LED Backlight Driving Circuit Suppressing Screen Flicker
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Solution Overview
Problem
The existing PWM dimming control method for LED backlights causes screen flicker due to low PWM signal frequencies, which are near the scanning frequency of liquid crystal panels, and increasing the frequency to reduce flicker requires high-performance power supplies with design constraints such as coil thermal capacitance and efficiency losses.
Innovation Solution
A driving circuit with a power supply, output capacitor, and detection resistor that controls the power supply based on a pulse modulation signal to gradually reduce the driving voltage, preventing sudden current changes and thus suppressing screen flicker, by determining the capacitance and resistance values to achieve a suitable slope for the voltage fall.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the PWM signal frequency is increased to reduce screen flicker, then the flicker is suppressed, but the power supply design becomes more complex and efficiency decreases
Solution Approach 1:
The patent applies preliminary action by pre-charging the output capacitor during the on-period before the off-period begins. This ensures that when the PWM signal transitions to the off-state, the capacitor already holds sufficient charge to maintain LED current, preventing sudden current drops that cause flicker. The capacitor is prepared in advance to handle the upcoming off-period requirement.
Solution Approach 2:
The output capacitor serves as a beforehand cushion by storing energy during the on-period to compensate for the upcoming off-period. This cushioning effect smooths out current variations by providing continuous charge to the LED string even when the power supply is temporarily disconnected, thereby eliminating flicker without requiring high-frequency switching.
2Object-affected harmful factors
If the PWM signal frequency is increased to reduce screen flicker, then the flicker is suppressed, but the switching regulator efficiency is reduced
Solution Approach 1:
The patent employs periodic action by using low-frequency PWM switching (on-period followed by off-period) to control LED luminance. The output capacitor bridges the periodic on/off cycles, maintaining continuous current flow through the LED string. This periodic charging and discharging of the capacitor enables flicker-free operation at low switching frequencies without the energy losses associated with high-frequency switching regulators.
3Object-affected harmful factors
If the PWM signal frequency is increased to reduce screen flicker, then the flicker is suppressed, but constraints due to coil thermal capacitance increase
Solution Approach 1:
The output capacitor is pre-charged during the on-period before the off-period begins, ensuring sufficient energy is stored to maintain LED current during the subsequent off-state. This preliminary charging action eliminates the need for high-frequency switching that would generate excessive heat in the coil, thereby avoiding thermal capacitance constraints while still preventing flicker.
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 effectively suppresses screen flicker without increasing the PWM signal frequency, relaxing design constraints and maintaining efficient operation by gradually changing the luminance of the LED string, thereby preventing flicker and improving the backlight's performance.
Implementation Method 1
a power supply configured to supply a driving voltage from an output terminal thereof to a first end of the LED string, and comprising an output capacitor arranged between an output terminal thereof and a fixed voltage terminal
Implementation Method 2
a detection resistor arranged between a second end of the LED string and a fixed voltage terminal
Data Source
AI summary
An LED driving circuit drives a display backlight LED string including multiple LEDs connected in series. A power supply supplies a driving voltage from an output terminal thereof to a first end of the LED string. A detection resistor is arranged between a second end of the LED string and a fixed voltage terminal. The LED driving circuit receives a pulse modulation signal having a duty ratio that corresponds to the luminance. In the on period, in which the PDIM signal is the first level, the LED driving circuit controls the power supply such that the voltage drop across the detection resistor approaches a predetermined target value. In the off period, in which the PDIM signal is the second level, the LED driving circuit stops the control operation for the power supply.