Designing an uninterruptible power supply based on the high efficiency push–pull converter
University of Technology,VNU-HCM
(Manuscript Received on July 08th, 2013, Manuscript Revised September 03rd, 2013)
This paper presents an implementation of the DC/DC push–pull converter for an uninterruptible power supply (UPS). Some classical DC/DC converters are presented and analyzed for pointing out their advantages and drawbacks. Besides, an original system based on a push-pull converter associated with a dynamic modulation control is chosen. The main advantage is
the possibility to control the delivered electric power in a wide range from very low level to high level of voltage within the same basic architecture. It can reduce the switching power losses and increase the power conversion efficiency. This paper proposed a new control scheme of the DC/DC converter and DC/AC inverter. The suggested system consists of a high efficiency DC/DC converter and a single-phase DC/AC inverter has been simulated using Matlab/Simulink and designed basing on the DSP TMS320F28027. Both results show high performances of the DC link and AC load voltages, when load changes from zero to rated. The performance of the proposed system has been verified through a 1kW prototype of the system for a 50 Hz/220-230 VAC load sourcing by two series connected batteries of 12V. The proposed DC/DC converter achieves a high efficiency of 93.0%. The system including the DC/DC converter and DC/AC inverter achieves an efficiency of 91.2% and Total Harmonic Distortion (THD) of AC load voltage reached 1.9%.
Key words: DC/DC converter, push–pull converter, uninterruptible power supplies (UPS), inverter.
An uninterruptible power supply is an electronic device that provides an alternative electric power supply to connect electronic equipment when the primary power source is not available. The UPS systems provide
for a large number of applications in a variety of industries. Conventional UPSs, which are shown in Figure 1, have very low efficiency, about 70-80%. Because of the topology consists of AC transformer, its power loss is very high. Moreover, semiconductors of the DC/AC inverter work with high current, leading power loss decreases. Some new UPSs have efficiency of 87% - .
To correct disadvantages of conventional UPS and increase their efficiency, the paper proposed a new topology of UPS, a block scheme is shown in Figure 2.
In the new proposed topology, in UPS applications, they need dischargers to draw power from batteries. The voltage level of batteries is much lower than that of DC-Link bus; and in order to generate 50 Hz, 220-230Vac. Thus, a converter with a high step-up voltage ratio is required for the dischargers. Furthermore, to effectively utilize the energy stored in batteries, the dischargers should be designed with high efficiency and the output voltage of the DC/DC converter should be high enough to generate the DC-link voltage , ,. Up to now, various DC/DC converters have been investigated for high step-up applications. These DC/DC converters are divided into five basic topologies: Fly-back Converter, Forward Converter, Half-Bridge Converter, Full-Bridge Converter and Push-Pull Converter . After analysis advances and drawbacks of these converters, a push-pull DC/DC converter has been chosen.
In this paper, a push–pull converter for UPS applications is proposed. The proposed converter is depicted ina DC-link voltage controller is described for a constant DC-link voltage regulation. A digital model has been designed on DSP TMS320F28027, and prototype 1kW with a 50 Hz/220 Vac has been implemented. Experimental results show that ripple of Dc link is less than 1.5%, total efficiency is 91.2%.
Conventional UPS topology
2. ANALYSIS OF DC/DC TOPOLOGIES
Because the output voltage of the DC/DC converter should be high enough to generate the DC-link voltage around 350 V, and battery has a low voltage ranges from 20 V to 24 V, so a DC/DC converter has been investigated for high step-up applications. For this application, below DC/DC converters are chosen. These DC/DC converters provide an electrical isolation between the input and output of the converter. Selection of a topology depends on careful analysis of the design specifications, cost and size requirements of the converter. Operation of each of the above topologies is described in the following sections of this application note. Details of the topology selection and hardware design are provided in below sections.,
2.1. Forward Converter
A forward converter, which can be a step-up or step-down converter, is shown in Figure 3(a). When the transistor Q is ON, VIN appears across the primary, and then generates output voltage determined by equation 1. The diode D1 on the secondary ensures that only positive voltages are applied to the output circuit while D2 provides a circulating path for inductor current if the transformer voltage is zero or negative. A third winding is added to the transformer of a forward converter, also known as a “reset winding”. This winding ensures that the magnetization of the transformer core is reset to zero at the start of the switch conduction. This winding prevents saturation of the transformer.
Where: D is the duty cycle of the transistor Q and N2/N1 is the secondary –to-primary turns ratio of the transformer
Figure 2. A DC/DC Forward converter (a); And DC/DC Fly-back converter (b)
One problem with the Forward topology is that the primary switch voltage can rise essentially unconstrained. When the switch turns off, energy stored in the transformer primary wants to cause current to continue to flow toward the FET drain. And resetting of a transformer is a cause to limit the maximum duty cycle that a Forward converter can operate. These issues tend to limit the power level, that a Forward converter is best used for power levels between 30W and 150W ,. Moreover, Forward Converter hashigh input ripple current. So it tends to decrease a quality of output voltage and current
2.2. Fly-back Converter
Figure 3(b) shows a fly-back converter circuit. When transistor Q1 is ON, due to the winding polarities, the diode D1 becomes reverse-biased. Therefore, transformer core flux increases linearly. When transistor Q1 is turned OFF, energy stored in the core causes the current to flow in the secondary winding through the diode D1 and flux decreases linearly. Output voltage is given by
The fly-back converter suffers from high power losses due to its hard-switching operation. Especially, for the high step-up applications like battery, the hard-switching operation causes high power losses. Considering its operating conditions for the harshest environment, the life time of the UPS system is reduced ,,.
