PWM LED Driver Circuit With Non-Zero OFF-State Voltage
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Solution Overview
Problem
Constant-voltage systems for LED lighting face issues with reverse voltage phenomena due to capacitive currents from AC mains and PWM modulation, leading to potential electrical overstress and deterioration of low-power LEDs, which existing solutions like shunt diodes or integrated ESD diodes increase production costs.
Innovation Solution
Reducing the amplitude of the PWM signal swing and using a dual voltage generator setup to maintain a non-zero voltage during the OFF time, allowing the PWM signal to swing between a high and a low value, with the low value set below the LED's turn-off threshold, thereby reducing reverse voltage and preventing negative voltage on LEDs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If shunt diodes or integrated ESD diodes are used to protect LEDs from reverse voltage, then LED reliability is improved, but manufacturing cost increases
Solution Approach 1:
The harmful capacitive current path is extracted and redirected through a dedicated protection circuit connected to a second voltage generator, separating the protection function from individual LED components. This eliminates the need for shunt diodes on each LED while providing systematic protection against reverse voltage.
Solution Approach 2:
A second voltage generator acts as an intermediary component that provides a reference voltage during PWM OFF time, preventing negative voltage swings that would otherwise directly affect the LEDs. This mediator component protects LEDs without requiring expensive per-LED protection devices.
2Reliability
If PWM signal amplitude is reduced to prevent reverse voltage, then LED reliability is improved, but dimming range is reduced
Solution Approach 1:
The system changes the voltage reference parameter by introducing a second voltage generator that sets the OFF-time voltage level. This allows the PWM signal to swing between high and non-zero low values, maintaining positive voltage throughout the cycle while preserving full dimming control through duty cycle adjustment.
Solution Approach 2:
The protection mechanism dynamically adapts to different dimming levels by maintaining the voltage swing within positive bounds. The second voltage generator dynamically adjusts the reference level based on PWM timing, allowing full adaptability across the dimming range without compromising LED protection.
3Reliability
If a dual voltage generator setup is used to maintain non-zero voltage during OFF time, then reverse voltage is prevented, but device complexity increases
Solution Approach 1:
The second voltage generator serves multiple functions simultaneously: it provides a reference voltage during PWM OFF time, establishes the low-level voltage swing, prevents negative voltage on LEDs, and sets the baseline for capacitive current redirection. This multi-functionality justifies the additional component.
Solution Approach 2:
The second voltage generator creates an equipotential reference that prevents voltage from dropping below a safe threshold during PWM OFF time. By maintaining the low-level voltage at a non-zero value, it ensures that the voltage across LEDs remains positive throughout the entire PWM cycle, simplifying the protection mechanism.
Data Source
Figure 1~2b
Figure 3~4
Figure 5~6
AI summary
A method for driving a unidirectional current through an electrical load, such as one or more LED modules by applying thereto a PWM-modulated signal (Vpwm) that swings between a high value (Von) and a low value (Voff) envisages choosing said high value (Von) and said low value (Voff) as nonzero values having a same sign.