SMPS Synchronous Switch for Fast LED Current Decay
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Solution Overview
Problem
Switch mode power supplies (SMPS) used for powering LEDs in applications like automotive headlights cause noticeable light color shifts due to slow decay of LED current, leading to undesirable wavelength shifts and color changes during dimming.
Innovation Solution
A method and system where a synchronous switch is used to quickly discharge the output capacitor, allowing for bi-directional energy control and eliminating tail current, thereby enabling fast turn-off of LED current without color shifts, and dynamically controlling the output voltage to produce substantially square current pulses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a dimming switch is used to turn LED current ON and OFF quickly, then the LED current decay speed is improved, but the circuit complexity increases
Solution Approach 1:
The patent extracts the dimming switch from the circuit by using the synchronous switch's natural operation to achieve fast current decay. The synchronous switch is turned off at the end of each PWM cycle, which naturally creates the fast decay path without requiring an additional dimming switch component.
Solution Approach 2:
The synchronous switch serves multiple functions: it provides the fast decay path for current termination, enables bi-directional energy control for output voltage positioning, and eliminates the need for separate dimming control circuitry. This multi-functionality resolves the contradiction by achieving fast decay without adding circuit complexity.
2Object-affected harmful factors
If the output capacitor is discharged quickly to achieve fast turn-off of LED current, then the color shift is reduced, but the energy control becomes unidirectional
Solution Approach 1:
The patent inverts the conventional unidirectional energy flow by enabling bi-directional energy control through the synchronous switch. Energy can flow from the output capacitor to the input during discharge (fast decay) and from the input to the output during charging (voltage positioning), resolving the contradiction between fast discharge and energy control flexibility.
Solution Approach 2:
The synchronous switch provides dynamic control over energy flow direction based on operational requirements. The controller can selectively enable forward energy transfer for voltage positioning or reverse energy transfer for fast discharge, achieving both fast turn-off and adaptability through dynamic operation.
3Measurement precision
If a synchronous switch is used for bi-directional energy control, then the output voltage positioning is improved, but the device complexity increases
Solution Approach 1:
The patent merges the voltage positioning function with the existing synchronous rectification switch. The same synchronous switch used for rectification is controlled to provide the fast decay path and enable bi-directional energy control for voltage positioning, eliminating the need for separate voltage positioning circuitry and reducing overall device complexity.
Solution Approach 2:
The synchronous switch serves itself by performing multiple functions: rectification, fast decay path provision, and voltage positioning control. This self-service approach achieves precise voltage positioning without requiring additional dedicated components, thereby improving precision while avoiding increased complexity.
4Device complexity
If the LED current is allowed to decay slowly through the output capacitor, then the circuit simplicity is maintained, but the wavelength shift in LED light occurs
Solution Approach 1:
The patent applies preliminary anti-action by pre-establishing a fast decay path through the synchronous switch before the LED current naturally decays through the output capacitor. When the synchronous switch is turned off at the end of the PWM cycle, it immediately creates the low-impedance discharge path that prevents the slow decay and associated color shift, while maintaining circuit simplicity.
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 solution prevents color shifts, improves dimming range, and simplifies LED drive applications by eliminating the need for dimming switches and timing circuit calibration, enhancing current transient response and reducing variability in light output.
Implementation Method 1
a synchronous switch is used to quickly discharge the output capacitor, allowing for bi-directional energy control and eliminating tail current, thereby enabling fast turn-off of LED current
Data Source
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AI summary
A switch mode power supply (SMPS) converter is periodically run backwards by using a synchronous switch instead of the normally used commutating diode. By running the SMPS converter backwards the SMPS output capacitor can be discharged very quickly to provide a fast turn off of (no current through) the LED's, thereby solving the color shift problem. This enables positioning the output voltage of the SMPS up or down by actively charging or discharging the bulk output capacitor. Having the capability of actively charging or discharging the bulk output capacitor allows generation of a current source comprising substantially square, e.g., substantially full current when on and substantially no current when off, current pulses that are preferable for driving LED lighting applications.