any component that can severely degrade the converters’ performance at high temperature, they are especially useful for high-temperature applications. Besides, the proposed converters are capable of delivering bidirectional power, which is a key requirement for emerging applications with battery storages. Different aspects of the proposed converters, including a simple auxiliary power supply circuit for the MOSFETs’ drivers, will be discussed in this paper. A 9-time SC converter prototype that operates with 20 V input voltage, 100 W output, and at 75 kHz, is constructed and tested. Experiment results show that the maximum efficiency achievable with this prototype is over 98% (without driver’s loss) and the efficiency over the entire load range between 25 W and 100 W is over 95.5% including the driver’s loss. The output voltage ripple of the SC converter is less than 1%. When the SC converter is open-loop controlled, the load voltage regulation is relatively well kept at less than 5% between full load and no load conditions.
EXISTING SYSTEM:
The magnetic-less converter known as the switched-capacitor (SC) converter, which is composed of only switches and capacitors, is known for its light weight, high efficiency and high power density. There are many available types of SC converters. The series-parallel SC converter is of high efficiency, simple to control, and good for power extension. However, it requires too many components when used in the high-gain-conversion application. The ladder-type SC converter is also of high efficiency. However, for high-voltage-gain conversion, it also needs a large number of switches and capacitors. The Fibonacci SC converter and the exponential SC converter require fewer components. However, the voltage conversion ratio achievable is rigid and not continuously incremental with an increase in the number of its basic SC cell. This severely limits their range of applications. However, the number of control phases of this multilevel SC converter increases proportionally with the conversion ratio of the converter. Therefore, the control of this converter is complex when the conversion ratio is high. A modified multilevel modular capacitor-clamped SC converter (MMCCC) that is based on the multilevel SC converter in which has only two switching phases and that is easy to control, is proposed. The voltage stress on the switches of this converter is kept at a constant low level even when the conversion ratio is increased to a very high level. However, the number of required modules in the converter increases linearly with a higher conversion ratio. To achieve a high-voltage-gain conversion, this converter will require a lot of power components. The complexity and cost of its associated driver circuits are also high. The double-wing multilevel SC converter that is based on MMCCC, which requires fewer switches and still achieving low voltage stress on the switches, is proposed. This step-up SC converter is named as N_ SC converter in . While it uses fewer components and has a flexible conversion ratio, the number of the capacitors and switches required are still relatively high for high-gain conversion applications. Besides, the output of the converter is not of common ground with the input voltage source. This limits the application of the converter to those not needing common ground and exclude those that needs it, e.g. telecommunication application. A bridge modular SC converter (BMSCC), which can further reduce the number of switches as compared to the MMCCC, is proposed . However, the achievable conversion ratio is rigid and highly discrete. Moreover, the output and input of the converter are not of common ground. These drawbacks limit its application.
PROPOSED SYSTEM:
The proposed SC converters are composed of a basic SC cell, followed by the cascade of one or more basic SC cells. The input to the SC cells is part of or all of the stacked bypass capacitors of previous cells. For discussion, the step-up mode SC converter is used as an example to introduce the proposed bidirectional SC topologies, where the input power source is connected to V L . The number 2 and 3 in the following discussion will represent the 2-time and 3-time SC cells, respectively. The proposed family of SC converters has the following characteristics. (1) The numbers of switches and capacitors are both relatively low. Reducing the number of switches also reduces the number of their driver circuits and associate circuits. This reduces the cost of the components and the complexity of the SC converters. If the switches and their associate circuits of the SC converter are fabricated into an integrated circuit (IC) chip, then the size of the SC converter is mainly determined by the required size and number of the capacitors. Therefore, reducing the required capacitor size and number is necessary. (2) The maximum voltage stress of switches and capacitors are both relatively low. Switches with a lower voltage stress has a better performance, which in turns improve the performance of the SC converter. (3) The overall voltage stress of switches and capacitors are relatively low. (4) The conversion ratio can be of any integer, which makes application more flexible. (5) The control is easy. The converter is controlled by a pair of complementary PWM signals. (6) Good load regulation is achievable even with only open-loop control. Line regulation is disabled as the capacitors in the proposed SC converter are fully charged. (7) Allows bidirectional power flow, which fits the requirement of battery application that are commonly used as storage elements in DERs.
