Boost Converter Average Current Control Stability

Solution Overview

Problem

Peak current mode controlled boost converters with parabolic slope compensation have maximum average inductor current dependent on slope compensation, making output power strongly dependent on boost voltage, leading to instability and subharmonic oscillations at high duty cycles.

Innovation Solution

Implementing average current mode control with a peak current level function that varies over time, combining a target peak value derived from a target average inductor current and a periodically varying slope compensation function, independent of boost voltage, to maintain constant average current and prevent voltage-dependent output power.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If parabolic slope compensation is used in peak current mode control, then current loop stability is improved, but maximum average inductor current becomes dependent on boost voltage

Engineering Contradiction:

Improvecurrent loop stabilityVSAvoidoutput power independence from boost voltage

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The patent changes the control parameter from peak current to average current. By controlling the average inductor current instead of the peak current, the maximum output power becomes independent of the boost voltage while maintaining current loop stability through average current mode control with appropriate slope compensation.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

Instead of using peak current mode control with slope compensation (the conventional approach), the patent inverts the approach by using average current mode control. This inversion allows the maximum average current to be set independently of the boost voltage, resolving the contradiction between stability and voltage independence.

Inventive Principle:

Principle #13The other way round (Inversion)

2Ease of operation

If peak current mode control is used, then input power limitation is simplified, but subharmonic oscillations occur at high duty cycles

Engineering Contradiction:

Improveinput power limitationVSAvoidstability at high duty cycles

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

The patent changes the control parameter from peak current to average current. By controlling the average inductor current instead of the peak current, the maximum output power becomes independent of the boost voltage while maintaining current loop stability through average current mode control with appropriate slope compensation.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent implements average current mode control with feedback mechanisms including slope compensation and, in some embodiments, hysteretic feedback. This feedback ensures stability at high duty cycles by preventing subharmonic oscillations while maintaining the ability to limit input power through average current control.

Inventive Principle:

Principle #23Feedback

3Power

If maximum average inductor current is set constant, then output power is limited, but current loop stability is compromised at high conversion ratios

Engineering Contradiction:

Improveoutput power limitationVSAvoidcurrent loop stability

Core Design Contradiction:

PowerVSReliability

Solution Approach 1:

The patent makes the peak current reference dynamic by deriving it from the average current reference and the actual inductor current waveform. The peak current level function varies over time based on the switching period and duty cycle, allowing the system to maintain stability at high conversion ratios while preserving the output power limitation through average current control.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent changes the control parameter from peak current to average current. By controlling the average inductor current instead of the peak current, the maximum output power becomes independent of the boost voltage while maintaining current loop stability through average current mode control with appropriate slope compensation.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentEP2775599B1Boost converter

Publication Date: 2018.09.05 NXP BV

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AI summary

A boost converter control based on average current and parabolic slope compensation. The converter combines maximum speed of the current loop without the voltage dependent maximum output power that usually accompanies the parabolic slope compensation. Especially for high conversion ratio ranges this significantly simplifies the maximum power control.