LED Backlight Driver Transfer Switch for Short Dimming Cycles
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Solution Overview
Problem
LED backlight driving circuits face challenges in maintaining normal operation during short dimming cycles due to energy inefficiencies caused by leakage currents and inadequate charging when PWM signals are OFF for extended periods.
Innovation Solution
An LED backlight driving circuit with a transfer switch that switches between the LED lightbar output and a feedback voltage, ensuring continuous power supply to the capacitor even when the PWM is OFF, and adjusts the duty ratio to ensure adequate energy delivery when PWM is ON, using a power supply module with a boost circuit and comparator to regulate brightness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If PWM is OFF for a long time to reduce power consumption, then energy saving is improved, but the capacitor charges insufficiently and LEDs cannot operate normally when PWM turns ON again
Solution Approach 1:
The invention introduces a transfer switch that performs preliminary action by switching the feedback voltage source before the PWM dimming cycle ends. When the dimming signal is detected to be in the OFF state for an extended period, the transfer switch proactively switches from the LED lightbar feedback voltage to a lower feedback voltage, ensuring the capacitor is recharged in advance. This preliminary action prevents the capacitor from being fully depleted during the OFF period, guaranteeing that sufficient energy is available when the PWM turns ON again, thus maintaining LED operation normality while allowing longer OFF periods for energy saving.
Solution Approach 2:
The transfer switch acts as an intermediary component between the feedback voltage source and the power supply module. It mediates the conflict between energy saving and reliable operation by selecting different feedback voltage sources based on the dimming cycle state. During normal operation, it uses the LED lightbar feedback voltage for precise dimming control; during extended OFF periods, it switches to a lower feedback voltage to maintain minimum capacitor charging, thus ensuring both energy efficiency and operational reliability.
2Speed
If PWM dimming cycle is shortened to improve response speed, then response speed is improved, but the ON time becomes too short to achieve required energy for LED operation
Solution Approach 1:
The invention ensures continuity of useful action by maintaining capacitor charging during the PWM OFF period through the transfer switch mechanism. Even when the PWM dimming cycle is shortened for faster response, the transfer switch continuously monitors the dimming state and switches to the lower feedback voltage source when extended OFF periods are detected, ensuring the capacitor receives continuous charging current. This continuous action guarantees that energy accumulation is maintained regardless of how short the ON period becomes, enabling fast response while ensuring sufficient energy for LED operation.
Solution Approach 2:
The invention dynamically changes the feedback voltage parameter based on the dimming cycle characteristics. When the PWM OFF period exceeds a threshold, the transfer switch changes the feedback voltage from the normal LED lightbar voltage to a lower voltage, which allows the capacitor to be recharged during the OFF period. This parameter change enables the system to adapt to shortened dimming cycles by ensuring the capacitor accumulates sufficient energy even with reduced ON time, thus supporting faster response speeds.
3Device complexity
If conventional LED driver is used with simple PWM control, then device complexity is reduced, but the circuit cannot operate reliably within short dimming cycles due to capacitor discharge
Solution Approach 1:
The transfer switch serves as an intermediary component that adds minimal complexity to the conventional LED driver circuit. It is inserted between the feedback voltage source and the power supply module, monitoring the dimming signal state and automatically switching between different feedback voltage sources. This simple intermediary structure enables the circuit to reliably operate within short dimming cycles by ensuring the capacitor is continuously recharged during OFF periods, without significantly increasing overall circuit complexity.
Solution Approach 2:
The transfer switch implements self-service by automatically detecting the dimming signal state and making decisions about which feedback voltage source to use, without requiring external control signals or complex control logic. The circuit monitors its own operating state and autonomously adjusts the feedback voltage selection to maintain capacitor charging during extended OFF periods, thereby enabling reliable short dimming cycle operation while keeping the control system simple.
Data Source
AI summary
The invention discloses an LED backlight driving circuit, an LCD device and a driving circuit. The LED backlight driving circuit includes a power supply module, an LED lightbar coupled with an output terminal of the power supply module, a dimming module coupled with the output terminal of the LED lightbar, and the LED backlight driving circuit further includes a transfer switch. A first input terminal of the transfer switch is coupled to the output terminal of the LED lightbar, and a second input terminal of the transfer switch is coupled to a feedback voltage. The output terminal of the transfer switch is coupled to the power supply module. The transfer switch is switched to the first input terminal when the dimming module is ON, and the transfer switch is switched to the second input terminal when the dimming module is OFF. Therefore, the LEDs of the invention can operate normally within the short dimming cycle. The invention only adds one transfer switch, the circuit is simple and does not increase the circuit area, and the cost is low.