Lighting Device Delay Circuit Low Luminance Control
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Solution Overview
Problem
Conventional lighting devices struggle to maintain constant current operation at very low PWM signal duties, leading to inadequate luminance control and difficulty in displaying low luminance levels, especially in applications like liquid crystal display backlights.
Innovation Solution
A lighting device incorporating a drive circuit, delay circuit, and DC/DC converter that generates a delayed PWM signal to control the switching element, ensuring a longer on-duty period and maintaining constant current operation even at low PWM signal duties, thereby enabling lower luminance levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If PWM control is used to dim the solid-state light emitting device, then luminance can be controlled, but the lowest gradation value is relatively large and very low luminance cannot be achieved
Solution Approach 1:
The delay circuit performs preliminary action by delaying the PWM signal before it reaches the DC/DC converter. This delay ensures that the switching element has sufficient time to complete its switching cycle and establish stable current flow, even at very low duty cycles. By advancing the PWM signal timing, the system can achieve lower minimum luminance levels while maintaining constant current operation.
Solution Approach 2:
The invention changes the timing parameter of the PWM signal by introducing a delay. This parameter modification allows the DC/DC converter to properly respond to very low duty cycle signals, enabling luminance control down to 0.2% duty cycle while maintaining stable constant current operation. The delay effectively transforms the PWM signal characteristics to be compatible with the converter's switching dynamics.
2Illumination intensity
If the on-duty is reduced to achieve lower luminance, then luminance decreases, but the constant current operation cannot be maintained below a certain on-duty threshold
Solution Approach 1:
The delay circuit performs preliminary action by advancing the PWM signal timing before it reaches the DC/DC converter. This ensures that the switching element receives the control signal early enough to complete its switching cycle and establish stable current flow, even at very low duty cycles. The preliminary timing adjustment prevents the loss of constant current operation that would otherwise occur at low luminance levels.
Solution Approach 2:
The system uses feedback from the drive circuit to monitor the actual current flow status. The feedback signal indicates whether the required voltage for constant current operation is applied to the solid-state light emitting device. This feedback mechanism allows the delay circuit to adjust the PWM timing dynamically, ensuring constant current operation is maintained across the full luminance range including very low levels.
3Illumination intensity
If the PWM signal timing is advanced to enable lower luminance, then very low luminance can be achieved, but the control circuit complexity increases
Solution Approach 1:
The delay circuit serves as an intermediary component between the drive circuit and the DC/DC converter. It receives the PWM signal from the drive circuit, applies the necessary timing delay, and outputs the delayed signal to the converter. This intermediary function isolates the complexity of timing adjustment from both the drive circuit and the converter, maintaining relative simplicity in each component while achieving the overall timing optimization needed for very low luminance operation.
Data Source
AI summary
A lighting device includes a DC/DC converter, a drive circuit, and a delay circuit. The drive circuit generates a feedback signal and a PWM signal. The feedback signal indicates whether or not a voltage required for a constant current to flow in a solid-state light emitting device is applied to the solid-state light emitting device. The PWM signal indicates a current supply period during which a current flows in the solid-state light emitting device. The delay circuit delays at least one of a start timing and an end timing of the current supply period with respect to the PWM signal. The DC/DC converter includes: a switching element, and a control circuit which performs control such that the switching element switches in accordance with the feedback signal generated by the drive circuit, during an on-duty period determined by the PWM signal the timing of which has been delayed by the delay circuit.