SMPS Input Filter Capacitor Overvoltage Protection Circuit
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Solution Overview
Problem
Conventional switch mode power supplies (SMPS) are vulnerable to high voltage surges, which can damage input filter capacitors and the entire power supply system, leading to increased costs and larger board sizes due to the need for higher voltage-rated capacitors.
Innovation Solution
A control circuit that includes a transistor to disconnect the input filter capacitor during overvoltage events, providing overvoltage protection while reducing the bill of materials cost and circuit board size by using a transistor instead of multiple capacitors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If filter capacitors with higher voltage ratings are used to protect against high voltage surges, then the power supply can withstand higher breakdown voltages, but the cost and board size increase
Solution Approach 1:
The patent divides the protection function into two parts: a transistor that disconnects the filter capacitor during overvoltage events, and a smaller capacitor that operates during normal conditions. This segmentation allows the system to use a smaller capacitor while maintaining protection capability through the transistor's switching action.
Solution Approach 2:
The transistor acts as an intermediary device between the input voltage source and the filter capacitor. It monitors the voltage and intervenes by disconnecting the capacitor when overvoltage conditions are detected, thereby protecting the capacitor without requiring it to be rated for the maximum surge voltage.
2Reliability
If filter capacitors with higher voltage ratings are used to protect against high voltage surges, then the power supply can withstand higher breakdown voltages, but the cost increases
Solution Approach 1:
The protection function is segmented between a transistor and a smaller capacitor. The transistor, which is a low-cost component, handles the overvoltage protection by disconnecting the capacitor, allowing the use of a cheaper, smaller capacitor instead of an expensive high-voltage-rated capacitor.
Solution Approach 2:
The transistor serves as a sacrificial protection device that can be easily replaced if needed, while the main capacitor operates at lower voltage ratings and can be a more economical component. This approach uses cheaper components to achieve the same protection level.
3Reliability
If more or larger filter capacitors are used to withstand greater energy from high voltage surges, then the power supply can absorb more surge energy, but the board size and cost increase
Solution Approach 1:
The system segments the energy handling function: the transistor manages the high-voltage surge by disconnecting the capacitor, while the smaller capacitor handles normal operating voltages. This allows the capacitor to be smaller while the transistor provides the surge protection capability.
Solution Approach 2:
The transistor serves as an intermediary that protects the capacitor from excessive energy by disconnecting it during surge events. This allows the use of a smaller capacitor that would otherwise be overwhelmed by high-voltage surges.
Data Source
AI summary
One or more embodiments relate to a control circuit for an input filter circuit in a switch mode power supply comprising a power switch and a switch controller to control the power switch to provide a regulated output voltage and current to a load. The control circuit, also referred to as a filter control circuit, can be used to detect a high voltage surge at its input and disconnect a capacitor in the input filter circuit from an input return, thereby protecting the input filter capacitor and the SMPS from damage. According to certain aspects, the control circuit can be integrated with the switch controller. Additionally, the control circuit can provide power to the switch controller at start-up.