DC-DC Converter PLL Spread Spectrum EMI Reduction

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Solution Overview

Problem

Higher switching frequencies in DC-DC converters lead to increased electromagnetic interference (EMI), which is challenging to manage without adding external components or significant silicon area, as existing spread spectrum clock techniques require additional resources.

Innovation Solution

A phase locked loop configuration with a controlled oscillator, filter, charge pump, and phase frequency detector, along with a comparator, enables spread spectrum modulation of the DC-DC converter clock, allowing for reduced EMI emission without external components or additional silicon area by switching off the phase frequency detector and using the comparator to control the charge pump for modulation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If spread spectrum clock techniques are used to reduce EMI, then EMI emission is reduced, but external components and additional silicon area are required

Engineering Contradiction:

ImproveEMI emissionVSAvoidadditional components and silicon area

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

The phase frequency detector is designed to perform dual functions: in normal operation it synchronizes the VCO to the reference clock, and in EMI reduction mode it generates the spread spectrum modulation signal by controlling the charge pump. This multi-functionality eliminates the need for separate EMI reduction components while achieving the same effect.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The invention merges the EMI reduction functionality into the existing phase frequency detector and charge pump circuitry. The phase frequency detector's output is routed to control the charge pump in a specific mode, combining the synchronization and modulation functions into a single integrated circuit without adding external components.

Inventive Principle:

Principle #5Merging (Combining)

2Productivity

If higher switching frequencies are used in DC-DC converters, then power efficiency and component size are improved, but electromagnetic interference increases

Engineering Contradiction:

Improveswitching frequencyVSAvoidelectromagnetic interference

Core Design Contradiction:

ProductivityVSObject-affected harmful factors

Solution Approach 1:

The invention applies periodic modulation to the switching frequency through spread spectrum techniques. The phase frequency detector generates a modulated control signal that periodically varies the VCO frequency, distributing the electromagnetic energy across a broader frequency spectrum and reducing peak EMI levels while maintaining the average switching frequency for power conversion efficiency.

Inventive Principle:

Principle #19Periodic action

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 solution effectively reduces EMI emission by implementing spread spectrum clocking with minimal additional components, simplifying production and reducing chip area, while maintaining high switching frequencies and power efficiency.

Implementation Method 1

The phase locked loop comprises a controlled oscillator, a filter with an integration capacitor coupled to a control input of the controlled oscillator, a charge pump, and a phase frequency detector

Methodology Applied

Scientific EffectPhase locked loop:

Implementation Method 2

The controlled oscillator (e.g. a voltage controlled oscillator), the filter, the charge pump and the phase frequency detector are configured to operate as the phase locked loop in the slave mode

Methodology Applied

Scientific EffectVoltage controlled oscillation:

Implementation Method 3

a filter with an integration capacitor coupled to a control input of the controlled oscillator

Methodology Applied

Scientific EffectCapacitance integration: Capacitance

Data Source

PatentUS8618850B2Device and method for generating clock signals for DC-DC converters

Publication Date: 2013.12.31 TEXAS INSTRUMENTS INC

AI summary

An electronic device includes a DC-DC converter for voltage conversion in a slave mode an in a master mode and including a phase locked loop. The phase locked loop comprises a controlled oscillator, a filter having an integration capacitor coupled to a control input of the controlled oscillator, a charge pump, and a phase frequency detector. In the slave mode, the controlled oscillator, the filter, the charge pump and the phase frequency detector are coupled to operate as the phase locked loop. There is a comparator coupled with an input to a control input of the controlled oscillator and with an output to the charge pump. In the master mode, the comparator is configured to control the charge pump in response to a control signal at the control input of the controlled oscillator when the phase frequency detector is switched off so as to perform a modulation of the control signal at the control input of the controlled oscillator by charging and discharging the integration capacitor.