Separate AC DC Current Sensing Paths for Step-Down Converter Noise

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Solution Overview

Problem

Current DC/DC converters face issues with switching noise and low signal-to-noise ratio in current sensing, particularly with low resistance value sensing elements, leading to imprecise switching and increased power dissipation.

Innovation Solution

The implementation of separate AC and DC current sensing paths with distinct RC circuits, where the AC path has a lower time constant than the inductor's DCR to enhance signal-to-noise ratio and the DC path includes a low pass filter to eliminate switching noise, allowing for precise detection of the inductor current.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Loss of energy

If a low resistance value sensing element is used, then power dissipation is reduced, but signal-to-noise ratio deteriorates leading to imprecise switching

Engineering Contradiction:

Improvepower dissipationVSAvoidsignal-to-noise ratio

Core Design Contradiction:

Loss of energyVSMeasurement precision

Solution Approach 1:

The current sensing function is divided into two separate paths: an AC sensing path for detecting ripple current and a DC sensing path for detecting average current. Each path is optimized independently - the AC path prioritizes signal-to-noise ratio while the DC path prioritizes power efficiency, resolving the contradiction between these two requirements.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different sensing elements with different resistance values are used in different parts of the system. The AC sensing path uses a higher resistance element optimized for signal detection, while the DC sensing path uses a lower resistance element optimized for power efficiency. Each local sensing element has quality tailored to its specific function.

Inventive Principle:

Principle #3Local quality

2Productivity

If switching frequency is increased, then converter efficiency is improved, but switching noise increases causing false triggering

Engineering Contradiction:

Improveconverter efficiencyVSAvoidswitching noise

Core Design Contradiction:

ProductivityVSObject-affected harmful factors

Solution Approach 1:

RC filtering circuits are introduced as intermediary elements between the sensing elements and the PWM comparator. These filters act as mediators that smooth out high-frequency switching noise while preserving the essential current information, preventing false triggering while allowing high-frequency operation for improved efficiency.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The switching noise, which is normally a harmful byproduct of high-frequency operation, is converted into a beneficial signal by using it to charge the RC filtering capacitors. The noise charges the capacitors to voltages that represent the average current levels, transforming the harmful high-frequency fluctuations into useful low-frequency control signals.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Measurement precision

If separate AC and DC sensing paths are implemented, then current sensing precision is improved, but device complexity increases

Engineering Contradiction:

Improvecurrent sensing precisionVSAvoidcircuit complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The RC filtering circuits serve multiple functions simultaneously: they filter high-frequency switching noise, integrate current signals over time, and generate voltage signals proportional to average current levels. This multi-functionality reduces the need for separate dedicated components for each function, offsetting the complexity increase from having separate AC and DC sensing paths.

Inventive Principle:

Principle #6Universality (Multi-functionality)

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

This approach reduces jitter and improves the signal-to-noise ratio, enabling more accurate duty cycle control and reduced power dissipation by effectively filtering out switching noise and increasing the precision of current sensing.

Implementation Method 1

The sensed current signal is filtered by an RC circuit having a time constant that is lower than the inductor's DCR time constant

Methodology Applied

Scientific EffectRC circuit filtering: Filter (electronic)

Implementation Method 2

The DC component of the sensed current signal is filtered by a low pass filter to reduce the effect of switching noise

Methodology Applied

Scientific EffectLow pass filtering: Filter (electronic)

Data Source

PatentEP2546966B1Robust current control for step-down converter

Publication Date: 2015.09.02 LINEAR TECHNOLOGY CORP

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AI summary

In a current mode controlled switching power supply, current through an inductor (L1) is sensed to determine when to turn off or on the switching transistors (86, 88), the inductor current (IL) having a higher frequency AC component and a lower frequency DC component. The AC current feedback path (AC PATH), sensing the ramping ripple current, is separate from the DC current path (DC PATH), sensing the lower frequency average current. Separating the current sensing paths allows the signal to noise ratio of the AC sense signal (Vsen(ac)) to be increased and allows the switching noise to be filtered from the DC sense signal (K*Vsen(dc)). The gain (K) of the DC sense signal is adjusted so that the DC sense signal (K*Vsen(dc)) has the proper proportion to the AC sense signal (Vsen(ac)). The AC sense signal (Vsen(ac)) and the DC sense signal (K*Vsen(dc)) are combined by a summing circuit (72). The composite sense signal (Sum) is applied to a PWM comparator (50) to control the duty cycle of the switch.