Capacitive Charge Measurement for LED Current Control
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Solution Overview
Problem
Existing constant current control methods for light emitting diodes (LEDs) using DC-DC converters face challenges in accurately monitoring current variations among LEDs, leading to excessive design margins and losses from sense resistors.
Innovation Solution
An electronic device with a light emitting element, a capacitor, a measurement circuit to measure charge changes, and a control circuit that adjusts light emission based on the charge difference and a reference value, eliminating the need for sense resistors and allowing for precise current control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If constant current control is executed with a DC-DC converter without direct current monitoring, then the control circuit is simpler, but the current flowing through the light emitting diode varies when variations occur between light emitting diodes
Solution Approach 1:
The patent introduces a capacitor as an intermediary element between the DC-DC converter and the LED string. The capacitor accumulates charge during a charging period, and the change in charge amount serves as a proxy measurement for current flow through the LED. This intermediary approach enables indirect current monitoring without requiring direct sensing components in the LED circuit path.
Solution Approach 2:
The patent replaces the traditional electrical sensing method (using sense resistors to measure voltage drops) with a capacitive charge measurement method. Instead of using Ohm's law (V=IR) to infer current through voltage measurement across a resistor, the system uses the relationship Q=CV (charge=capacitance×voltage) to measure current indirectly through charge accumulation on a capacitor over time.
2Measurement precision
If a sense resistor is used to monitor current flowing through light emitting diode, then current control precision is improved, but power loss increases due to the sense resistor
Solution Approach 1:
The capacitor serves as a non-dissipative intermediary for current measurement. Unlike a sense resistor that continuously dissipates power (P=I²R) whenever current flows, the capacitor only temporarily stores charge during measurement intervals and then discharges it back into the circuit, creating no net energy loss from the measurement process itself.
Solution Approach 2:
The system creates a parallel measurement path using the capacitor that copies the current flow information without the original current path being disrupted or degraded by a sense resistor. The charge accumulated on the capacitor during the charging period provides a copy of the current flow data that can be used for control without introducing resistance into the main LED current path.
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 enables accurate control of light emission, reduces power consumption, and eliminates the need for excessive design margins, while avoiding losses associated with sense resistors, by measuring charge changes in a capacitor to simulate current measurement.
Implementation Method 1
a capacitor coupled to the light emitting element, a measurement circuit that measures a change amount in charge of the capacitor
Data Source
AI summary
An electronic device including a light emitting element. A capacitor is coupled to the light emitting element. A measurement circuit measures a change amount in charge of the capacitor. A control circuit controls a light emission amount of the light emitting element in accordance with a difference of the change amount in the charge and a reference value.