Switching Control Circuit for Accurate PFC Transistor On-Time
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Solution Overview
Problem
Switching control circuits in power factor correction circuits struggle to accurately determine the on-width of transistors due to the limited control range of comparators, especially when output voltages are low, leading to incorrect determination of transistor switching times.
Innovation Solution
A switching control circuit that includes a driver circuit to turn on and off transistors based on inductor current values, an estimation circuit to differentiate between different AC voltage levels, and output circuits to adjust the slope of the output signal based on these levels, ensuring accurate transistor control across varying AC voltage conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a comparator is used to determine transistor on-width by comparing output voltage with a ramp wave, then the switching control can be simplified, but the control range is limited and accurate determination fails when output voltage is low
Solution Approach 1:
The patent changes the control parameter from direct voltage comparison to time-period-based control. The comparator compares a time period signal (derived from oscillation frequency) with a reference value to determine transistor on-width, rather than directly comparing output voltage with ramp wave. This parameter transformation expands the control range and maintains accuracy across different output voltage levels.
Solution Approach 2:
The patent introduces dynamic adjustment of the oscillation frequency based on the detected AC voltage level. When AC voltage is high, the oscillation frequency increases, allowing the time period signal to remain within the comparator's effective control range even when output voltage is low. This dynamic adaptation resolves the limitation of fixed control range.
2Device complexity
If the comparator control range is limited, then the circuit design is simpler, but it cannot correctly determine on-width when output voltage is low
Solution Approach 1:
The oscillation frequency is dynamically adjusted based on the detected AC voltage level. When AC voltage is high, frequency increases proportionally, which keeps the time period signal within the comparator's effective range across all output voltage conditions. This dynamic behavior enables the simple comparator circuit to adapt to varying operating conditions.
Solution Approach 2:
The patent introduces a time period signal as an intermediary between the output voltage and the comparator decision. Instead of directly comparing voltage levels, the system converts voltage information into a time domain signal through oscillation, which then serves as the input to the comparator. This intermediary transformation expands the effective control range.
3Device complexity
If AC voltage level is not accurately detected, then the circuit operation is simpler, but transistor switching control becomes inaccurate leading to potential damage
Solution Approach 1:
The system uses its own operating parameters (oscillation frequency and time period) to detect the AC voltage level, rather than requiring separate sensing circuits. The detection is performed selflessly through the existing signal paths and components, simplifying the overall circuit while maintaining reliable voltage level detection for safe transistor operation.
Data Source
AI summary
A switching control circuit for a power supply circuit, including: a driver circuit that turns on the transistor after an inductor current reaches a first value, and turns off the transistor in response to a first time period having elapsed; an estimation circuit that estimates whether an effective value of the AC voltage is a first level or a second level, based on a second time period, the first time period, and an output voltage; a first output circuit that, in response to an inductor current reaching the first value, sets a slope of an output thereof to first and second slopes, respectively when the effective value is the first and second levels; and a second output circuit that outputs, as the first time period, a period from when the inductor current reaches the first value to when the output of the first output circuit reaches a predetermined level.