Load Adaptive Frequency Jittering for Switching Power Supply EMI
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Solution Overview
Problem
Switching mode power supplies generate high-frequency noise that causes Electromagnetic Interference (EMI), which is not adequately mitigated by existing frequency jittering schemes, especially at light load conditions, leading to unacceptable noise floors in sensitive applications like high-fidelity audio systems.
Innovation Solution
A frequency jittering device comprising a Variable State Machine, Time delay generator, Digital Control Pulse Density Generator, PWM Control Current Source, and Current Control Oscillator, which generates a variable frequency clock signal with adaptive pulse density and delay, allowing for low-cost EMI filtering while maintaining a low noise floor at light loads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If frequency jittering is applied to reduce EMI, then peak EMI is reduced, but average noise floor increases
Solution Approach 1:
The patent implements dynamic frequency jittering where the frequency variation range is adaptively adjusted based on load conditions. At light loads, the frequency jitter range is minimized to maintain low noise floor, while at heavy loads, the frequency jitter range is maximized to achieve effective EMI reduction. This dynamic adaptation resolves the contradiction by making the frequency modulation behavior conditional rather than fixed.
Solution Approach 2:
The patent changes the parameter of frequency variation amplitude based on operating conditions. By monitoring load current and adjusting the frequency jitter magnitude accordingly, the system maintains optimal performance across different operating points - using minimal frequency variation at light loads to preserve low noise floor, and aggressive frequency variation at heavy loads to suppress peak EMI.
2Object-affected harmful factors
If aggressive frequency jittering is used to reduce peak EMI, then EMI filtering requirements are reduced, but noise floor becomes unacceptable for sensitive applications
Solution Approach 1:
The system dynamically adjusts the frequency jittering aggressiveness based on real-time load monitoring. During light load conditions when sensitivity to noise is highest, the frequency jitter is minimized or suspended entirely. During heavy load conditions when EMI is most problematic, the frequency jittering is applied more aggressively. This temporal and conditional adaptation allows the system to satisfy both requirements at different operating points.
3Device complexity
If simple EMI filter is used with frequency jittering, then device complexity is reduced, but EMI reduction effectiveness is insufficient at light loads
Solution Approach 1:
The frequency jittering circuit serves as a self-adjusting EMI mitigation mechanism that automatically adapts its behavior to load conditions without requiring complex external filtering. The circuit monitors its own operating state and adjusts frequency variation accordingly, providing effective EMI reduction across the full range of operating conditions using only the inherent frequency modulation capability of the power supply controller.
Data Source
AI summary
The present invention relates to a frequency jittering device and method, and a switching power supply employing such frequency jittering device. Said method comprises: S1 generating a variable logic number; S2 generating a delay signal; S3 generating a PWM control signal according to the variable logic number and the delay signal; S4 generating an output signal according to the PWM control signal; and S5 generating a clock signal with variable frequency according to the output signal; wherein, the clock signal is fed back to update the variable logic number, and a jittering clock signal modified in each clock cycle is produced. The benefit of the present invention is not only can apply small low cost EMI filter but also can keep the noise floor level low enough at light load condition.