LED Drive Circuit Variable Off-Time Control
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Solution Overview
Problem
Existing drive circuits for loads, particularly LEDs, suffer from significant ripple and audible whistling due to fixed peak inductor current and off-time, which are undesirable in applications like LED torches, and have limited supply voltage range and temperature regulation capabilities.
Innovation Solution
A circuit with a controllable switching device and sensing resistors that adjusts peak inductor current and off-time based on demand and temperature signals, allowing continuous operation across a wide voltage range and preventing thermal damage, using voltage-to-current converters and integrating means to monitor and control the current flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the circuit operates in skip (start-stop) mode with fixed off-time, then regulation is maintained, but audible whistling occurs due to fixed frequency switching
Solution Approach 1:
The patent applies dynamics by making the off-time variable rather than fixed. The control circuit adjusts the off-time duration based on real-time feedback from the output voltage, allowing the switching frequency to adapt dynamically. This prevents the circuit from operating at a fixed audible frequency while maintaining regulation, thereby eliminating the whistling sound.
Solution Approach 2:
The patent changes the parameter of off-time from a fixed value to a variable parameter that can be adjusted by the control circuit. By modifying the off-time duration based on output voltage conditions, the switching frequency becomes variable, which prevents sustained operation at audible frequencies and eliminates the whistling effect while preserving voltage regulation.
2Power
If the circuit operates in skip mode to control excessive power, then power regulation is achieved, but large ripple and electromagnetic radiation are produced
Solution Approach 1:
The patent implements feedback by continuously monitoring the output voltage and using this information to adjust the switching duty cycle and off-time. This closed-loop control allows the circuit to regulate power output smoothly without resorting to extreme on-off cycling, thereby reducing voltage ripple and electromagnetic radiation while maintaining effective power control.
Solution Approach 2:
The patent uses periodic switching action with variable duty cycle and off-time to control power output. Instead of long stop periods followed by rapid cycling, the circuit employs continuous periodic switching with adjusted parameters, which smooths the power delivery and reduces ripple and electromagnetic interference while maintaining power regulation.
3Device complexity
If the circuit uses fixed threshold voltages for switching control, then simple control logic is maintained, but adaptation to varying supply voltage and temperature is limited
Solution Approach 1:
The patent changes the control parameters from fixed threshold voltages to variable reference values that can adapt to supply voltage and temperature conditions. The control circuit incorporates temperature compensation and supply voltage scaling, allowing the switching thresholds to change dynamically with environmental conditions while maintaining regulation accuracy across a wide operating range.
Solution Approach 2:
The patent applies dynamics by making the control thresholds adaptive rather than static. The control circuit continuously adjusts its reference voltages and switching parameters based on real-time measurements of supply voltage and temperature, enabling the system to adapt to varying conditions while maintaining simple overall control architecture through automated adjustment.
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 reduces audible whistling by controlling peak coil current and off-time, enabling continuous operation across the entire power range without audible noise, and provides efficient temperature management to prevent thermal damage, allowing operation from 1.2V to 18V and adjustable LED brightness.
Implementation Method 1
Current then ramps up in the inductor L1, the switching transistor Q1... While the switching transistor is off, the inductor discharges to the load causing the output voltage VOUT to rise
Implementation Method 2
a voltage across the switching device current sensing resistor is indicative of a current flowing through the switching device
Data Source
AI summary
A circuit comprises an inductor, a load, and a controllable switching device arranged in either a first configuration: switchable between the load being bypassed and the inductor energised, and current flowing through the load, and the inductor discharging energy into the load; or, a second configuration: switchable between current being permitted to flow through the load, inductor and controllable switching device in series, energising the inductor, and the current flowing through the load, and the inductor discharging energy into the load. The circuit further comprises control means for controlling the switching device, a load current sensing resistor connected in series with the load, and a demand signal input. Monitoring means is arranged to monitor the demand signal and the voltages across the sense resistors, and generate a monitor signal. The control means is arranged to receive the monitor signal and to switch the switching device in response the monitor signal exceeding a predetermined threshold.