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This application note introduces a scheme where the main power supply and backup battery are connected to the load via a diode "OR" logic circuit. While this architecture is easy to understand, when the battery voltage exceeds the main power supply voltage, the diode "OR" logic circuit will connect the battery power supply, failing to reasonably select the main power supply. This article provides a solution to this problem. The design uses the MAX931 comparator, which has an internal 2% reference.
The main power supply and backup battery are connected to the load through a simple diode "OR" logic circuit. However, when the battery voltage exceeds the main power supply voltage, the diode "OR" logic circuit will connect the battery power supply, failing to properly prioritize the main power supply. Figure 1 shows a solution to this problem. The voltage range of the main switch power supply is 7V to 30V, and the backup power supply is a 9V battery.
![Figure 1. IC1 MAX931 comparator monitors the main power supply voltage. When the main power supply voltage drops below 7.4V, it connects the backup battery by grounding the negative terminal of the battery.]()
Figure 3. When restoring main power supply quickly, there are no transient disturbances at the output of the circuit shown in Figure 1.
In today’s electronic systems, power management is critical for ensuring reliable operation across various scenarios. A common challenge arises when trying to seamlessly switch between a primary power source and a backup battery. Traditional diode-based "OR" logic circuits can fail to handle this transition effectively, especially when the backup battery voltage exceeds that of the primary source. This issue can lead to inefficient power usage and potential system instability.
The solution presented here utilizes the MAX931 comparator, which offers precise monitoring capabilities. By comparing the voltages of the two power sources, the comparator ensures that the system always draws power from the higher voltage source, thereby optimizing performance and reliability. This approach is particularly beneficial in applications where uninterrupted power supply is essential, such as medical devices, telecommunications equipment, and automotive electronics.
Moreover, the design's ability to quickly recover from temporary power outages without introducing any transient disturbances makes it ideal for environments where power stability is paramount. The integration of high-precision components like the MAX931 comparator enhances the overall robustness of the power management system, providing engineers with a dependable tool to address complex power challenges.
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