Class D Amplifier Clipping Control for Stable PWM Duty Cycles
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Solution Overview
Problem
Class D amplifiers face issues with clipping and slow recovery due to integrator drift and lack of sign change in the output signal, especially at high or low duty cycles, which can lead to distorted demodulated outputs and improper functioning of bootstrap capacitors.
Innovation Solution
A clipping control unit is introduced to control the input signal, ensuring the amplifier operates within its normal range by limiting the duty cycle and maintaining regular sign changes in the output signal, eliminating the need for an oscillator when receiving digital input signals like PDM or PWM.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the feedback loop gain is increased to compensate for amplitude loss when supply voltage decreases, then the modulation depth increases to maintain output amplitude, but the output signal clips to the supply voltage and stops changing sign, causing integrator drift and distortion
Solution Approach 1:
The clipping control unit preemptively limits the modulation depth by controlling the input signal before clipping can occur. It detects when the output signal approaches the supply voltage and proactively reduces the input signal amplitude, preventing the integrators from drifting and ensuring the output continues to change sign without distortion
Solution Approach 2:
The system uses feedback from the output signal to the clipping control unit, which monitors the output amplitude and adjusts the input signal accordingly. This closed-loop control ensures that the output remains within valid ranges while maintaining proper sign changes, preventing integrator drift and signal distortion
2Reliability
If a pulse width modulation limiter is added to limit modulation depth and prevent clipping, then signal integrity is improved, but the device complexity increases and oscillators are required
Solution Approach 1:
The invention extracts and removes the oscillator component from the traditional PWM limiter architecture. By implementing clipping control through direct manipulation of the input signal based on output feedback, the system achieves modulation depth limiting without requiring separate oscillators or complex PWM limiter circuits
Solution Approach 2:
The clipping control unit performs multiple functions: it limits modulation depth, prevents integrator drift, ensures proper sign changes, and eliminates the need for oscillators. This multi-functional approach reduces overall device complexity while maintaining signal integrity
3Reliability
If the output signal duty cycle becomes extreme (very high or very low), then the amplifier may operate outside normal range causing integrator drift, but adding complex control circuits increases device complexity
Solution Approach 1:
The clipping control unit takes preliminary action by limiting the input signal amplitude before it can cause extreme duty cycle conditions. By preemptively controlling the input signal based on feedback, the system prevents integrator drift and ensures the output remains within the normal operating range without requiring complex real-time correction circuits
Data Source
AI summary
A class D amplifier (1) comprises an input unit (11) for receiving a digital input signal (Vin), a pulse shaping unit (12) for producing pulse shaped signals in dependence of the input signal (Vin), a comparator unit (13) for comparing the pulse shaped signals and producing a comparator signal, a driver unit (14) for producing driver signals in dependence of the comparator signal, a switching output unit (15) for producing a pulse width modulated output signal (Vout) in dependence of the driver signals, and a feedback unit (16) for feeding the output signal (Vout) back to the pulse shaping unit (12). The input unit (11) comprises a clipping control unit (10) for controlling the duty cycle of the pulse width modulated output signal (Vout).