PFC Circuit Switch Timing for Light Flicker Prevention
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Solution Overview
Problem
Boost PFC circuits face challenges in detecting zero crossings at low loads due to decaying voltage oscillations, leading to potential switch failure and output voltage flicker in lighting applications.
Innovation Solution
A PFC circuit with a control unit that calculates a switch on time (Ton) after a local minimum of the discharge voltage, switching on at the next local minimum if Ton is less than a reference time (Tref), or switching on at Ton if it exceeds Tref, to prevent light flicker by efficiently determining local minima and controlling the switch.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the switch is clocked at local minimums of discharge voltage in DCM mode, then power factor correction is achieved, but detection reliability deteriorates due to decaying oscillation amplitude
Solution Approach 1:
The control unit calculates the expected switch-on time (Ton) in advance based on the first detected local minimum, then proactively searches for the next local minimum before Ton expires. This preliminary calculation and proactive searching approach ensures that the switch is reliably turned on at the correct timing, even when oscillation amplitude has decayed and local minimums become difficult to detect.
Solution Approach 2:
The invention implements a safety mechanism where the control unit monitors whether the calculated Ton expires before detecting the next local minimum. If Ton expires without detecting a local minimum, the control unit defaults to switching on at Ton. This beforehand cushioning prevents detection failures from causing operational errors, ensuring reliable switch control under all conditions.
2Reliability
If the switch is switched on after a safety margin time period, then switch protection is ensured, but output voltage stability deteriorates causing light flicker
Solution Approach 1:
The control unit calculates the expected switch-on time (Ton) in advance and actively searches for the next local minimum before Ton expires. This preliminary action ensures that the switch is turned on at the precisely calculated timing rather than waiting for a safety margin period, thereby maintaining output voltage stability and preventing light flicker while still protecting the switch through proper timing control.
3Productivity
If the oscillation amplitude decays below the discrimination threshold, then zero crossing detection fails, but continuous operation is required
Solution Approach 1:
The control unit calculates Ton in advance based on the first local minimum and then actively searches for the next local minimum before Ton expires. This preliminary calculation combined with active searching ensures that zero crossing detection succeeds even when oscillation amplitude has decayed below the discrimination threshold, maintaining continuous operation without detection failures.
Solution Approach 2:
The invention provides a fallback mechanism where if the next local minimum is not detected before Ton expires, the control unit defaults to switching on at Ton. This beforehand cushioning ensures continuous operation is maintained even when measurement precision deteriorates due to low oscillation amplitude, preventing detection failures from stopping the system.
Data Source
AI summary
The invention relates to a power factor correction (PFC) circuit (20), comprising an inductor (21) which is configured to provide a discharge current, a capacitor (23) which is connected to the inductor (21) via a switch (24) and which can be charged with said discharge current, a control unit (14) which is configured to alternately switch the switch (24) on and off based on a feedback control, wherein the control unit (14) has an input interface (42) for receiving a feedback signal (ZXCS) which represents a discharge voltage of the inductor (21), wherein the control unit (14), in a DCM mode, is further configured to calculate a switch on time (Ton) of the switch (24) which is after a first local minimum of the discharge voltage, and wherein, after switching off the switch (24), the control unit is configured to: either switch on the switch (24) at a next or closest local minimum of the inductor voltage after Ton, in case Ton is less than a directly or indirectly set reference time (Tref), or close the switch (24) at Ton, in case Ton is equal to or exceeds Tref.