A design scheme for power failure backup power supply-EEWORLD

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This paper introduces a design of a power failure backup power supply. It uses supercapacitors as energy storage elements for long-term floating charge and high-current discharge, which increases the service life. It uses a boost topology and optimizes the supercapacitor capacity configuration. It can work continuously for 10s under 5V@5A conditions. When the capacitor stops working due to undervoltage, it can quickly shut down the output. The output voltage decreases monotonically without oscillation. The electrical performance indicators meet the requirements.

1 Introduction

Important units or key components such as measuring instruments, data acquisition systems, servo systems, and robots need to record the status and perform necessary system configurations when the power is abnormally lost. The battery life is often reduced due to long-term floating charge and needs to be replaced regularly. Supercapacitors have the advantages of high power density of conventional capacitors and high specific energy of rechargeable batteries. They can be charged and discharged quickly with high efficiency and can be floated for a long time. They are superior to batteries in terms of high current charging and discharging, number of charging and discharging, and life. They are developing into a new, efficient, and practical energy storage device, and are a new type of energy device between rechargeable batteries and capacitors. This paper uses supercapacitors to design a high-efficiency, high-current Boost power failure backup power supply.

2 Selection of supercapacitor capacity and topology

To achieve short-term power-off protection, the configuration of this power supply needs to be optimized, that is, to use the smallest possible capacitor capacity to obtain the longest possible use time. Using the Buck structure, the efficiency will be improved, but there will be a large amount of capacitor charge that cannot be used; it will be difficult to directly use the reverse pressure generated by the Buck-Boost structure; the topology isolated by the high-frequency transformer has certain limitations in terms of economy, efficiency, power density, etc. In summary, this article adopts a non-isolated boost topology that can make the capacitor capacity more reasonable. The main technical indicators are as follows: the supercapacitor voltage can be used in the range of 3V-5V, the maximum input current is 18A~20A, the output voltage is +5V@5A, and the holding time is 10 seconds. Due to the short power-off protection time, the power component does not need to be too harsh when it is used.

As a storage element, the supercapacitor is powered by a 5V power supply under normal circumstances, and the supercapacitor is charged at the same time. When the external power supply is powered off, all power supply needs of the system are met by the supercapacitor. In this design, the supercapacitor part is composed of two capacitors with a withstand voltage of 2.7V and a capacitance of 220F in series. In order to achieve a better voltage balancing effect, two 1M resistors are used to balance the two supercapacitors.

3 Backup power supply main power design

3.1 Design of main power topology

The topology of the main power circuit adopts the Boost boost circuit. The circuit is shown in Figure 1, which mainly includes three parts: supercapacitor, boost topology and LC filter.

In the Boost power topology, the current borne by the inductor and MOSFET is large, up to 20A, and the current resistance of the MOSFET and the necessary heat dissipation measures must be considered. The inductor value should be selected appropriately (2.2uH is selected in this article). Since the necessary gain needs to be obtained when the input voltage is low, the internal resistance of the MOSFET and the inductor will affect the voltage gain, that is, there is a maximum duty cycle. When the duty cycle exceeds this value, the voltage gain decreases instead, the efficiency becomes low, and it is easy to cause the inductor saturation due to excessive inductor current, thereby burning the MOSFET or inductor. The MOSFET needs to have a small on-resistance, and the DC impedance of the inductor also needs to be very small.

The LC filter part mainly includes inductance and capacitance, and the filter level can be selected through experiments. This design uses 0.9uH inductance as the filter inductance, and the filter capacitor is a parallel connection of 2200uF and 0.1uF.

Figure 1 Main power circuit schematic

3.2 Drive control design The

drive control adopts UCC2813, the switching frequency is 100K, as shown in Figure 2, the output Gate1 of the chip directly drives the MOSFET.

Figure 3 Schematic diagram of shutdown circuit Figure

4 Reliable shutdown circuit design

After the task is completed, it can be powered off reliably, that is, the power-off voltage drops rapidly and monotonically. The principle of the shutdown circuit is shown in Figure 3, which mainly includes two parts: TL431 reference circuit and LM339 op amp comparison circuit. By detecting the voltage across the super capacitor and comparing it with the setting, a hysteresis loop is formed to complete the output cutoff of the circuit. In the figure, the hysteresis comparator shuts down the circuit when the capacitor voltage is less than 3.5V.

5 Experimental results The

output is tested under the conditions of full load 5A and no load. The output and control duty cycle waveforms, voltage ripples and shutdown voltage waveforms are shown in Figures 4, 5 and 6 respectively, and the voltage tube section waveform is shown in Figure 7.

Figure 4: No-load output waveform, the average voltage is 5.0V (left) Full-load output waveform, the average voltage is 4.98V (right).

Figure 5 Output voltage waveform (2) and duty cycle waveform (1) (left) when no-load. Output voltage waveform (2) and duty cycle waveform (1) (right) when full-load.

Figure 6 Output voltage ripple waveform at no load (left) and output voltage ripple waveform at full load (right).

Figure 7 Waveform when the voltage is turned off.

When the output is unloaded, the voltage is 5.0V, and the ripple peak-to-peak value is 50mV; when the output current is 5A, the voltage is stable at 4.98V, and the ripple peak-to-peak value is 150mV during the full working period. The load adjustment rate is less than 1%, and the duty cycle regulation is stable; the shutdown circuit works normally and can instantly shut down the output. The waveform is monotonic and does not produce oscillation. The supercapacitor drops from 5V to 3.5V, which can provide the device with 5A for continuous power supply for 10s, meeting the design requirements. 6 Conclusion

This paper introduces a design of a power-off backup power supply. The supercapacitor is used as an energy storage element for long-term floating charge and large current discharge, which improves the service life; the boost topology is adopted to optimize the supercapacitor capacity configuration, which can work continuously for 10s under 5V@5A conditions, and when the capacitor stops working due to undervoltage, the output can be quickly shut down. The output voltage drops monotonically without oscillation, meeting the needs of most devices.

Introduction: This paper introduces the design of a power-off backup power supply. It uses supercapacitors as energy storage elements for long-term floating charge and large current discharge, which increases the service life. It adopts a boost topology and optimizes the supercapacitor capacity configuration. It can work continuously for 10s under 5V@5A conditions, and can quickly shut down the output when the capacitor stops working due to undervoltage. The output voltage decreases monotonically without oscillation, and the electrical indicators meet the requirements.

Reference address：A design scheme for power failure backup power supply

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