Design scheme to maintain backup power supply in case of power failure

　　1 Introduction

　　Important units or key components such as measuring instruments, data acquisition systems, servo systems, and robots need to record their status and perform necessary system configurations when power is off abnormally. Batteries used often have their life reduced due to long-term floating charge and need 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 and efficiently, 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-off backup power supply.

　　2 Selection of supercapacitor capacity and topology

　　The power supply realizes short-term power-off protection, and its configuration needs to be optimized, that is, to use the smallest possible capacitor capacity to obtain the longest possible use time. The Buck structure will improve efficiency, but there will be a large amount of capacitor charge that cannot be used; the reverse pressure generated by the Buck-Boost structure will be difficult to use directly; the topology of high-frequency transformer isolation 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 available range of supercapacitor voltage is 3V-5V, the maximum input current is 18A~20A, the output voltage is +5V@5A, and the holding time is 10 seconds. Since the power-off protection time is short, the power component derating does not need to be too harsh.

　　As a storage element, the supercapacitor is normally powered by a 5V power supply and charges the supercapacitor at the same time. When the external power supply is disconnected, all power supply requirements 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 connected in series. In order to achieve a better voltage balancing effect, two 1M resistors are used to balance the voltage of 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 circuit. The circuit is shown in Figure 1, which mainly includes three parts: super capacitor, boost topology and LC filtering.

　　In the Boost power topology, the current that the inductor and MOSFET bear is relatively large, up to 20 A, 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, and the efficiency becomes low. It is easy to cause the inductor to saturate 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

　　Figure 2 Control drive schematic diagram

　　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

　　4 Reliable shutdown circuit design

　　After the task is completed, it can be powered off reliably, that is, the power-off voltage decreases 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 supercapacitor 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

　　When the output is tested under full load 5A and no load conditions, the output and control duty cycle waveforms, voltage ripple and shutdown voltage waveforms are shown in Figure 4, Figure 5 and Figure 6 respectively, and the voltage 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) at no load (left) Output voltage waveform (2) and duty cycle waveform (1) at full load (right).

　　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 complete working period. The load adjustment rate is less than 1%, and the duty cycle adjustment is stable; the shutdown circuit works normally and can instantly shut down the output. The waveform is monotonous and does not oscillate. The supercapacitor drops from 5V to 3.5V, which can provide the device with 5A continuous power supply for 10s, meeting the design requirements.

　　6 Conclusion

　　This article introduces the design of a power-off 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 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, meeting the needs of most devices.