Buck-Boost Converter Segmentation Reduces Switching Losses
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Solution Overview
Problem
Conventional buck-boost DC-DC converters suffer from higher inductive switching losses and lower efficiency due to larger switching transistors required to handle higher voltage amplitudes, leading to lower output current to average inductor current ratios and increased inductor losses, especially during extreme duty cycle operations.
Innovation Solution
The proposed buck-boost converter circuit incorporates a control circuit and a switching circuit with five transistors, utilizing an external buck-boost inductor and flying capacitor to operate in multiple modes (buck, boost, and buck-boost) by alternating between different phase configurations, reducing voltage stress on transistors and inductors, and optimizing switching operations based on feedback signals and clock signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional buck-boost converter topology is used, then output voltage flexibility is improved, but switching transistor size and voltage stress increase
Solution Approach 1:
The converter is divided into two separate transistor legs, each handling only a portion of the voltage stress. The first transistor leg handles input voltage while the second transistor leg handles output voltage, preventing either transistor from experiencing the full sum of both voltages as in conventional topologies.
Solution Approach 2:
A coupled inductor structure acts as an intermediary element between the two transistor legs, enabling energy transfer while isolating the voltage stresses. The coupled inductors with specific turns ratios allow voltage transformation without requiring any single transistor to block the complete voltage sum.
2Reliability
If larger switching transistors are used to handle higher voltage amplitudes, then voltage stress is reduced, but switching losses increase
Solution Approach 1:
The voltage rating parameter of the switching transistors is changed from needing to withstand Vin+|Vout| to only needing to withstand Vin or |Vout| respectively. This parameter reduction allows selection of transistors with lower voltage ratings that have smaller on-resistances and lower switching losses.
3Reliability
If higher inductor voltages are used, then voltage stress on transistors is improved, but inductive switching losses increase
Solution Approach 1:
The inductor voltage stress is segmented across two separate coupled inductor structures. Each inductor experiences only a portion of the total voltage stress, reducing the voltage amplitude across each inductor and thereby reducing inductive switching losses while maintaining adequate voltage handling capability.
4Device complexity
If conventional two-transistor topology is used, then device complexity is reduced, but efficiency decreases due to higher losses
Solution Approach 1:
The converter uses two transistor legs with multiple transistors per leg, segmenting the voltage handling function. Although this increases transistor count from two to four or more, each transistor operates at lower voltage stress with lower on-resistance, resulting in reduced conduction losses and improved overall efficiency that compensates for the increased device count.
Data Source
AI summary
Disclosed examples include inverting buck-boost DC-DC converter circuits with a switching circuit to alternate between first and second buck mode phases for buck operation in a first mode, including connecting an inductor and a capacitor in series between an input node and a reference node to charge the inductor and the capacitor in the first buck mode phase, and connecting the inductor and the capacitor in parallel between an output node and the reference node to discharge the inductor and the capacitor to the output node. For boost operation in a second mode, the switching circuit alternates between connecting the inductor and the capacitor in series between the input node and the reference node to discharge the inductor and charge the capacitor in a first boost mode phase, and connecting the inductor between the input node and the reference node to charge the inductor and connecting the capacitor between the first output node and the reference node to discharge the capacitor to deliver power to the output node in a second boost mode phase.