Buck Converter Current Detection for Low-Side Reverse Current
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Solution Overview
Problem
Current detection methods for low-side transistors in switching circuits face challenges when operating in current source mode, as the reverse current flow generates a negative voltage, making it difficult to detect the current based on the on-resistance of the low-side transistor.
Innovation Solution
A current detection circuit utilizing a capacitor and switch circuit that connects the capacitor to a ground line in two phases, allowing the detection of the low-side transistor current by converting the negative voltage into a positive voltage for accurate signal generation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If current detection is performed based on the on-resistance of the low-side transistor in current source mode, then the current flowing through the low-side transistor can be monitored, but a negative voltage is generated at the output node making detection difficult
Solution Approach 1:
The patent inverts the detection approach by detecting the voltage at the source node instead of the drain node, and by using a sensing resistor connected to the source node rather than relying on the transistor's on-resistance. This inversion allows current detection in current source mode where conventional methods fail due to negative voltage generation.
Solution Approach 2:
The patent introduces a sensing resistor as an intermediary element connected between the source node and ground. This sensing resistor serves as a mediator that converts the difficult-to-detect negative voltage into a measurable voltage signal without requiring direct measurement at the problematic output node.
2Measurement precision
If a sensing resistor is inserted in the current path to detect voltage drop, then current can be detected, but the circuit complexity increases
Solution Approach 1:
The sensing resistor serves multiple functions: it acts as a current sense element, provides a reference potential for the source node, and enables current detection in current source mode. This multi-functionality reduces the need for additional dedicated components, thereby limiting the increase in circuit complexity.
3Device complexity
If the on-resistance of the low-side transistor is used for current detection, then no additional components are needed, but accurate detection in current source mode becomes impossible due to reverse current flow
Solution Approach 1:
The patent segments the current detection function from the power switching function by using a dedicated sensing resistor separate from the low-side transistor. This segmentation allows the sensing resistor to be optimized for detection accuracy while the transistor maintains its switching function, resolving the conflict between component simplicity and detection accuracy.
Data Source
AI summary
The present disclosure provides a current detecting circuit, a synchronous rectification type DC/DC buck converter and a control circuit thereof. The current detection circuit is used with a push-pull switching circuit to detect a source current flowing in a low-side transistor. The switching circuit, (i) in a first phase, connects a first end of the capacitor to a ground line, and connects a second end of the capacitor to a switch line connecting a high-side transistor and the low-side transistor. The switching circuit, (ii) in a second phase, sets the first end of the capacitor to a high impedance, and connects a second end of the capacitor to the ground line. An output circuit has a high impedance input and outputs a current detection signal based on a voltage generated at the first end of the capacitor in the second phase.