Switch Mode Power Supply Transient Voltage Suppression
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Solution Overview
Problem
Switch mode power supplies, particularly those employing a two-switch buck-boost converter, face challenges in transient voltage protection and MOSFET driving protection due to the limitations of varistors, which can lead to damage from transient voltages and free-oscillating conditions, as MOSFET switches are often not designed to withstand the high voltages generated during abnormal line conditions and free-oscillation events.
Innovation Solution
The implementation of a transient voltage suppression device using a series connection of a diode and a varistor, and a free-oscillation suppression device involving NPN transistors and resistors, to clamp and regulate voltages across MOSFET switches, ensuring they operate within their withstanding limits and preventing damage from transient and free-oscillating conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a standard varistor is used for transient voltage suppression, then the varistor can suppress transient voltages, but the clamp voltage exceeds the MOSFET withstanding voltage causing damage
Solution Approach 1:
The transient voltage suppression function is divided into two stages: the varistor provides initial suppression and limits voltage rise, while the series diode takes over to maintain the clamp voltage within MOSFET withstanding limits. This segmentation allows each component to operate in its optimal range without exceeding component ratings.
Solution Approach 2:
The series diode acts as an intermediary element between the varistor and the MOSFET. It modifies the varistor's clamp characteristic by conducting in a specific direction, thereby reducing the effective clamp voltage seen by the MOSFET while still utilizing the varistor's transient suppression capability.
2Reliability
If MOSFET switches are designed to withstand higher voltages, then transient voltage protection is improved, but device cost and complexity increase
Solution Approach 1:
The circuit provides beforehand cushioning by placing the varistor-diode suppression network in parallel with the MOSFET before transient voltages occur. This pre-positioned protection mechanism is always ready to activate when transient voltages appear, preventing the MOSFET from being exposed to damaging voltage levels without requiring the MOSFET itself to be over-engineered for higher voltage tolerance.
3Reliability
If free-oscillation suppression is added to the converter driver, then MOSFET driving protection is improved, but device complexity increases
Solution Approach 1:
The free-oscillation suppression function is extracted as a separate, dedicated circuit module within the converter driver. By isolating this protection function into its own circuitry with specific components (resistors, capacitors, diodes), the design can address free-oscillation issues without complicating the main switching control logic, allowing independent optimization of each function.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution effectively suppresses transient voltages and prevents damage to MOSFET switches by ensuring the voltages across them remain below their withstanding limits, even during abnormal conditions, and stabilizes drive voltages during free-oscillation, thereby enhancing the reliability and safety of switch mode power supplies.
Implementation Method 1
a transient voltage suppression device operable to suppress the rectified supply voltage in response to an abnormal line condition
Implementation Method 2
a free-oscillation suppression device operable to suppress the drive voltage(s) in response to a free-oscillating condition of the converter driver
Data Source
AI summary
A switch mode power supply (15) employs a rectifier (20), a converter (50) and converter driver (60). The rectifier (20) generates a rectified supply voltage (VRS) based on an in-line voltage (VLN), and the converter driver (60) generates one or more driving voltages (VDR) to facilitate a conversion by the converter (50) of the rectified supply voltage (VRS) to a DC bus voltage (VDC) based on the driving voltage(s) (VDR). The converter (50) may include a transient voltage suppression device (52) to suppress the rectified supply voltage (VRS) in response to an abnormal line condition of the switch mode power supply (15), and the converter driver (60) may include a free-oscillating suppression device (61) to suppress the one or more driving voltages (VDR) in response to a free-oscillating condition of the converter driver (60).