Switching Control Circuit for Critical-Mode PFC Waveform Shaping
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Solution Overview
Problem
In power factor correction circuits operating in critical mode, the limitation of switching frequency to prevent losses in power transistors and inductors results in an increased off-time period, making it difficult to shape the inductor current waveform similarly to the rectified AC voltage waveform, thereby deteriorating the power factor.
Innovation Solution
A switching control circuit that includes a correction circuit to adjust the on-time period of the transistor, using a timer to measure the elapsed time since the transistor is turned on and correcting the command value to increase the on-time period, ensuring the inductor current waveform resembles the rectified AC voltage waveform even when the switching frequency is limited.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the switching frequency is limited to prevent losses in power transistors and inductors, then losses are reduced, but the off-time period increases making it difficult to shape the inductor current waveform similarly to the rectified AC voltage waveform
Solution Approach 1:
The patent applies dynamics by making the on-time period variable through a correction circuit that dynamically adjusts the transistor's on-time based on the relationship between off-time and switching frequency. This allows the system to adapt to changing operating conditions, maintaining waveform shaping precision even when switching frequency is limited to reduce losses.
Solution Approach 2:
The patent changes the parameter of on-time period through the correction circuit, which modifies the transistor's on-time to compensate for the increased off-time caused by limited switching frequency. This parameter adjustment ensures that the inductor current waveform continues to follow the rectified AC voltage waveform shape despite the frequency limitation.
2Loss of energy
If the switching frequency is limited, then losses in power transistor and inductor are reduced, but the power factor correction performance deteriorates
Solution Approach 1:
The patent implements feedback through a correction circuit that monitors the switching frequency and off-time period, then adjusts the transistor's on-time period accordingly. This feedback mechanism ensures that power factor correction performance is maintained even when switching frequency is limited to reduce switching losses.
3Loss of energy
If the off-time period increases due to limited switching frequency, then switching losses are reduced, but the inductor current waveform cannot be shaped similarly to the rectified AC voltage waveform
Solution Approach 1:
The patent changes the on-time period parameter through the correction circuit to compensate for the increased off-time. By adjusting this parameter, the system maintains the triangular waveform shape of the inductor current that follows the rectified AC voltage, even when switching frequency is limited and off-time is extended.
Data Source
AI summary
A switching control circuit for a power supply circuit, including: a first command value output circuit outputting a first command value; a correction circuit correcting the first command value, to output a second command value; a first timer circuit measuring a first time period starting from a first timing at which a transistor of the power supply circuit is turned on; and a driving signal output circuit outputting a driving signal to turn on the transistor in response to an inductor current of the power supply circuit reaching a predetermined value and the first time period having elapsed since the first timing, and outputting the driving signal to turn off the transistor based on the second command value. The correction circuit causes an on time period of the transistor to increase, when the first time period has elapsed since the first timing, after the inductor current reaches the predetermined value.