Variable-Slope PWM Carrier for Class-D Amplifier Linearity
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Solution Overview
Problem
Voltage-feedback class-D amplifier circuits face challenges in linearity, frequency characteristic, and noise performance, particularly when driving loads like actuators or motors, due to dead time issues that lead to precision errors and noise components, especially at small input signal amplitudes.
Innovation Solution
The solution involves adjusting the slew rate of the triangular-wave signal used in pulse width modulation to improve the frequency characteristic by reducing the gradient near the central amplitude, allowing for precise load driving without noise, and enhancing the closed-loop frequency response.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a dead time is provided to avoid induced current and prevent short-circuiting, then reliability is improved, but manufacturing precision deteriorates due to PWM dead zone appearing at small input signal amplitudes
Solution Approach 1:
The triangular wave gradient (slew rate) is made variable rather than constant. The gradient changing section dynamically adjusts the slope of the triangular wave based on its amplitude level, using a smaller gradient near the central value and larger gradients at higher amplitude levels. This dynamic adjustment allows the system to maintain reliable dead time protection while eliminating the PWM dead zone effect that plagues fixed-gradient implementations.
Solution Approach 2:
The patent changes the key parameter of the triangular wave from a fixed gradient to a variable gradient that depends on the amplitude level. By modifying the gradient parameter adaptively - using a first (smaller) gradient for low-amplitude triangular waves and a second (larger) gradient for high-amplitude triangular waves - the system resolves the contradiction between maintaining dead time protection and achieving precise PWM signal generation across all input signal amplitudes.
2Device complexity
If a fixed gradient triangular wave is used for PWM comparison, then device complexity is reduced, but frequency characteristic deteriorates due to PWM dead zone at small input amplitudes
Solution Approach 1:
The triangular wave generation circuit incorporates a gradient changing section that dynamically modifies the wave slope based on amplitude levels. This adds moderate complexity to the circuit but dramatically improves frequency response by eliminating the PWM dead zone, allowing the system to accurately track high-frequency input signals across all amplitude ranges.
Solution Approach 2:
Different portions of the triangular wave cycle are given different gradient qualities. The portion near the central value uses a smaller gradient to prevent dead zone effects, while portions at higher amplitude levels use larger gradients. This local differentiation of gradient quality improves overall frequency characteristic without requiring complete redesign of the entire wave generation system.
3Manufacturing precision
If the gradient of triangular wave is reduced near central value, then linearity is improved by eliminating dead zone, but device complexity increases due to gradient changing section
Solution Approach 1:
The patent implements a gradient changing section that modifies the triangular wave parameter (gradient/slew rate) based on amplitude levels. This adds circuit complexity but achieves superior linearity by eliminating the PWM dead zone, ensuring that small input signal amplitudes are accurately represented in the PWM output without the nonlinear distortion that plagues fixed-gradient systems.
Data Source
AI summary
The frequency characteristic of a voltage-feedback class-D amplifier circuit for driving an output load is improved. A triangular-wave correction circuit which compensates a gradient of a triangular wave is provided to a triangular-wave signal generator which supplies a triangular wave signal used as a PWM carrier to a comparison circuit for performing PWM modulation of an input signal. In an area where a duty of a command value for an output circuit drive becomes about 50%, a slew rate (gradient) of the triangular wave is decreased.