Class D Amplifier Current-Mode Control Without Current Sensors
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Solution Overview
Problem
Class D amplifiers using voltage mode control suffer from filter resonance issues that lead to undesirable frequency response peaking and drooping, which can damage speakers and degrade audio accuracy, and existing current mode control solutions require explicit current sensors that introduce noise and complexity.
Innovation Solution
An audio amplifier design implementing current mode control without a separate current sensor, utilizing feedback loops and integrators to provide current mode control, eliminating filter resonance and simplifying the circuit by repurposing existing integrators and reducing operational amplifiers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If voltage mode control is used in class D amplifier, then the amplifier can be implemented with standard control circuits, but filter resonance causes frequency response peaking and drooping that degrades audio accuracy
Solution Approach 1:
The patent replaces voltage mode control with current mode control, substituting the control mechanism to eliminate filter resonance issues. The current mode controller directly controls the output current through the inductor, bypassing the voltage control path that causes resonance peaking and drooping in the frequency response.
2Manufacturing precision
If explicit current sensors are added to achieve current mode control, then filter resonance is eliminated, but noise and circuit complexity increase
Solution Approach 1:
The patent makes the output filter inductor serve dual functions: as the filtering element and as the current sensing element. By measuring the voltage across the inductor and integrating it, the system derives current information without requiring a separate current sensor, thus eliminating filter resonance while avoiding additional noise and complexity from explicit current sensing components.
Solution Approach 2:
The inductor in the output filter is made multi-functional by using its inherent voltage-current relationship for both filtering and current sensing purposes. The same inductor that filters the PWM output also provides the voltage signal that, when integrated, gives the current information needed for current mode control, eliminating the need for dedicated current sensing hardware.
3Manufacturing precision
If explicit current sensors are added to achieve current mode control, then filter resonance is eliminated, but noise is introduced into the system
Solution Approach 1:
The patent makes the output filter inductor serve dual functions: as the filtering element and as the current sensing element. By measuring the voltage across the inductor and integrating it, the system derives current information without requiring a separate current sensor, thus eliminating filter resonance without introducing additional noise from external current sensing components.
4Device complexity
If traditional voltage mode control is used, then the amplifier design is simpler, but speaker damage risk increases due to filter resonance
Solution Approach 1:
The patent implements current mode feedback control where the integrated current signal is fed back to control the PWM modulator. This feedback mechanism actively regulates the output current to prevent filter resonance conditions that could cause speaker damage, while maintaining a relatively simple overall architecture by using the existing integrator circuit.
Data Source
Figure 1A
Figure 1B
Figure 2A
AI summary
An audio amplifier that implements current mode control without the use of an explicit or separate current mode sensor is disclosed. The audio amplifier may include a pair of feedback loops that provide current from a node located before an inductor of an output filter and current from a node located after the inductor of the output filter to an integrator circuit. The integrator circuit may be formed from existing circuitry of the audio amplifier controller. Thus, current mode control can be implemented without a separate current mode sensor.