AC Switch Thermal Protection via Zero-Cross Sampling
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Solution Overview
Problem
Conventional temperature detection circuits for A.C. power supply switches often trigger falsely due to transient overheating during initial powering-on, leading to premature inhibition of switch operation, which can be problematic for bidirectional switches that cannot be turned off.
Innovation Solution
A method and circuit that samples the temperature signal at a frequency corresponding to an even multiple of the A.C. supply voltage frequency, delaying deactivation by an odd number of zero crossings to differentiate between transient and sustained overheating, using a comparator and flip-flop synchronized with the A.C. voltage or current zero crossings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If temperature sampling is performed continuously, then sustained overheating is detected, but transient overheating during power-on causes premature inhibition
Solution Approach 1:
The patent uses periodic sampling synchronized with the AC voltage frequency to detect temperature conditions. By sampling at an even multiple of the AC frequency, the system performs temperature checks at optimal intervals that align with the AC cycle, allowing transient power-on heating to subside before the first sampling occurs, thus avoiding premature inhibition while still detecting sustained overheating.
Solution Approach 2:
The patent applies preliminary action by pre-synchronizing the sampling timing with the AC voltage zero-crossings. The sampling is scheduled to occur at specific phases of the AC cycle before transient power-on effects persist, allowing the system to wait out temporary heating conditions while being ready to detect and respond to sustained overheating conditions that persist beyond the initial transient period.
2Measurement precision
If sampling frequency is increased, then temperature monitoring precision is improved, but sensitivity to transient spikes increases causing false triggering
Solution Approach 1:
The patent resolves this contradiction by making the sampling frequency an even multiple of the AC voltage frequency rather than using a continuously high sampling rate. This periodic sampling approach maintains sufficient measurement precision for detecting sustained overheating while naturally filtering out transient power-on spikes that occur between sampling intervals, thus avoiding false triggering.
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
Effectively avoids untimely triggerings by distinguishing between transient and sustained overheating, ensuring the switch operates correctly while preventing damage from sustained overheating.
Implementation Method 1
a voltage representative of a temperature in the vicinity of the switch is compared with a threshold
Implementation Method 2
the result of this comparison is sampled at a frequency corresponding to an even multiple of the frequency of the A.C. supply voltage
Data Source
AI summary
A method and a circuit for detecting an overheating of an electronic switch of power supply of a load by an A.C. voltage, in which a voltage representative of the temperature in the vicinity of the switch is compared with a threshold, the result of this comparison being sampled at frequency corresponding to an even multiple of the frequency of the A.C. power supply voltage, to provide a signal indicative of the fact that a temperature threshold has been exceeded.