SMPS Switch Thermal Protection Using MOSFET RDS(on) Sensing
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Solution Overview
Problem
Switched mode power supplies (SMPS) face efficiency issues due to poor temperature sensing methods, which can lead to overheating and damage of the power switch, especially when dealing with fluctuating input voltages and varying load conditions.
Innovation Solution
A circuit is introduced that measures the transistor's ON state resistance (R DS(on) ) to detect temperature, using a comparator and a delay circuit to generate an overtemperature signal, allowing the SMPS to adjust switching frequency or enter bypass mode when the temperature exceeds a preset threshold, thereby protecting the power switch.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If indirect temperature sensing methods are used in SMPS, then the power switch may be damaged due to overheating, but adding direct temperature sensing increases device complexity
Solution Approach 1:
The power MOSFET's intrinsic body diode and parasitic capacitances are utilized as the temperature sensing mechanism, eliminating the need for external temperature sensors. The device's own electrical characteristics serve the dual purpose of power switching and temperature detection, reducing overall device complexity while improving reliability
Solution Approach 2:
The patent uses the gate-source capacitance and body diode characteristics as intermediary elements that indirectly reveal temperature information through electrical measurements. These inherent device structures act as mediators between the power switch and temperature detection functions without requiring separate sensing components
2Power
If linear power supply is used for delivering larger output current, then it can meet the power delivery requirement, but efficiency becomes detrimentally poor
Solution Approach 1:
The patent implements dynamic switching between linear and bypass modes based on real-time temperature conditions. The power MOSFET operates in switched-mode for normal conditions to maintain high efficiency, while automatically transitioning to linear mode only when temperature thresholds are exceeded, optimizing the balance between power delivery and energy efficiency
Solution Approach 2:
The system dynamically changes the operating parameters of the power supply by switching between different operational modes (switched-mode vs. linear mode) based on temperature conditions. This parameter change allows the system to maintain high efficiency during normal operation while providing thermal protection when needed
3Adaptability or versatility
If temperature threshold is fixed, then the protection circuit is simple, but it cannot adapt to varying load conditions and temperature ranges
Solution Approach 1:
The protection circuit dynamically adjusts the temperature threshold based on real-time operating conditions including load current and ambient temperature. The threshold is not fixed but adapts continuously to maintain optimal protection levels across varying operating conditions, enhancing system versatility
Solution Approach 2:
The system incorporates feedback mechanisms that continuously monitor operating conditions and adjust the temperature threshold accordingly. This feedback loop allows the protection circuit to adapt to varying load conditions and temperature ranges, ensuring appropriate protection levels without requiring complex manual configuration
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 protects the power switch from overheating by accurately measuring temperature changes and triggering protective measures, enhancing the reliability and efficiency of SMPS operations.
Implementation Method 1
A circuit for measuring temperature of an external switch (132) includes a variable resistor (102), a switch (104) coupled to the variable resistor in series, a fixed value resistor (122) coupled to the variable resistor in parallel, and a comparator (110) coupled between the variable resistor and the fixed value resistor
Data Source
Figure 1~2
AI summary
A circuit for measuring temperature of an external switch is disclosed. The circuit includes a variable resistor, a switch coupled to the variable resistor in series, a fixed value resistor coupled to the variable resistor and a comparator coupled between the variable resistor and the fixed value resistor. The circuit is configured to compare voltage drop between a drain and a source of the external switch when the external switch is in ON state with voltage drop at the variable resistor and output a signal to indicate an overtemperature based on the comparing.