PWM Signal Spectrum Spreading via Dual-Use Capacitor

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Solution Overview

Problem

Switching power supplies generate electrical noise at the frequency of the clock oscillator, known as radio frequency interference (RFI) or electromagnetic interference (EMI), which is not fully suppressed by existing techniques, especially when the clock frequency falls within the operating frequency range of the powered circuit, causing subjective impairment such as diagonal lines in displayed images.

Innovation Solution

A method and apparatus that utilize a single capacitor initially for the soft start circuit during startup and subsequently for a spreading oscillator to modulate the frequency of the clock oscillator, providing spread spectrum operation without the need for an additional frequency-determining capacitor, allowing independent control of timing and frequency parameters.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a spreading oscillator uses an additional frequency-determining capacitor, then the spreading function can be implemented, but the component count increases and integration becomes difficult

Engineering Contradiction:

Improvenoise reduction effectivenessVSAvoidcomponent count

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent makes the existing capacitor C1 serve dual functions: first as the soft-start timing capacitor during startup, then as the frequency-determining capacitor for the spreading oscillator after startup completes. This eliminates the need for a separate frequency-determining capacitor, reducing component count while maintaining spreading functionality for noise reduction

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The patent combines the soft-start timing function and the spreading oscillator frequency determination function into a single capacitor C1. By merging these two functions into one component, the design avoids adding extra components and maintains integration feasibility while achieving both soft-start and spectrum spreading objectives

Inventive Principle:

Principle #5Merging (Combining)

2Device complexity

If the clock oscillator frequency is fixed, then the PWM control is simple, but electrical noise at that frequency cannot be fully suppressed

Engineering Contradiction:

Improvecontrol simplicityVSAvoidelectrical noise

Core Design Contradiction:

Device complexityVSObject-generated harmful factors

Solution Approach 1:

The patent introduces periodic frequency modulation of the clock oscillator by using the spreading oscillator to vary the oscillator control voltage. This causes the clock frequency to periodically deviate from its nominal value, spreading the PWM spectrum and reducing concentrated electrical noise at any single frequency while maintaining overall system simplicity

Inventive Principle:

Principle #19Periodic action

3Adaptability or versatility

If spreading oscillator frequency is set using RC time constant, then frequency can be adjusted, but additional components and pins are required

Engineering Contradiction:

Improvefrequency adjustabilityVSAvoidintegration difficulty

Core Design Contradiction:

Adaptability or versatilityVSDevice complexity

Solution Approach 1:

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The patent changes the functional role of capacitor C1 from solely a soft-start timing element to a dual-role component that also determines spreading oscillator frequency. By changing the parameter usage of existing components rather than adding new ones, the design achieves frequency adjustability without increasing integration difficulty

Inventive Principle:

Principle #35Parameter changes

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 effectively reduces electrical noise by modulating the frequency of the clock oscillator, thereby minimizing RFI/EMI interference, and is suitable for integrated circuits where additional capacitors are too large and require extra pins, enhancing noise reduction without increasing component count.

Implementation Method 1

utilize a single capacitor initially for the soft start circuit during startup and subsequently for a spreading oscillator which modulates the frequency of the clock oscillator

Methodology Applied

Scientific EffectCapacitance: Capacitance

Data Source

PatentUS8115523B2Circuit to spread the spectrum of a PWM signal

Publication Date: 2012.02.14 TEXAS INSTRUMENTS INC

AI summary

An apparatus is provided. The apparatus comprises a first current source and a second current source that charge and discharge a capacitor. Coupled between the capacitor and the second current source is a switch that can be actuated and deactuated by a controller. Preferably, the controller is coupled to the capacitor and receives a first threshold voltage and a second threshold voltage so that it can actuate the switch if the voltage across the capacitor is greater than the first threshold voltage and deactuate the switch if the voltage across the capacitor is less than the second threshold voltage. Additionally, there is a comparator that is coupled to the capacitor that compares the voltage across the capacitor to a reference voltage, and there is a a multiplexer that is coupled to the capacitor and that is coupled to the comparator. The multiplexer outputs the voltage across the capacitor if the voltage across the capacitor is greater than the reference voltage and outputs the reference voltage if the voltage across the capacitor is less than the reference voltage.