Pulse Edge PWM Spectrum Shaping for Class D EMI Suppression
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Class D amplifiers generate electromagnetic interference (EMI) that interferes with AM frequency bands, particularly due to the common mode carrier frequency and its harmonics overlapping with the AM band, causing interference with nearby AM receivers and violating FCC emission limits, and existing mitigation techniques either fail to suppress EMI effectively or compromise baseband signal integrity.
Innovation Solution
A pulse edge control circuit that selectively swaps or chops PWM signals with their logic-inverted, duty cycle complements at discrete time intervals to produce modulated PWM signals with altered power spectra, suppressing common mode carriers while maintaining differential mode performance and baseband signal integrity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If class D amplifiers use PWM modulation to achieve high power efficiency, then power efficiency is improved, but electromagnetic interference (EMI) in the AM frequency band increases
Solution Approach 1:
The patent applies spectral shaping techniques to convert the harmful EMI emissions into beneficial outcomes by redistributing the PWM carrier energy away from the AM frequency band. Through careful design of the modulation spectrum, the carrier energy is concentrated in frequency regions that do not interfere with AM receivers, thus converting the harmful EMI into a controlled spectral distribution that maintains power efficiency while eliminating interference.
Solution Approach 2:
The patent modifies the spectral parameters of the PWM carrier signal by adjusting the modulation index and carrier frequency relationships. By changing these parameters, the power spectrum is shaped such that carrier harmonics fall outside the AM band, resolving the contradiction between maintaining high power efficiency through PWM and avoiding EMI in the AM frequency range.
2Object-generated harmful factors
If dithering technique is used to suppress EMI, then average EMI suppression is improved, but baseband signal integrity deteriorates and maximum modulation index is limited
Solution Approach 1:
The patent applies spectral shaping selectively to specific frequency regions rather than uniformly across the entire spectrum. By targeting only the carrier and harmonic frequencies for spectral redistribution while leaving the baseband signal region unchanged, the invention achieves EMI suppression without degrading baseband signal integrity or limiting the modulation index.
3Object-generated harmful factors
If PWM carrier frequency is adjusted to avoid AM band, then interference with co-resident AM receivers is reduced, but interference with non co-resident AM receivers cannot be avoided and EMI compliance is not achieved
Solution Approach 1:
The patent employs periodic spectral shaping applied to the PWM carrier signal to systematically redistribute carrier energy away from the AM band. This periodic modulation technique ensures that carrier harmonics are consistently positioned outside the AM frequency range, providing both protection against co-resident and non co-resident AM receivers while meeting FCC emission compliance requirements.
Data Source
AI summary
In a particular embodiment, a circuit device includes a pulse edge control circuit to receive at least one pulse-width modulated (PWM) signal from a PWM source. The pulse edge control circuit is adapted to selectively invert and swap the at least one PWM signal with a logic-inverted duty-cycle complement of the at least one PWM signal at discrete time intervals to produce at least one modulated PWM signal having a changed power spectrum. The pulse edge control circuit provides the at least one modulated PWM signal to at least one output of the pulse edge control circuit.