EL Driver Circuit Using PWM Boost Converter for Noise Reduction
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Solution Overview
Problem
Electroluminescent (EL) lamps used for keypad backlighting in mobile phones generate audible noise and electrical interference, which is undesirable in thin, flexible phone designs, and existing solutions fail to effectively minimize these issues.
Innovation Solution
A circuit comprising a full H-bridge and a pulse width modulated boost converter generates a high voltage AC waveform with slow rising and falling edges to drive the EL lamp, reducing noise and interference by controlling the voltage and frequency of the waveform applied to the EL lamp.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If EL lamps are used for keypad backlighting to achieve thin and flexible design, then the phone thickness is reduced and light uniformity is improved, but audible noise and electrical interference are generated
Solution Approach 1:
The patent applies periodic action by using pulse width modulation (PWM) to drive the EL lamp at a specific frequency (e.g., 100 Hz). This periodic driving approach allows the lamp to operate efficiently while the frequency is chosen to be below the audible range, thereby reducing noise. The PWM technique also enables precise control of the voltage applied to the EL lamp, minimizing electrical interference with the keypad sensing circuitry.
Solution Approach 2:
The patent changes the electrical parameters of the EL lamp drive signal by using a full H-bridge circuit with controlled PWM output. The voltage and frequency parameters are adjusted to optimize both the lighting performance and noise reduction. By controlling the duty cycle and voltage levels, the system achieves thin design benefits while minimizing harmful emissions.
2Speed
If fast rising and falling edges are used to drive the EL lamp, then the response speed is improved, but audible noise and electrical coupling to keypad circuitry increase
Solution Approach 1:
The patent uses periodic PWM driving at a controlled frequency to replace abrupt fast edges. The periodic nature of the signal allows for smoother transitions and reduces the high-frequency components that cause noise and electrical coupling. The frequency is selected to be low enough to avoid audible noise while maintaining adequate response for keypad backlighting.
Solution Approach 2:
The patent incorporates feedback mechanisms through the H-bridge circuit and PWM control system. The feedback loop monitors the drive signal and adjusts the voltage and duty cycle to maintain optimal operation. This feedback control ensures that the rising and falling edges are not too abrupt, thereby reducing noise and electrical interference while maintaining sufficient response speed for the application.
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 minimizes audible noise and electrical interference to the keypad sensing circuitry, allowing for the use of EL lamps in thin mobile phone designs without increasing thickness, while maintaining uniform light for the keypad.
Implementation Method 1
A boost converter is coupled to the full H-bridge circuit. The boost converter is pulse width modulated to generate a gradual increase in voltage
Implementation Method 2
an electroluminescent (EL) lamp... The EL lamp's light uniformity eliminates the need for a light diffuser
Data Source
AI summary
An EL driver circuit has a full H-bridge circuit. A boost converter is coupled to the full H-bridge circuit. The boost converter is pulse width modulated to generate a gradual increase in voltage to be applied to the full H-bridge circuit to drive an EL lamp.