Temporary Negative Rail Circuit for Low-Voltage LED Drive
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Solution Overview
Problem
Electronic circuits face challenges in maintaining sufficient voltage headroom for LEDs when operating from low-voltage power sources, as the difference between the positive system voltage and ground is insufficient to forward bias the LEDs, leading to inadequate light emission or insufficient light output.
Innovation Solution
A novel circuit that generates a temporary, self-generated negative voltage rail using a capacitor, diode clamp, and reset transistor, synchronized with the LED's enable and disable signal, to increase the voltage difference across the LED without requiring additional power supplies or complex ICs, thereby enhancing the operating headroom.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a conventional power supply is used, then the circuit is simple, but the voltage difference across the LED is insufficient to forward bias the LED
Solution Approach 1:
Instead of generating a higher positive voltage to forward bias the LED, the patent inverts the approach by generating a negative voltage rail. This negative rail is created by charging a capacitor to the positive supply voltage and then disconnecting it, allowing the capacitor to discharge through the LED in reverse, creating a negative voltage that adds to the positive supply voltage to achieve the required forward bias voltage across the LED.
Solution Approach 2:
The patent employs periodic switching of the transistor to charge and discharge the capacitor in cycles. During the charging phase, the transistor is on and the capacitor charges to the positive supply voltage. During the discharge phase, the transistor is off and the capacitor discharges through the LED, creating the negative voltage pulse. This periodic action allows the circuit to maintain operation with a simple power supply while achieving the necessary voltage difference.
2Strength
If a switched-mode power supply or charge pump is used to create a higher voltage rail, then the voltage difference is sufficient, but the cost and complexity increase
Solution Approach 1:
The patent extracts only the essential function needed to generate the negative voltage rail, rather than implementing a complete switched-mode power supply or charge pump circuit. By removing unnecessary components and simplifying the circuit to just a capacitor, diode, and transistor, the solution achieves the voltage multiplication effect at a fraction of the cost and complexity of conventional approaches.
Solution Approach 2:
The patent uses inexpensive, readily available components (capacitor, diode, transistor) to create the voltage multiplication effect. These are simple, short-lived components that can be easily replaced if needed, avoiding the use of expensive, complex integrated circuits while achieving the same functional result.
3Reliability
If a negative voltage rail is generated continuously, then the LED can always be driven, but the power consumption increases
Solution Approach 1:
The negative voltage rail is generated periodically rather than continuously, synchronized with the LED's enable and disable signals. The transistor switches on during the charging phase and off during the discharge phase, creating negative voltage pulses only when needed to drive the LED. This periodic generation significantly reduces average power consumption compared to continuous generation while ensuring the LED receives sufficient voltage during operation.
Solution Approach 2:
The capacitor is charged in advance during the transistor's on-state before the negative voltage is needed. This preliminary charging action stores energy that will be released during the LED's active period, ensuring the negative voltage is available when required without continuous power consumption.
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 effectively increases the voltage difference across the LED, allowing it to emit light even with low-voltage sources, while avoiding the costs, complexity, and noise associated with conventional methods, and operates at low frequencies to minimize power consumption and noise.
Implementation Method 1
The capacitor may be electrically coupled to the LED. The diode clamp may be electrically coupled to the LED and the capacitor. The negative rail generator may be configured to create a voltage difference across the LED that is greater than the supply voltage level
Implementation Method 2
The diode clamp may be electrically coupled to the LED and the capacitor
Implementation Method 3
The reset transistor may be electrically coupled to the capacitor and ground. The negative rail generator may be configured to create a voltage difference across the LED that is greater than the supply voltage level when the reset transistor is turned on
Data Source
AI summary
The present concepts relate to a negative rail generator that temporarily self-generates a negative voltage rail to increase the voltage difference across a light emitting diode (LED) to be greater than the positive source voltage that is available. As such, the voltage difference provides sufficient headroom to exceed the minimum forward voltage required to conduct the LED with constant current. In one example, the negative rail generator may include a capacitor, a diode clamp, and a transistor. The negative rail generator and the LED may be operated in synchronization by a common PWM signal. The negative voltage rail can be generated without adding a switched-mode power supply (SMPS) or a charge pump.