Flyback Converter Input Voltage Detection via Timer
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Solution Overview
Problem
Indirect input voltage detection in flyback converters is affected by process variations such as magnetizing inductance, bulk capacitance, load, and sense resistance, leading to reduced detection accuracy and increased costs due to the need for protection circuitry and dedicated terminals.
Innovation Solution
A flyback converter controller that uses a timer to measure the detection time between the power switch transistor cycling and the sense resistor voltage exceeding a threshold, with a memory to store the minimum detection time per half-cycle and a logic circuit to compare this time to thresholds to characterize the rectified input voltage, thereby improving detection accuracy and eliminating the need for high voltage protection circuitry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If direct input voltage detection is used, then detection accuracy is improved, but device complexity and cost increase due to high voltage protection circuitry and dedicated terminals
Solution Approach 1:
The patent uses the sense resistor voltage as an intermediary parameter to indirectly detect input voltage conditions. Instead of directly measuring the high voltage input, the system measures the voltage across the sense resistor which is proportional to the primary winding current, which in turn reflects the input voltage state. This intermediary measurement approach eliminates the need for high voltage protection circuitry while maintaining detection capability.
2Device complexity
If indirect input voltage detection using sense resistor voltage is used, then device complexity is reduced, but measurement precision deteriorates due to process variations
Solution Approach 1:
The system performs preliminary measurements by capturing the sense resistor voltage at multiple specific time points during the switching cycle (at the minimums of the sinusoidal variation of input voltage). By pre-identifying and storing these critical measurement moments, the system ensures that measurements are taken when the input voltage is at its minimum, providing the most reliable indication of brownout conditions while accounting for the dynamic nature of the rectified voltage.
Solution Approach 2:
The patent implements a feedback mechanism where the controller continuously monitors the sense resistor voltage and compares it against stored minimum values from previous cycles. The system uses this feedback to dynamically adjust its detection decisions, comparing current measurements with historical minimums to account for normal voltage variations. This feedback loop enables accurate brownout detection by distinguishing between normal voltage dips and actual brownout conditions based on the pattern and magnitude of voltage variations over time.
Data Source
AI summary
A flyback converter is disclosed that times a detection time for each cycle of a power switch transistor. The detection time ends when a sense resistor voltage exceeds a threshold voltage. Over a half cycle of an AC input voltage to the flyback converter measures a series of detection time and determines a minimum detection time from the series of detection times. If the minimum detection time exceeds a brownout delay, a brownout condition exists. If the minimum detection time is less than an overvoltage delay, an overvoltage condition exists.