DCM LED Dimming Circuit Eliminates Op-Amp Deviation
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Solution Overview
Problem
Existing dimming circuits using PWM dimming signals suffer from poor dimming accuracy and unstable current supply to LED loads when the duty cycle of the PWM dimming signal is relatively small.
Innovation Solution
A dimming method and circuit that operate in the DCM working mode, where a power switch transistor is controlled based on the inductance current and PWM dimming signal, avoiding the use of operational amplifiers and processing small signals to achieve stable current supply to LED loads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If PWM dimming signal with small duty cycle is used, then LED brightness is reduced, but dimming accuracy deteriorates due to operational amplifier deviation and low-frequency PFM mode noise
Solution Approach 1:
The patent changes the working mode parameter from PFM (pulse frequency modulation) to DCM (discontinuous conduction mode) to eliminate the noise and instability issues that occur at low duty cycles. This parameter change allows the circuit to maintain stable operation and high dimming accuracy even when the LED brightness is reduced to low levels.
Solution Approach 2:
The patent replaces the operational amplifier-based error amplification mechanism with a direct DCM control mechanism. By eliminating the operational amplifier from the control loop and using current-mode control with DCM operation, the patent removes the source of deviation and noise that degrades dimming accuracy at small duty cycles.
2Adaptability or versatility
If operational amplifier is used for error amplification, then circuit can operate in PFM mode, but switching frequency becomes unstable and current to LED load becomes unstable
Solution Approach 1:
The patent substitutes the operational amplifier-based control system with a DCM-based current mode control system. This replacement eliminates the instability and noise problems associated with PFM mode while maintaining the ability to provide stable current to the LED load across all operating conditions.
Solution Approach 2:
The patent extracts and removes the operational amplifier from the control circuit, eliminating the source of instability. By taking out the operational amplifier and its associated PFM control mechanism, the patent achieves stable switching frequency and stable LED current without sacrificing working mode flexibility.
3Use of energy by moving object
If duty cycle is reduced to dim LED, then power consumption is reduced, but dimming error increases and dimming accuracy deteriorates
Solution Approach 1:
The patent changes the control parameter from duty-cycle-based PWM to current-based DCM control. This parameter change enables the circuit to maintain high dimming accuracy across the entire brightness range, including low power consumption states, by directly controlling the inductor current rather than relying on small duty cycle signals that are susceptible to operational amplifier deviation.
Data Source
AI summary
A dimming method and a dimming circuit for driving LED load are provided. The dimming circuit includes a power stage circuit, and the power stage circuit includes a power switch transistor. After the power stage circuit enters the DCM working mode, it obtains a first integral value according to the inductance current in a switch period of the power switch transistor, and obtains second time according to the first integral value and a duty cycle of a PWM dimming signal; when the switch period reaches the second time, the power switch transistor is controlled to be turned on to start the next switch period; a first upper limit voltage is set to a fixed voltage; the power switch transistor is controlled to be turned off when a sampling signal of the inductance current representing the inductance current of the power stage circuit reaches the first upper limit voltage.