CONCLUSIONS
This paper presents a family of N-time (N is a positive integer) DC–DC bidirectional SC converters of high conversion ratio and high efficiency. These proposed SC converters use fewer components as compared with other SC converters with the same conversion ratio. The number of switches and the capacitors are relatively fewer and the converters are light weight due to the absence of magnetic components. The control is simple as it is implemented through a pair of PWM signals with duty ratio of 0.5 for each SC module. A prototype of the 9-time SC converter at 20 V input, 100 W output, 75 kHz, has been constructed for verification. Results show that the converter has a low output voltage ripple, which is less than 1%. It can achieve good voltage regulation of less than 5% between full load and no load even without feedback control. The open-loop dynamic response between full load and no load is less than 2 ms. these properties make them highly suitable for applications with a high temperature environment as an unregulated DC–DC converter in grid-tie inverter systems used in distributed energy resources.
REFERENCES
[1] W. C. Li, X. Xiang, C. Li, W. H. Li, and X. N. He, “Interleaved high step-up ZVT converter with built-in transformer voltage doubler cell for distributed PV generation system,” IEEE Trans. on Pow. Electron., vol. 28, no. 1, pp. 300–313, Jan. 2013.
[2] I. Laird and D. D. C. Lu, “High step-up DC–DC topology and MPPT algorithm for use with a thermoelectric generator,” IEEE Trans. on Pow. Electron., vol. 28, no. 7, pp. 3147–3157, July 2013.
[3] L. Barreto, P. Prac¸a, D. Oliveira, and R. Silva, “High-voltage gain boost converter based on 3-state commutation cell for battery charging using PV panels in a single conversion stage,” IEEE Trans. on Pow. Electron., vol. 29, no. 1, pp. 150–158, Jan. 2014.
[4] W. H. Li and X. N. He, “Review of non-isolated high-step-up DC–DC converters in photovoltaic grid-connected applications,” IEEE Trans. on Indust. Electron., vol. 58, no. 4, pp. 1239–1250, Apr. 2011.
[5] A. Ioinovici, “Power electronics and energy conversion systems–volume 1: Fundementals and hard-switching converters,” John Wiley & Sons, United kindom, 2013.
[6] Q. Zhao and F. C. Lee, “High-efficiency, high step-up DC–DC converters,” IEEE Tran. on Pow. Elect., vol. 18, no. 1, pp. 65–73, Jan. 2003.
[7] Y. Zhao, W. H. Li, and X. N. He, “Single-phase improved active clamp coupled-inductor-based converter with extended voltage doubler cell,” IEEE Tran. on Pow. Elect., vol. 27, no. 6, pp. 2869–2878, June 2012.
[8] L. S. Yang, T. J. Liang, H. C. Lee, and J. F. Chen, “Novel high step-up DC–DC converter with coupled-inductor and voltage-doubler circuits,” IEEE Tran. on Ind. Elect., vol. 58, no. 9, pp. 4196–4206, Sept. 2011.
[9] R. D. Middlebrook, “Transformerless DC-to-DC converters with large conversion ratios,” IEEE Tran. on Pow. Elect., vol. 3 no. 4, pp. 484–488, Oct. 1988.
[10] T. Liang, S. Chen, L. Yang, J. Chen, and A. Ioinovici, “Ultra-large gain step-up switched-capacitor DC–DC converter with coupled inductor for alternative sources of energy,” IEEE Tran. on Cir. and Sys. I, vol. 59, no. 4, pp. 864–874, Apr. 2012.