LED Display Flicker Elimination via PWM-Synchronized Spread Spectrum Modulation
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Solution Overview
Problem
LED-based displays powered by switching regulators often experience visible flicker due to harmonics introduced by direct-sequence spread spectrum modulation, even when dimming frequencies exceed 200 Hz, which is not effectively addressed by existing technologies without degrading electromagnetic interference (EMI) performance.
Innovation Solution
A system comprising a driver circuit, a modulator circuit, and a reset circuit that uses direct sequence spread spectrum modulation to eliminate visible flicker by repeating a frequency hopping sequence at each pulse of the brightness control signal, thereby eliminating harmonics at frequencies less than the dimming frequency, while maintaining EMI performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If direct-sequence spread spectrum modulation is used to reduce EMI, then EMI performance is improved, but visible flicker is introduced due to harmonics
Solution Approach 1:
The patent applies periodic action by resetting the spread spectrum modulator at each pulse of the PWM dimming signal. This causes the frequency hopping sequence to repeat periodically at the dimming frequency, which eliminates harmonics at frequencies less than the dimming frequency while maintaining EMI reduction benefits. The periodic resetting creates a synchronized relationship between the spread spectrum modulation and the PWM dimming signal.
2Object-affected harmful factors
If PWM dimming frequency is increased above 200 Hz to reduce flicker, then visible flicker is reduced, but EMI performance is degraded
Solution Approach 1:
The patent uses periodic resetting of the modulator at each PWM pulse to create harmonics only at frequencies above the dimming frequency. This allows the system to operate at lower PWM frequencies (reducing EMI) while still eliminating visible flicker, because the periodic synchronization prevents the generation of harmful low-frequency harmonics that would otherwise be present.
Solution Approach 2:
The patent changes the temporal parameters of the spread spectrum modulation by periodically resetting the modulator state at each PWM pulse. This parameter change transforms the continuous frequency hopping sequence into a periodic sequence that repeats at the dimming frequency, fundamentally altering the harmonic spectrum to eliminate low-frequency components while maintaining high-frequency EMI reduction.
3Object-generated harmful factors
If frequency hopping sequence is varied continuously to maintain EMI reduction, then EMI performance is improved, but harmonics at low frequencies are generated causing flicker
Solution Approach 1:
The patent resolves this contradiction by introducing periodic resetting of the frequency hopping sequence at each PWM pulse. This creates a periodic structure in the modulation that eliminates low-frequency harmonics while preserving the spread spectrum EMI reduction. The sequence varies within each PWM period to maintain EMI reduction, but repeats identically at each pulse to eliminate low-frequency flicker.
Data Source
AI summary
A system includes a driver circuit, a modulator circuit, and a reset circuit. The driver circuit drives a plurality of light emitting diodes via a switch. The switch is controlled by a first signal having a first frequency. The driver circuit controls brightness of the light emitting diodes based on a second signal including a plurality of pulses. The modulator circuit modulates the first signal using a direct sequence spread spectrum modulation. The direct sequence spread spectrum modulation uses a sequence generated based on the first signal. The reset circuit resets the modulator circuit at each of the plurality of pulses of the second signal. The modulator circuit repeats the sequence at each of the plurality of pulses of the second signal.