Fixed-off-time PFC Controller Modulates Switching Frequency
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Solution Overview
Problem
Traditional power factor correction (PFC) devices in forced-switching power supplies result in distorted line voltage and increased energy dissipation due to pulsed current absorption, leading to low power factor and high total harmonic distortion, which is inefficient and wasteful.
Innovation Solution
A control device for PFC that modulates the switch-off period of a power transistor based on the instantaneous line voltage, maintaining a constant switch-off time and adjusting the switch-on time to maintain a constant operating frequency, thereby ensuring sinusoidal current absorption and reducing harmonic distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If traditional capacitive filter is used in power supply, then the circuit is simple, but the power factor is low (0.4-0.6) and total harmonic distortion is high (>100%)
Solution Approach 1:
The patent implements a dynamic switching control system that adjusts the switch-on and switch-off times of the power transistor based on instantaneous line voltage conditions. The control device continuously monitors the line voltage and dynamically modulates the switching parameters to maintain constant switching frequency while ensuring sinusoidal current absorption, transforming the static capacitive filter approach into a dynamic active power factor correction system.
2Device complexity
If fixed switch-off time control is used, then the control is simple, but the switching frequency varies with line voltage causing distorted current absorption
Solution Approach 1:
The patent employs a feedback control mechanism where the control device monitors the instantaneous line voltage and uses this information to adjust the switching parameters. The system measures the line voltage conditions and feeds this information back to the switching control logic, enabling the power transistor switching timing to be synchronized with the line voltage waveform, thereby achieving sinusoidal current absorption and maintaining constant switching frequency.
3Stability of the object's composition
If pulsed current absorption is used, then the capacitor can maintain small ripple voltage, but the current waveform is distorted with high harmonic content
Solution Approach 1:
The patent changes the switching parameters (switch-on time and switch-off time) as functions of the instantaneous line voltage. By dynamically adjusting these parameters based on the voltage waveform, the system transforms the fixed pulsed current absorption into a controlled sinusoidal current draw, eliminating harmonic distortion while maintaining voltage stability through the continuous switching action.
4Productivity
If switch-on time is extended to increase power transfer, then the current pulses become narrower and more distorted, but if switch-on time is reduced to improve current form factor, then the power transfer efficiency decreases
Solution Approach 1:
The patent dynamically adjusts the switch-on time based on the instantaneous line voltage conditions. During voltage peaks, the switch-on time is naturally extended to maximize power transfer, while during voltage zero-crossings, the switch-on time is reduced to maintain sinusoidal current form. This dynamic adjustment resolves the contradiction by optimizing both power transfer efficiency and current quality simultaneously through voltage-synchronized switching control.
Data Source
AI summary
A control device for a power factor correction device in forced switching power supplies is disclosed; the device for correcting the power factor comprises a converter and said control device is coupled with the converter to obtain from an input alternating line voltage a regulated output voltage. The converter comprises a power transistor and the control device comprises a driving circuit of said power transistor; the driving circuit comprises a timer suitable for setting the switch-off period of said power transistor. The timer is coupled with the alternating line voltage in input to the converter and is suitable for determining the switch-off period of the power transistor in function of the value of the alternating line voltage in input to the converter.