Light Emitting Control Circuit Back-EMF Suppression
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Solution Overview
Problem
The existing light emitting apparatus experiences a significant back electromotive force that swings negatively immediately after the transition from the off period to the on period in PWM dimming control, requiring high-voltage components and increasing costs and circuit area.
Innovation Solution
A control circuit that resets the oscillator based on the transition of the first pulse signal from the off state to the on state, synchronizing the cyclic signal, and includes a reset circuit to manage the timing of oscillator reset, preventing excessive back electromotive force.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the light emitting apparatus uses PWM dimming control without oscillator reset synchronization, then the dimming control function is achieved, but a significant negative back electromotive force occurs immediately after transition from off period to on period
Solution Approach 1:
The oscillator is reset in advance based on the transition timing of the PWM dimming control signal from off period to on period. This preliminary action synchronizes the switching transistor's on-period start timing with the oscillator's cyclic signal, preventing the generation of excessive negative back electromotive force that would otherwise occur during unsynchronized transitions.
2Reliability
If high-voltage components are used to withstand the back electromotive force, then the circuit can handle the voltage swing, but the component breakdown voltage requirement increases and circuit area increases
Solution Approach 1:
By resetting the oscillator in advance based on PWM signal transitions, the switching transistor is ensured to start its on-period synchronized with the oscillator's cyclic signal. This prevents excessive voltage swings and back electromotive force, allowing the use of lower breakdown voltage components and reducing overall circuit area.
3Speed
If the switching transistor turns on immediately after off period in PWM dimming, then the response speed is fast, but the back electromotive force swings excessively in negative direction
Solution Approach 1:
The oscillator reset mechanism uses feedback from the PWM dimming control signal transition to adjust the switching transistor's on-period start timing. This feedback ensures synchronization between the PWM signal and the oscillator's cyclic signal, maintaining fast response while preventing excessive negative back electromotive force swings.
Data Source
AI summary
A first driver switches on and off a driving current ILED according to a first pulse signal which is pulse-width modulated according to an externally supplied target luminance level of a light emitting element. An oscillator generates a cyclic signal SOSC having a predetermined frequency. A pulse modulator generates a second pulse signal which is switched to an on level that corresponds to the on state of a switching transistor in synchronization with the cyclic signal SOSC, and which is switched to an off level that corresponds to the off state according to a feedback voltage. A second driver switches on and off the switching transistor according to the second pulse signal. The oscillator is configured to be reset according to a first edge which is transition of the first pulse signal from a first level that corresponds to the off state to a second level that corresponds to the on state.