Design of a Redundant Hot Backup Power Supply

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Design of a Redundant Hot Backup Power Supply [Copy link]

When designing a high-reliability computer system, its supporting power supply is required to adopt a redundant design. Generally speaking, the solutions that can be adopted include capacity redundancy, redundant cold backup, parallel current-sharing N +1 backup, and redundant hot backup.

Capacity redundancy means that the maximum load capacity of the power supply is greater than the actual load, which is called "a big horse pulling a small cart". Its disadvantage is that it is not conducive to improving the efficiency of the power supply and has little significance for improving the reliability of the power supply.

Redundant cold backup means that the power supply consists of two or more unit modules with the same functions. After the power supply is started, one of the unit modules supplies power to the device. When the working unit fails, the backup unit immediately starts to supply power to the device. The disadvantage of this method is that it takes a certain amount of time from the start-up of the backup unit to the establishment of the output voltage, which can easily cause a large gap in the output voltage, which will affect the powered equipment.

The N +1 backup mode of parallel current sharing means that the power supply is composed of multiple units with the same functions. The sum of the output power of all units is greater than the power required by the system. The outputs of each unit are connected in parallel through an OR gate diode. Sometimes the output adopts a current sharing control circuit. This mode is currently used more frequently. In the N +1 backup mode, since multiple units supply power to the equipment at the same time, the failure (failure) of a single unit generally does not affect the output voltage. However, if the output line fails, it is easy to affect all units.

Redundant hot backup means that the power supply is composed of multiple units with the same function. When the power supply is started, all units work at the same time, and the pre-set unit supplies power to the device. The backup unit is in a no-load state. When the unit supplying power to the device fails, the backup unit immediately supplies power to the device to maintain the stability of the output voltage. The advantage of this method is that after the working unit fails, the backup unit has a fast output response speed, which can ensure that the output voltage fluctuates only within a very small range.

This article discusses in detail the power supply design scheme using redundant hot backup mode.

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1 Working Principle The main circuit of the redundant hot standby structure consists of two units with the same function and in working state at the same time. The switching circuit controls one of them to supply power to the equipment and the other to be unloaded. When the unit supplying power to the equipment fails, the switching circuit immediately acts to make the other unit supply power to the equipment and cut off the output of the faulty unit. The main circuit topology adopts a forward converter, which consists of an input filter circuit, a power conversion circuit, a control circuit, an output filter circuit, and a monitoring switching circuit. The power supply block diagram is shown in Figure 1. The DC 28V input is filtered and provided to the power conversion circuit. The control circuit controls the power conversion circuit through real-time detection to achieve an output isolated and stable 5V voltage, and at the same time, the output voltage is protected against overvoltage and overcurrent. Figure 1 Power supply block diagram The redundant hot standby function is realized by output monitoring and switching switches. Under normal conditions, one of the two units supplies power to the equipment. When a failure occurs, the other unit in hot standby immediately supplies power to the equipment and cuts off the output of the faulty unit. If the output monitoring circuits of the two units fail at the same time, the two units supply power to the equipment at the same time. Since the output end of each unit is connected to an OR gate diode, this is a backup in the parallel mode of power output. 2 Circuit Implementation of Monitoring and Switching Function As a redundant hot backup power supply, the main problem is the fault judgment of the working unit. If sensors are set for each possible fault point of the power supply, fault judgment is performed through intelligent chips or discrete chipsets, and then corresponding switching control is adopted, the complexity of the entire power supply will increase, and the reliability of the fault detection and judgment part may not be higher than the power supply itself. Since the main faults of the power supply will be reflected in the output voltage, the standard for fault judgment is to monitor whether the output voltage of the working unit is within the set range. The circuit of the monitoring and switching function is shown in Figure 2. Figure 2 Schematic diagram of output monitoring and switching circuit In Figure 2, R18, R19, V10, and D1 form the 5V output switching switch of unit 1. When D1 is turned on, the 5V output of unit 1 is cut off; R018, R019, V010, and D01 form the 5V output switching switch of unit 2, and its function is the same as that of unit 1. R34, R35, R33, AJ4 (TL431), R20*, R22, C20, R21*, AJ2 (TL431), D2, R24, R23, C21, V13, and R30 realize the output voltage monitoring of unit 1 and the function of controlling the output switching switch of unit 2: when the output voltage of unit 1 is higher or lower than the set voltage range (the set voltage range can be changed by adjusting the resistance values of R34, R35, R20*, and R21*), no current flows through pins 1 and 2 of the optocoupler D2, so that pins 4 and 5 of D2 are cut off, the base voltage of V13 becomes low, and no current flows through pins 1 and 2 of D01, so that the switching switch of unit 2 is opened, and unit 2 outputs voltage to the device. At the same time, when the output voltage of unit 2 is within the set voltage range (adjusting the resistance values of R034, R035, R020*, and R021* can change the set voltage range), current flows through pins 1 and 2 of the optocoupler D02, causing pins 4 and 5 of D02 to conduct, the base voltage of V013 becomes high, and current flows through pins 1 and 2 of D01, causing the switch of unit 1 to turn off, and unit 1 does not output voltage to the device. Similarly, the components in the symmetrical position of unit 2 realize the same output voltage monitoring and control unit 1 output switch functions as in unit 1. By setting the capacitance values of C21 and C021, it is set which unit will supply power to the device first when the power is started. During the power startup process, pins 4 and 5 of D2 and D02 will be turned on for a very short time, and the unit with the smaller capacitance value of C21 and C021 will first cut off the path for the other unit to supply power to the device. It can be seen from Figure 2 that among the two units of this power supply, if unit 1 outputs the correct voltage first, the output of unit 2 is turned off; if the output voltage of unit 1 is incorrect, the output switch of unit 2 is turned on to supply power to the device, while the output voltage of unit 1 is cut off. Vice versa.

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3 Experimental results Using an oscilloscope to observe the output terminals of the two units, it can be observed that unit 1 outputs voltage to the device, while unit 2 does not output voltage to the device; disconnecting the input voltage of unit 1, it can be observed that the output voltage does not change, and the device is powered by unit 2. If the input voltage is first supplied to unit 2, then to unit 1, and then the input voltage to unit 2 is disconnected, the situation is the same. Figure 3 shows the waveform when the backup unit immediately supplies power to the device due to a fault or voltage drop in the working unit. It can be seen that the output voltage returns to the standard value within 10ms and will not cause the computer to restart. Figure 3 Switching waveform

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This article presents a method for implementing redundant hot backup power supply. Its circuit is simple and helps to improve the overall reliability of the power supply. However, in high-power applications, the switch should be carefully selected to reduce its impact on the output voltage. At the same time, consider using a multi-point output voltage feedback method to compensate for the impact of the switch on the output voltage.