Understanding the Role of a Large Resistor Before a Bridge Rectifier in an SMPS

Switched-mode power supplies (SMPS) are widely used in electronic devices due to their efficiency and compactness. These power supplies are designed to convert alternating current (AC) from the mains supply into direct current (DC) for use in electronic components. A critical component in the SMPS design is the bridge rectifier, but the functionality of a large resistor placed before the bridge rectifier is often overlooked. This article will explain the reasons behind using such a resistor and its impact on the operation of the SMPS.

Introduction to SMPS and Bridge Rectifiers

Switched-mode power supplies (SMPS) are designed to provide stable and efficient power to electronic devices. They work by chopping up the AC supply into smaller, high-frequency pulses that are then filtered to produce a DC output. One key component in an SMPS is the bridge rectifier, which converts the AC input into a pulsating DC voltage.

The Importance of a Large Resistor Before the Bridge Rectifier

Before the bridge rectifier, there may be a large DC filter capacitor. During the initial turn-on of the system, this capacitor is effectively discharged. As the filter capacitor begins to charge, a large initial current surge can flow into the rectifier. This surge current can be so high that it can damage the rectifier, which is a critical component. To avoid this, an additional component, such as a large resistor, is placed in series before the bridge rectifier to limit the input surge current.

Lowering Inrush Current

The primary function of the large resistor is to lower the inrush current. Inrush current is the surge of current that flows into the circuit immediately after the power supply is turned on. In the context of an SMPS, this surge current can be problematic because it can lead to overheating and potential damage to the rectifier. By placing a resistor in series with the rectifier, the flow of initial current is restricted, thus protecting the rectifier from damage.

Necessary Voltage Drop

Another role of the large resistor is to provide a necessary voltage drop. As the resistor limits the inrush current, it also creates a small voltage drop across its terminals. This voltage drop is important in the context of an SMPS because it ensures that the voltage at the rectifier’s input is within an acceptable range before the filter capacitor has fully charged. Without this voltage drop, the input voltage could be too high for the rectifier to handle safely.

Anti-Surge Disk-Shaped Thermistor

While a resistor is commonly used to control inrush current, there is an alternative component known as an anti-surge disk-shaped thermistor. This thermistor can also be placed in series before the bridge rectifier to limit the switch-on current. The switch-on current surge occurs when the filter electrolytic capacitor on the output side of the rectifier begins to charge.

Purpose of the Anti-Surge Thermistor

The purpose of the anti-surge thermistor is to protect the rectifier from the high currents that can occur when the system is powered on at the peak of the mains cycle. By limiting the initial surge current, the thermistor helps to ensure that the rectifier is not subjected to excessive stress and current, reducing the risk of damage. This component is particularly useful in applications where the power supply is frequently turned on and off, as it can help to extend the life of the rectifier and the overall system.

Conclusion

Understanding the role of a large resistor or an anti-surge thermistor before a bridge rectifier in an SMPS is crucial for the proper design and operation of electronic power supplies. These components are essential in limiting inrush current and preventing overvoltage situations, thus protecting the rectifier and ensuring the longevity and reliability of the SMPS. Proper design and selection of these components can significantly enhance the performance and durability of electronic devices that rely on SMPS technology.

References

1. Electronics CookBook: Understanding the Role of a Large Resistor in an SMPS

2. All About Circuits: Understanding Thermistors

3. CMU ECE460 Lecture Notes: Power Supply Design