Synchronous Converter Dimming Control via On-Time Adjustment
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Solution Overview
Problem
Existing synchronous converters for LED lighting face challenges in efficiently controlling current to achieve dimming while maintaining high efficiency and reducing audible noise, particularly in adjusting light output from 1% to 100% of maximum without drastic changes in switching frequency.
Innovation Solution
A synchronous converter with a threshold control circuit for the first switch and an on-time control circuit for the second switch, allowing adjustment of current through the load by varying the on-time of the second switch, enabling dimming of LEDs with reduced audible noise and improved efficiency through zero voltage switching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the on-time of the second switch is varied to control current for dimming, then LED light output can be adjusted from 1% to 100%, but switching frequency changes drastically
Solution Approach 1:
The patent implements dynamic control of the second switch's on-time duration based on the desired dimming level. The controller adjusts the on-time dynamically within each switching cycle, allowing precise control of average current through the LED while maintaining a relatively stable switching frequency. This dynamic adjustment enables the LED output to be varied from 1% to 100% without drastic changes in switching frequency, resolving the contradiction between illumination control and frequency stability.
2Illumination intensity
If conventional dimming control is used, then LED light output can be adjusted, but audible noise increases
Solution Approach 1:
The patent employs periodic switching action with carefully controlled on-time intervals for the second switch. By regulating the on-time duration within each periodic cycle rather than changing the overall switching frequency, the patent achieves dimming control while keeping the switching frequency within a range that minimizes audible noise. The periodic nature of the switching, combined with controlled on-time variation, allows smooth dimming without generating significant audible electromagnetic interference or acoustic noise from inductor vibration.
3Loss of energy
If the second switch is turned off at zero current, then efficiency is improved, but the switching frequency becomes unstable
Solution Approach 1:
The patent incorporates feedback control mechanisms that monitor the current through the inductor and the timing of switch operations. The controller uses this feedback information to precisely determine when to turn off the second switch, ensuring it occurs at or near zero current to minimize losses. Simultaneously, the feedback loop adjusts the on-time duration in subsequent cycles to maintain stable switching frequency, resolving the contradiction between efficiency maximization through zero-current switching and frequency stability.
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
The solution allows for precise control of LED light output, reducing audible noise and maintaining high efficiency across a wide range of light output levels without significant changes in switching frequency, enhancing the performance of synchronous converters in LED driving applications.
Implementation Method 1
At the beginning of a cycle, the first switch is turned on, and current rises in the inductor. At the end of the rise, the first switch is turned off and the second switch is turned on consecutively and the current decays in the inductor.
Implementation Method 2
A second switch connected in parallel to the commutating diode is conducted so as not to be overlapped with the conduction period of the first switch. In the conduction period of the second switch, the current IL does not flow through the commutating diode, and it is possible to prevent lowering of efficiency caused by forward voltage drop.
Data Source
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AI summary
A synchronous converter for driving a load (4) comprises a first switch (M1) coupled in series with a second switch (M2) via a node (x), an inductor (L1) coupled to the node (x), input terminals (10, 11) for receiving an input voltage (Vin) from a power source, output terminals (12, 13) for supplying an output current and an output voltage to the load (4), a first mode wherein the first switch (M1) is in an on state and the second switch (M2) is in an off state, and wherein the first switch and the inductor (L1) form a series arrangement coupled between the input terminals (10, 11), a second mode wherein the first switch (M1) is in the off state and the second switch (M2) is in the on state, and wherein the second switch (M2) and the inductor (L1) form a series arrangement coupled between the output terminals (12, 13). The synchronous converter further comprises a control circuit (1) comprising a threshold control circuit (2) for generating a threshold control signal (iq2) for switching off one switch of the first switch (M1) and the second switch (M2) when a current threshold of a current through this one switch is exceeded, wherein the threshold control circuit has an input for receiving a current signal indicating a current through this one switch and a comparator for comparing the current signal with the current threshold to obtain the threshold control signal and an on-time control circuit (3) comprising a calculating unit for directly calculating an on-time duration for the other switch of the first switch (M1) and the second switch (M2), wherein the on-time duration is proportional to an average output current (ILED) and inversely proportional to an output voltage (Vout) and for adjusting a negative peak current (Ineg) to obtain a substantially constant switching frequency of the synchronous converter when the synchronous converter is in normal operating mode.