AC voltage regulating system based on full-bridge inverter

Abstract

A low cost indirect type AC voltage regulating system is proposed to overcome the grid pollution of silicon controlled regulating circuit and the high cost of chopping sine wave regulating circuits. The system consists of full bridge rectifier, filter and full bridge inverter. The bipolar SPWM control signal for inverter is generated by the STM32 micro controller. The circuits is developed and used in the dimming of 400 W high pressure sodium lamp inductive loads. Continuous dimming with slight distortion of the sine wave is obtained. The system is attractive in the replacement of silicon controlled dimming circuit, and promotes the energy saving project. Introduction China's urbanization continues to accelerate, owning to people’s greatly improved living standard and the development of the economy. As an important indicator of urban modernization, traffic lighting and landscape lighting is developing rapidly and consuming more and more energy. Energy shortage and environmental pollution problems is a common concern of all humanity, energy and environmental protection are increasingly being taken seriously. According to the external light intensity and flow of people, appropriate steps down lightening, can effectively save energy, thus reduce carbon emissions [1]. The AC regulating system is the core block for light dimming. The AC regulating circuit based on Silicon Controlled Rectifier (SCR) is used widely in dimming [2], with advantages of low cost and simple structure. However, the SCR based dimming circuit adjusts the output voltage by changing the conduction angle of SCR. Thus the output is not a standard sine wave, but contains a large number of high-order harmonics, which will disturb other equipment’s in the grid. With the development of electronic devices IGBT, dimming circuit based on chopping is proposed and used in the stage dimming, as well as hotels [3]. The output is pure sine wave and will not affect other equipment’s in the grid any more. However, the chopping based dimming circuit is relatively complex and the filter circuit depends on the load characteristics, which resulting in higher system costs and can only be applied in expensive entertainment places [4]. In this paper, a low cost AC regulating system with simple structure is proposed and realized. The output is continuously adjustable standard sine wave, and compatible for resistive, inductive and capacitive loads. System Design The system consists of full bridge rectifier, full-bridge inverter and filer circuit, as shown in Fig. 1. The input 220V/50HZ sine wave is rectified to DC voltage firstly, and then enters into the fullbridge inverter circuit and filter circuit, where the input DC voltage is inverted to AC voltage. The output voltage is adjusted by changing the control signal of full-bridge inverter. The output is insensitive with the load characteristics, owning to the inverter circuit. International Power, Electronics and Materials Engineering Conference (IPEMEC 2015) © 2015. The authors Published by Atlantis Press 996 Fig. 1: Schematic of the indirect regulating system The full-bridge inverter is the core block of the system. The inverter consists of 4 seriesand parallel -connected IGBT devices, as shown in Fig. 2. The 4 IGBTs are driven by SPWM signal generated by the STM32 microcontroller. According to detected voltage and current of the input and output ports, the STM32 microcontroller dynamically changes the SPWM driving signal to satisfy the output voltage. These are two types of SPWM driving signal, unipolar SPWM and bipolar SPWM. For inductive load in our application, bipolar SPWM is chosen for driving the IGBTs, since it is far easier to get a good output waveform. In Fig. 2, the left half-bridge consists of IGBT T1 and T, and the right half-bridge consists of IGBT T3 and T4. For bipolar IGBT driving circuit, the high-side and low-side IGBTs (e.g. T1 and T2) have complementary driving signal, and the diagonal IGBTs (e.g. T1 and T4) have the same driving signal. The output is close the peak of the sine signal as the duty cycle of the SPWM is close to 0% or 100%, meanwhile the output is close to 0 as the duty cycle of the SPWM is close to 50%, which is shown in Fig. 3. To simplify the system complexity and lower the cost, two complementary driving signals with dead zone are generated by the STM32. Fig. 2: Topology of full-bridge inverter Fig. 3: Bipolar SPWM driving signal T1