PLL Spread-Spectrum Modulation for Peak EMI Reduction
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Solution Overview
Problem
Phase-locked loops (PLLs) in DC-to-DC power converters generate electromagnetic interference (EMI) that can exceed regulatory limits, and existing techniques like spread spectrum clocking (SSC) struggle when the modulation frequency is outside the PLL's bandwidth, limiting the effectiveness of EMI mitigation.
Innovation Solution
A PLL configuration that includes a phase frequency detector, control circuit, modulation circuit, and oscillator, where the modulation frequency is de-tuned to modulate the control signal of the oscillator, causing the output frequency to vary around the target frequency without affecting the average frequency, thus distributing EMI across a range of frequencies and reducing peak EMI generation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If spread spectrum clocking is used to reduce EMI, then EMI compliance is improved, but the technique fails when modulation frequency is outside the PLL bandwidth
Solution Approach 1:
The patent implements dynamic frequency modulation by varying the oscillator frequency around a target frequency according to a spread spectrum modulation signal. The modulation frequency is deliberately set outside the PLL bandwidth to create frequency variations that distribute EMI energy across a wider spectrum, thereby reducing peak EMI while maintaining average frequency stability.
Solution Approach 2:
The patent changes the frequency parameter of the oscillator output by modulating it with a spread spectrum signal. The modulation causes the instantaneous frequency to deviate from the target frequency, creating a frequency distribution that reduces EMI concentration at any single frequency while maintaining compliance through average frequency control.
2Object-affected harmful factors
If the output frequency is modulated to distribute EMI, then peak EMI is reduced, but the frequency stability is affected
Solution Approach 1:
The patent applies periodic frequency modulation around the target frequency using a spread spectrum modulation signal. The modulation is designed to be symmetric around the target frequency, ensuring that the average frequency remains stable while the instantaneous frequency varies periodically to distribute EMI energy. The PLL's inherent filtering maintains long-term frequency stability despite short-term variations.
Data Source
AI summary
In some examples, a circuit includes a phase frequency detector (PFD) having a first input, a second input, and an output. The circuit also includes a control circuit having an input and an output, the control circuit input coupled to the output of the PFD. The circuit also includes a modulation circuit having an input and an output, the modulation circuit input coupled to the output of the control circuit. The circuit also includes an oscillator having an oscillator input and an oscillator output, the oscillator input coupled to the output of the modulation circuit and the output of the oscillator coupled to the second input of the PFD.