2.3. Half-Bridge Converter
Figure 4(a) shows the half-bridge converter. In this converter, the reversal of the magnetic field is achieved by reversing the direction of the primary winding current flow. In this case, two capacitors C1 and C2 are required to form the DC input mid-point. Transistors Q1 and Q2 are turned ON alternately to avoid a supply short circuit, in which case the duty cycle, d, must be less than 0.5. For the half-bridge converter, the output voltage VOUT equals that of equation 3.
A DC/DC Half-Bridge converter (a) and DC/DC Full-Bridge converter (b)
The half-bridge topology has some disadvantages: it features a split capacitor bus, twice the device current and twice transformer turn ratio, leading to increased transformer loss and size. The Half-Bridge DC/DC Converters have efficiency lower than the other one during a light load. They are working at half the supply voltage the switching transistors are working at twice the collector current compared with the basic push pull circuit. ,,.
The asymmetrical half-bridge fly-back converter has been utilized to reduce the switching power losses for low voltage DC source. It reduces the switching power losses by the soft-switching operation for the switching power devices ,. The half-bridge fly-back converter in , however, requires a large turns ratio of the transformer for generating a high DC-link voltage from the low voltage batteries. The large transformer turns ratio causes high voltage stresses on the switching power devices.
2.4. Full-Bridge Converter
The full-bridge converter topology is shown in Figure 4 (b), is basically the same as the half-bridge converter, where four transistors are used. Diagonal pairs of transistors (Q1-Q4 or Q2-Q3) conduct alternately, thus achieving current reversal in the transformer primary. Output voltage equals that of equation 4.
The full-bridge topology is well suited for applications that require a wide input voltage range. The full-bridge converter with a phase-shift control has been used for high step-up applications. However, the phase shifted full-bridge converters [10,11] require lots of power switching devices and the associated control circuits. So, the manufacturing cost of the system increases, which limit the practical utilization of the UPS system.
To optimize the global efficiency of the boost converters based on classical inverters described above, we designed a converter with a symmetric architecture using push-pull structure. The Push-Pull converters have several advantage characteristics in comparison of other topologies. A Push-Pull Converter is converter with a bi-directionally driven isolation transformer. Push-Pull transformers and filters are much smaller than others. A Push-Pull Converter has a low output ripple current, a lower input ripple current, and a simple gate drive. Moreover, the better core utilization and the lack of a realistic duty cycle limit in the Push-Pull architecture allows them to operate at significantly higher power levels. For moderate input voltages Push-Pull converters are useful to 500W and beyond ,,.
This paper has proposed a novel converter based on push–pull DC/DC converter for battery sourcing applications in transformer-less single phase inverter. This specific architecture provides high efficiency and high step-up DCDC conversion with the possibility of an independent adaptation single link to the converter ratio of the transformer. In the paper, analysis of the converter has been presented in detail, from which design equations and circuit parameters were derived. The proposed converter can be operated with PWM control and constant switching frequency 20 kHz. Experimental results have verified that the proposed converter can achieve high efficiency over a wide load range. The proposed converter achieves a high efficiency of 91.2 % for its rated power. This paper proposed a new control schemes for push-pull DC/DC converter anDC/AC inverter, which allows to keep DC link and output voltage stable under input voltage and load variations, that ripple of Dc link is less than 1.5%. Moreover, the control scheme reduced transience time. The suggested converter is expected to be a good candidate for a standalone UPS, and PV application.
Thiết kế bộ nguồn cung cấp liên tục trên cơ sở bộ biến đổi DC/DC Push-Full hiệu suất cao
Lê Minh Phương
Phan Quốc Dũng
Nguyễn Minh Huy
Nguyễn Hoài Phong
University of Technology,VNU-HCM
Bài báo trình bày việc sử dụng bộ biến đổi DC/DC push-pull cho bộ nguồn cung cấp liên tục. Trong đó có đánh giá phân tích các cấu hình DC/DC truyền thống, chỉ ra những ưu điểm và nhược điểm. Đồng thời, lựa chọn bộ DC/DC push-pull với điều chế động cho cấu hình, trong đó ưu điểm chính là khả năng điều khiển và phân phối công suất từ nơi có điện áp thấp đến nơi có điện áp cao. Nhờ cấu hình đề xuất, mà tổn hao đóng ngắt giảm và vì vậy tăng hiệu suất của hệ thống. Bài báo cũng trình bày sơ đồ điều khiển cho bộ DC/DC push-pull và bộ nghịch lưu áp 1 pha DC/AC. Cấu hình bao gồm bộ DC/DC push pull hiệu suất cao và bộ nghịch lưu áp DC/AC được mô phỏng bằng cách sử dụng phần mềm Matlab/Simulink và được tiến hành thực nghiệm trên cơ sở DSP Piccolo TMS320F28027. Kết quả cho thấy khả năng đáp ứng tốt của điện áp DC link và điện áp tải AC khi tải thay đổi từ không tải đến tải định mức. Hiệu suất của mô hình 1kW 50Hz, 220-230VAC với nguồn cung cấp là 2 ắc quy mắc nối tiếp đạt được là: bộ DC/DC Push-pull có hiệu suất 93,0%, và toàn bộ hệ thống đạt hiệu suất 91,2% trong đó độ méo dạng của điện áp tải AC đạt 1,9%.
Từ khóa: Bộ chuyển đổi DC/DC, bộ chuyển đổi push-pull.
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