PWM Carrier Spectrum Shaping for Class D Amplifier EMI Suppression
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Solution Overview
Problem
Class D amplifiers generate electromagnetic interference (EMI) that interferes with AM radio frequencies, causing jamming and violating emission compliance, and existing mitigation techniques either fail to suppress EMI effectively or compromise baseband signal integrity.
Innovation Solution
A circuit device and method that modify the PWM signal using a pulse edge control circuit and modulation sequence controller to phase-shift or chop the signal, selectively altering the output power spectrum and reducing carrier power and harmonics, thereby suppressing EMI without impacting the differential mode characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If class D amplifiers use PWM switching 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 parameter changes by modifying the PWM carrier signal parameters (frequency, phase, pulse width) through selective inversion and framing techniques. This alters the spectral distribution of the carrier signal to move energy away from the AM band while preserving the power efficiency benefits of PWM switching.
Solution Approach 2:
The patent converts the harmful EMI effect by using the same PWM switching mechanism to generate beneficial spectral notches in the AM band. The switching that causes interference is selectively inverted and framed to create frequency notches that suppress interference while maintaining the efficiency advantages of class D operation.
2Object-affected harmful factors
If the PWM carrier frequency is adjusted to avoid AM radio frequencies, then EMI to co-resident AM radios is reduced, but EMI to non-co-resident AM radios cannot be avoided and emission compliance is not achieved
Solution Approach 1:
The patent employs periodic action through selective inversion of PWM frames at regular intervals and application of inversion patterns. This periodic modification creates a spectral shaping effect that generates notches at specific frequencies, providing broad-spectrum EMI suppression across multiple AM frequencies rather than avoiding a single frequency.
Solution Approach 2:
The patent applies dynamics by making the PWM carrier signal characteristics variable and adaptive. The selective inversion and framing techniques dynamically alter the spectral distribution of the carrier signal, creating a flexible EMI mitigation approach that can suppress interference across different frequency bands rather than using a fixed carrier frequency.
3Object-generated harmful factors
If dithering is used to suppress EMI for emission compliance, then EMI suppression is achieved, but baseband signal integrity is compromised and maximum modulation index is limited
Solution Approach 1:
The patent applies segmentation by dividing the PWM signal into discrete frames and applying selective inversion to specific frames based on predetermined patterns. This segmented approach allows EMI suppression through spectral shaping without continuously dithering the signal, thereby preserving baseband signal integrity and maintaining maximum modulation index capability.
Data Source
AI summary
In a particular embodiment, a circuit device includes an input to receive a pulse-width modulated (PWM) signal and an output to send a modulated PWM signal. The circuit device further includes a pulse edge control circuit coupled between the input and the output. The pulse edge control circuit receives the PWM signal via the input and includes a control input to receive a modulation control signal. The pulse edge control circuit is adapted to modify the PWM signal to provide the modulated PWM signal with suppressed carrier power and associated harmonics to the output based on the modulation control signal. The circuit device further includes a modulation sequence controller adapted to provide the modulation control signal via the control input. The modulation control signal selectively controls a sequence of the modification of the PWM signal to selectively alter an output power spectrum of the modulated PWM signal.