Class-D Amplifier Logic Circuit for Lower EMI Switching
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Solution Overview
Problem
Conventional class D amplifiers generate increased Electromagnetic Interference (EMI) due to rapid switching, which is undesirable in portable electronic devices like smartphones and tablets, despite their efficiency advantages.
Innovation Solution
A class D amplifier design that includes first and second comparators converting analog audio signals into square waves, a logic block processing these signals to generate processed audio signals based on duty cycles, and output stages using PMOS and NMOS transistors to reduce switching by 50%, thereby minimizing EMI.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If class D amplifier uses rapid switching of amplifying devices to achieve high efficiency, then power consumption is reduced, but Electromagnetic Interference (EMI) is increased
Solution Approach 1:
The patent applies periodic action by using pulse-width modulation (PWM) where the amplifying devices switch periodically between conductive and non-conductive states based on a carrier wave. The switching frequency is maintained at a fixed periodic rate, and the audio signal is encoded in the duty cycle of these periodic pulses, achieving efficient power consumption while the periodic nature allows for controlled EMI characteristics through filtering.
Solution Approach 2:
The patent changes the parameter of switching frequency by operating the amplifying devices at a high fixed frequency (e.g., 400kHz) that is above the audio bandwidth. This parameter change allows the amplifier to maintain high efficiency through continuous switching while pushing the EMI to higher frequencies where it can be more easily filtered out, resolving the contradiction between efficiency and EMI generation.
2Productivity
If conventional class D amplifier switches transistors rapidly to amplify audio signals, then amplification efficiency is improved, but the number of switchings per period increases EMI
Solution Approach 1:
The patent uses periodic PWM switching where transistors are switched at a fixed high frequency with the audio information encoded in pulse width. This periodic action maintains high amplification efficiency through continuous switching operation while the regular periodic pattern allows for predictable EMI spectral characteristics that can be managed through filtering and shielding.
Solution Approach 2:
The patent introduces an intermediary low-pass filter between the switching amplifier output and the audio output. This intermediary component allows the amplifier to operate efficiently with rapid transistor switching while the filter mediates by blocking the high-frequency switching EMI from reaching the audio output, thus resolving the contradiction between amplification efficiency and EMI reduction.
3Loss of energy
If class D amplifier uses switching devices instead of linear gain devices, then power dissipation as heat is reduced, but EMI generation is increased
Solution Approach 1:
The patent employs periodic PWM switching action where the amplifying devices operate in discrete on/off states rather than linear regions. This periodic switching minimizes the time devices spend in high-power-dissipation transition states, reducing heat loss while the high-frequency periodic nature pushes EMI to frequencies that can be filtered, thus addressing both energy efficiency and EMI concerns.
Solution Approach 2:
The patent changes the operating parameter of the amplifying devices from linear analog operation to high-frequency digital-like switching operation. This parameter change from continuous analog control to discrete switching at frequencies above audio bandwidth reduces power dissipation by eliminating the resistive losses inherent in linear operation, while the high-frequency switching EMI is separated from the audio band through filtering.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design significantly reduces EMI by halving the number of transistor switches per period, improving the efficiency and fidelity of audio amplification in portable devices.
Implementation Method 1
first and second comparators configured to respectively convert first and second analog audio input signals into first and second audio square waves
Implementation Method 2
A logic block is coupled to receive the first and second audio square waves from the first and second comparators. The logic block is configured to generate a first processed audio signal representing a difference between the first audio square wave and the second audio square wave
Implementation Method 3
First and second output stages are coupled to the logic block and configured to generate first and second audio output signals respectively based upon the first and second processed audio signals
Data Source
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AI summary
A class-D amplifier (100) includes a signal processing block (140). The signal processing block generates a first processed signal (Vp) representing a difference between a first differential signal and a second differential signal, when a duty cycle of the first differential signal is greater than that of the second differential signal. The signal processing block generates the first processed signal representing a reference DC level, when the duty cycle of the first differential signal is less than that of the second differential signal. A second processed signal (Vn) representing a difference between the second differential signal and the first differential signal is generated when the duty cycle of the second differential signal is greater than that of the first differential signal, and the second processed signal representing the reference DC level is generated when the duty cycle of the second differential signal is less than that of the first differential signal.