BCM PFC Zero-Current Prediction for Stable Switch Timing
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Solution Overview
Problem
Conventional power factor correction converters face inaccuracies in sensing zero current time points due to constant threshold and delay period approaches, leading to unstable operation in boundary conduction mode (BCM) as rectified voltage changes.
Innovation Solution
A power factor correction converter with a zero current prediction circuit that generates a second period based on a first period between current sensing signal thresholds, allowing precise control of switches to accurately determine the zero current time point, incorporating comparators, timers, and integration capacitors to adjust switching states accordingly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a constant threshold and constant delay period are used to sense zero current time point, then the sensing circuit is simple, but the sensing accuracy deteriorates when rectified voltage changes
Solution Approach 1:
The patent applies dynamics by making the delay period variable instead of constant. The delay period is dynamically adjusted based on the rectified voltage level - when rectified voltage is high, the delay period is longer; when rectified voltage is low, the delay period is shorter. This dynamic adjustment compensates for the changing current waveform slopes at different voltage levels, maintaining accurate zero current time point detection across varying operating conditions.
Solution Approach 2:
The patent changes the parameter of delay period from constant to variable based on rectified voltage. By detecting the rectified voltage level and selecting appropriate delay periods from a plurality of predetermined delay periods, the system adapts the sensing parameters to match operating conditions, thereby maintaining measurement precision across different voltage levels without significantly increasing circuit complexity.
2Measurement precision
If the delay period is extended to accommodate lower rectified voltages, then sensing accuracy at low voltage improves, but switching accuracy at high voltage deteriorates
Solution Approach 1:
The system dynamically selects different delay periods based on the detected rectified voltage level. When rectified voltage is high, a shorter delay period is selected to maintain accurate switching timing. When rectified voltage is low, a longer delay period is selected to accommodate the slower current decay and maintain sensing accuracy. This dynamic parameter selection resolves the contradiction between low-voltage sensing accuracy and high-voltage switching accuracy.
Solution Approach 2:
The patent implements preliminary action by pre-establishing multiple delay period values corresponding to different rectified voltage levels. Before actual zero current detection, the system detects the current rectified voltage level and pre-selects the appropriate delay period from the predetermined set, ensuring optimal timing accuracy for the current operating condition without requiring real-time calculation or adjustment.
Data Source
AI summary
A power factor correction converter includes a power stage circuit, a current sensing circuit and a zero current prediction circuit. The power stage circuit converts a rectified power to an output power. The power stage circuit operates in a boundary conduction mode to correct a power factor of the rectified power. The current sensing circuit senses an inductor current to generate a sensing signal. The zero current prediction circuit controls at least one switch by: generating a second period according to a first period, wherein the first period is between when the sensing signal passes a first threshold and when the sensing signal passes a second threshold; and switching a state of the at least one switch at an end time point of the second period, wherein the end time point corresponds to a zero current time point at which the inductor current reaches zero.