LED Backlight Dimming Switch Varistor Protection Circuit
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Solution Overview
Problem
In liquid crystal display (LCD) devices, dimming controllable switches connected in series with LED lightbars are prone to damage due to high voltage differences when short-circuited, leading to reliability issues.
Innovation Solution
The proposed solution involves connecting the dimming controllable switch in series with LED lightbars and varistors in parallel, along with a clamping circuit comprising a storage capacitor and discharge resistor, to mitigate voltage stress and enhance reliability by reducing voltage and preventing reverse current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the dimming controllable switch is connected in series with LED lightbars for dimming control, then the dimming function is achieved, but the switch is prone to damage due to high voltage difference when short-circuited
Solution Approach 1:
A varistor is connected in parallel with the dimming controllable switch to provide beforehand cushioning against high voltage. When the switch is turned off and high voltage occurs, the varistor activates to clamp the voltage, protecting the switch from damage before the high voltage can cause harm.
Solution Approach 2:
The varistor acts as an intermediary protective element between the LED lightbar and the dimming controllable switch. It mediates the high voltage by providing an alternative path for current flow, preventing the full voltage stress from reaching the switch.
2Area of stationary object
If multiple LED lightbars are connected in parallel for large LCD devices, then the illumination area is increased, but the complexity of the driving circuit increases
Solution Approach 1:
The same varistor protection circuit is universally applied to multiple LED lightbars connected in parallel. Each lightbar branch is equipped with its own varistor, allowing the system to handle multiple parallel connections while maintaining consistent protection and control functionality across all branches.
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 configuration effectively reduces voltage stress on the dimming controllable switch during switching off, ensuring the switch's protection and enhancing the overall reliability of the LED backlight driving circuit.
Implementation Method 1
Two ends of the dimming controllable switch are connected with a varistor in parallel. When the dimming controllable switch is switched off and the voltage is overhigh, the clamping circuit can forcibly reduce the voltage of the dimming controllable switch.
Implementation Method 2
The clamping circuit comprises a storage capacitor and a discharge resistor which are connected in parallel. When the voltage of the output end of the LED lightbar (i.e. the voltage of the dimming controllable switch) exceeds the reference voltage, the storage capacitor absorbs the electric energy of a branch circuit of the LED lightbars and reduces the voltage of the dimming controllable switch.
Implementation Method 3
The storage capacitor and the discharge resistor are coupled to the output end of the LED lightbar through a diode. An anode of the diode is connected to the output end of the LED lightbar, and a cathode of the diode is connected with the storage capacitor and the discharge resistor. By adopting the diode, reverse current is avoided, a branch circuit of the LED lightbars is protected against disturbance of other control branch circuits, and system stability is enhanced.
Data Source
AI summary
A light emitting diode (LED) backlight driving circuit includes LED lightbar(s) and a control module; the control module includes a dimming controllable switch. The LED lightbars are connected with the dimming controllable switch in series. Two ends of the dimming controllable switch are connected with a varistor in parallel. Because two ends of the dimming controllable switch are connected with a varistor in parallel, when the dimming controllable switch is switched off, a current of the branch circuit is suddenly reduced, the voltage withstood by the LED lightbars is greatly reduced, and the voltage on two ends of the dimming controllable switch is increased. Then, under action of high voltage, resistance of the varistors connected in parallel is reduced, and the current of a new branch circuit formed by the LED lightbars and the varistor is also increased. Thus, the voltage withstood by the LED lightbars is increased again and the voltage on two ends of the dimming controllable switch is reduced.