Adaptive PFM Feedback Circuit for Audio-Band EMI Control

Solution Overview

Problem

Conventional pulse-frequency modulation (PFM) converters introduce electromagnetic interference (EMI) noise that can interfere with audio bands due to varying switching frequencies, especially under ultra-light load conditions, where reducing the peak inductor current value is not feasible.

Innovation Solution

A feedback control circuit for PFM converters that adaptively adjusts the on-time control signal based on the pulse interval between inductor current pulses, using an on-time timer circuit and detection circuit to maintain switching frequencies above the audio band (2 Hz to 20 kHz) by adjusting the on-time duration of the switch circuit.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Loss of energy

If the switching frequency is reduced under ultra-light load conditions, then power consumption is reduced, but electromagnetic interference noise interferes with audio bands

Engineering Contradiction:

Improvepower consumptionVSAvoidelectromagnetic interference noise

Core Design Contradiction:

Loss of energyVSObject-affected harmful factors

Solution Approach 1:

The patent implements dynamic switching frequency adjustment by introducing an audio band noise detection circuit that monitors for noise in the 20Hz-20kHz range. When noise is detected, the system dynamically switches between PFM mode (for power saving) and PWM mode (for noise-free operation), creating a adaptive frequency control mechanism that responds to real-time conditions rather than using a fixed frequency approach

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent employs feedback control through the audio band noise detection circuit that continuously monitors the output for electromagnetic interference. The detection circuit provides feedback to the control logic, which then adjusts the switching frequency or mode accordingly. This closed-loop feedback system ensures that when EMI noise enters the audio band, the system automatically corrects by switching to PWM mode or adjusting frequency, thereby eliminating the harmful effect while maintaining power efficiency

Inventive Principle:

Principle #23Feedback

2Object-generated harmful factors

If the peak inductor current value is reduced, then EMI noise is reduced, but stable operation cannot be maintained under ultra-light load conditions

Engineering Contradiction:

ImproveEMI noiseVSAvoidstable operation

Core Design Contradiction:

Object-generated harmful factorsVSReliability

Solution Approach 1:

The patent changes the control parameter from fixed peak current mode to adaptive on-time control. The on-time timer circuit dynamically adjusts the on-time duration based on feedback from the audio band noise detection circuit. This parameter change allows the system to maintain stable operation by ensuring minimum on-time requirements are met while simultaneously reducing EMI noise through frequency adjustment when operating in PFM mode

Inventive Principle:

Principle #35Parameter changes

3Loss of energy

If the switching frequency varies, then power efficiency is improved, but audio band interference occurs

Engineering Contradiction:

Improvepower efficiencyVSAvoidaudio band interference

Core Design Contradiction:

Loss of energyVSObject-affected harmful factors

Solution Approach 1:

The patent implements dynamic frequency control by making the switching frequency adaptive rather than fixed. The system uses the audio band noise detection circuit to monitor for interference and dynamically adjusts the switching frequency or mode between PFM and PWM. This dynamic adjustment maintains the power efficiency benefits of variable frequency operation while preventing audio band interference through real-time frequency management

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent employs feedback control through the audio band noise detection circuit that continuously monitors the output for electromagnetic interference. The detection circuit provides feedback to the control logic, which then adjusts the switching frequency or mode accordingly. This closed-loop feedback system ensures that when EMI noise enters the audio band, the system automatically corrects by switching to PWM mode or adjusting frequency, thereby eliminating the harmful effect while maintaining power efficiency

Inventive Principle:

Principle #23Feedback

Data Source

PatentUS12463620B2Feedback control circuit of pulse-frequency modulation converter that adaptively adjusts on-time control signal and associated feedback control method

Publication Date: 2025.11.04 AIROHA TECHNOLOGY CORPORATION

AI summary

A feedback control circuit of a pulse-frequency modulation (PFM) converter includes an on-time timer circuit and a detection circuit. The on-time timer circuit generates an on-time control signal for controlling an on-time duration of a switch circuit included in a power stage circuit of the PFM converter. The detection circuit controls the on-time timer circuit to adaptively adjust the on-time control signal according to a pulse interval between two successive inductor current pulses of the PFM converter.