Buck-Boost Converter Sample and Hold Current Loop Stability
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Solution Overview
Problem
Buck-boost converters using average current control face issues with residual ripple in the average inductor current demand signal, leading to sub-harmonic oscillations and mode switching during a switching cycle, which affects stability and transient response.
Innovation Solution
A sample and hold circuit is introduced within the current feedback loop to stabilize the average inductor current demand signal by locking in a sample of the signal at specific times during each switching cycle, preventing mode switching and stabilizing the loop over a wide bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a compensation circuit with slow time constant is used to filter the average inductor current demand signal, then noise immunity is improved, but transient response becomes too slow to react to transient conditions
Solution Approach 1:
The patent segments the control signal processing into two distinct parts: a slow compensation circuit for noise filtering and a fast sampled signal for transient response. The sample and hold circuit captures the compensated signal at specific instants, effectively separating the noise filtering function from the transient response function, allowing both requirements to be satisfied simultaneously.
Solution Approach 2:
The compensation circuit performs preliminary noise filtering on the average inductor current demand signal before the sample and hold circuit captures it. This preliminary action ensures that the signal fed to the PWM comparator is already cleaned of noise, while the sampling process preserves the fast transient response capability.
2Speed
If residual ripple is allowed in the average inductor current demand signal, then transient response is improved, but sub-harmonic oscillation and mode switching during switching cycle occur
Solution Approach 1:
The sample and hold circuit extracts only the necessary transient information from the compensated signal at specific sampling moments, while effectively removing the harmful residual ripple that causes sub-harmonic oscillation and unwanted mode switching. This extraction process preserves beneficial transient response while eliminating destabilizing ripple effects.
Solution Approach 2:
The sampling process rushes through the compensation circuit output at precise moments just before switching cycles, capturing the essential control information while skipping over the problematic residual ripple that would otherwise cause instability. This timing-based approach allows the system to respond quickly to transients without suffering from ripple-induced oscillations.
3Stability of the object's composition
If the compensation circuit time constant is increased to reduce ripple, then sub-harmonic oscillation is reduced, but the regulator cannot adequately react to transient conditions
Solution Approach 1:
The system dynamically switches between two signal representations: the slowly varying compensated signal for stability and the rapidly sampled signal for transient response. The sample and hold circuit creates a dynamic control signal that maintains the benefits of ripple reduction while restoring fast transient reaction capability through periodic sampling at optimal moments.
Data Source
AI summary
In an average-current mode control type buck-boost PWM converter, a sample and hold circuit is inserted in the current loop to avoid problems associated with ripple of the average inductor current demand signal. The rippling average inductor current is generated by a differential transconductance amplifier having applied to its inputs an error signal and a signal corresponding to the instantaneous current through the inductor, where the output of the amplifier is filtered. The rippling average inductor current is sampled and held at the beginning of each switching cycle, prior to the average inductor current demand signal being compared to buck and boost sawtooth waveforms. By using the sample and hold circuit, the feedback loops are easier to stabilize, and the converter cannot switch modes during a switching cycle.