Buck-Boost Converter Current Sensing via Offset Ramps
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Solution Overview
Problem
Buck-boost converters face inefficiencies and increased complexity due to sub-harmonic oscillations and the need for additional current-sensing components, particularly when operating at constant-frequency current-mode, which affects their ability to maintain constant output voltage with varying input voltages.
Innovation Solution
A constant-frequency current-mode buck-boost converter employs mutually offset ramp signals and control logic to determine duty cycles for switch operation, allowing current sensing only when the first switch is closed, thereby eliminating the need for extra current-sensing components and maintaining high efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If current sensing is performed continuously in current-mode buck-boost converter, then sub-harmonic oscillation is suppressed, but circuit complexity and cost increase due to additional current-sensing components
Solution Approach 1:
The patent extracts the current sensing function to only the periods when the first switch is closed, eliminating the need for continuous current sensing. This is achieved by using the existing current-sensing circuit only during the time when current actually flows through the first switch and inductor, rather than attempting to sense current during all switch states.
Solution Approach 2:
The patent implements periodic current sensing that occurs only during specific intervals (when the first switch is closed) rather than continuously. The control circuit periodically activates the current-sensing function synchronized with the switching cycle, enabling sub-harmonic oscillation suppression through periodic feedback without requiring continuous sensing hardware.
2Measurement precision
If additional current-sensing components are added to measure inductor current, then current control precision is improved, but manufacturing cost and circuit area increase
Solution Approach 1:
The patent makes the existing current-sensing circuit universal by using it for dual purposes: (1) measuring inductor current during the first switch closed period for precise current control, and (2) providing feedback for duty cycle adjustment during both buck and boost operations. This eliminates the need for separate current-sensing components for different operating modes.
Solution Approach 2:
The patent enables the existing current-sensing circuit to serve itself by configuring it to automatically sense current only when needed (when the first switch is closed and current flows through the inductor). The circuit uses its own existing components (switch, inductor, sense resistor) to perform current measurement without requiring external sensing hardware.
3Stability of the object's composition
If duty cycle is adjusted to maintain constant output voltage with varying input voltage, then output voltage stability is improved, but current consumption increases due to larger peak current requirements
Solution Approach 1:
The patent implements dynamic duty cycle adjustment that adapts to varying input voltages and operating modes (buck/boost). The control circuit continuously monitors the output voltage and input voltage conditions, dynamically modifying the duty cycle to maintain optimal efficiency across different operating points rather than using a fixed duty cycle.
Solution Approach 2:
The patent employs feedback control where the output voltage is monitored and compared against a reference, and the duty cycle is adjusted based on the error signal. Additionally, the current-mode control provides inner-loop feedback by sensing inductor current and using it to regulate the switching, creating a dual-feedback mechanism that maintains voltage stability while optimizing current consumption.
Data Source
AI summary
A converter including an inductor (L), a first switch (SW1, S1) connected between an input terminal (Vin) and the inductor, a diode/switch (D1, S2), connected between the first end of the inductor and ground, a diode/switch (D2, S3) connected between the inductor and an output terminal (Vout), and a second switch (SW2, S4) coupled between inductor and ground. A current sensor senses current in the first switch (SW1, S1) as a measure of inductor current. Waveform generators (31, 32) generate buck and boost slope compensation ramps (RMP-BUCK, RMP-BOOST). Control logic (10) opens and closes the switches every clock period at individual duty cycles determined using a feedback signal derived from the output terminal, the sensed current and the slope compensation ramps. The slope compensation ramps are mutually offset such that current sensing is needed only while the first switch (SW1, S1) is closed.