BCM PFC Controller Dynamic On-Time Adjustment for Valley Skipping
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Solution Overview
Problem
Boundary conduction mode (BCM) power factor correction (PFC) stages in switched mode power supplies (SMPS) experience high switching frequencies under light loads, leading to reduced power transfer and distorted input currents, which deviate from the ideal sine wave, resulting in a lower power factor.
Innovation Solution
The method involves adjusting the duration of the on-time in subsequent switching cycles based on the number of valleys skipped, using calculations such as geometric or arithmetic series, or a look-up table, to maintain a close approximation of the ideal switching cycle, thereby minimizing distortion and maximizing power factor correction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If valley skipping is implemented in BCM operation to limit switching frequency under light loads, then switching frequency is reduced, but input current distortion increases and power factor deteriorates
Solution Approach 1:
The patent implements dynamic adjustment of the on-time parameter based on the number of valleys skipped. The controller dynamically modifies the on-time of subsequent switching cycles in response to valley skipping events, creating a adaptive control system that responds to changing load conditions. This dynamic adjustment compensates for the distortion introduced by valley skipping while maintaining the frequency-limiting benefit.
Solution Approach 2:
The patent employs a feedback mechanism where the controller monitors the number of valleys skipped and uses this information to adjust the on-time of subsequent switching cycles. This closed-loop control approach allows the system to detect distortion-causing conditions (valley skipping) and automatically compensate by modifying switching parameters, thereby reducing current distortion while maintaining reduced switching frequency.
2Power
If the on-time is extended to compensate for skipped valleys, then power transfer is maintained, but the switching cycle duration increases
Solution Approach 1:
The patent changes the on-time parameter dynamically based on the number of valleys skipped. By adjusting this critical timing parameter, the system compensates for the extended cycle period caused by valley skipping, ensuring that the effective power transfer remains maintained despite the longer overall cycle duration.
3Loss of energy
If BCM operation is used for high efficiency, then energy efficiency is improved, but under light loads the switching frequency becomes excessively high
Solution Approach 1:
The patent implements valley skipping where the controller intentionally skips certain switching cycles (specifically, cycles where the inductor current would be at a valley point) under light load conditions. This skipping approach allows the system to maintain BCM operation for efficiency while avoiding the excessively high switching frequencies that would otherwise occur under light loads, as the skipped valleys effectively reduce the switching count without significantly impacting power transfer.
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 approach reduces the distortion of the input current, ensuring it closely follows a sinusoidal path and improves the power factor by maintaining a proportional relationship with the momentary mains voltage, even under varying load conditions.
Implementation Method 1
When the current has returned to zero, a resonance of the voltage at the switching node starts through the inductor and parasitic capacitance, the parasitic capacitance being mainly in the switch. This results in a ringing around zero inductor current.
Data Source
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Figure 3
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AI summary
For many applications an SMPS is designed to operate in boundary conduction mode. As the load decreases the switching frequency increases, and so the concept of valley skipping may be used in which the switching frequency is clamped, by delaying to turn on the time of the active switch, for an integral number of cycles of a resonant circuit in the SMPS. With further reduction of the load, additional valleys may be skipped. However, each change in the number of valleys skipped results in a step in the input current that is drawn, distorting the ideal mains sine wave, thereby increasing unwanted harmonics. A control method is disclosed, which reduces the steps: instead of a constant on-time for the switch, the duration of the on-time is increased each time an additional valley to be skipped. The predetermined increase may be either a fixed fractional increase or a further additional increment; it may be determined by a small regulation loop that multiplies the on-time from the main loop with a factor equal to the ratio between measured period time and the sum of primary and secondary stroke times.