DC-DC Converter Control Circuit for Low Load Efficiency
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Solution Overview
Problem
DC-DC converters face efficiency losses at low load currents due to inductor current reversal, which existing solutions fail to effectively block, requiring high-accuracy and high-speed comparators for timely turn-off of low-side transistors.
Innovation Solution
A DC-DC converter control circuit with digital detection and adjustment logic that determines the turn-off edge of the low-side gate drive signal, allowing for digital advancement or delay of the turn-off time, eliminating the need for a low-offset high-speed comparator and improving efficiency at low load currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a synchronous rectifier with low-side transistor is used to improve efficiency at high load currents, then efficiency is improved, but inductor current reversal occurs at low load currents causing power loss
Solution Approach 1:
The patent implements dynamic switching between synchronous rectifier mode and freewheeling diode mode based on load current conditions. The control circuit detects inductor current reversal and automatically transitions the low-side transistor from continuous conduction to controlled turn-off, adapting the rectifier topology to operating conditions to maintain efficiency across both high and low load ranges
Solution Approach 2:
The patent employs feedback mechanisms including zero-crossing detection of inductor current and body diode voltage monitoring to detect current reversal conditions. This feedback triggers the control circuit to advance the turn-off edge of the low-side gate drive signal, preventing reverse current flow and eliminating power loss at low load currents
2Measurement precision
If a low-offset high-speed comparator is used to detect zero-crossing for timely turn-off, then turn-off accuracy is improved, but device complexity and silicon area increase
Solution Approach 1:
The patent introduces an intermediary body diode between the low-side transistor and ground. This diode provides a natural voltage indicator during current reversal - when the inductor current reverses, the body diode becomes forward-biased and creates a detectable voltage drop. This intermediary element simplifies the detection mechanism, allowing standard comparators to accurately detect zero-crossing without requiring ultra-low offset or high-speed specifications
Solution Approach 2:
The patent uses the inherent body diode of the low-side MOSFET as a detection element rather than requiring a separate precision comparator circuit. The body diode's voltage characteristics during reverse current flow provide sufficient detection signal for standard comparators, eliminating the need for expensive, high-precision comparator components and reducing overall circuit complexity
Data Source
AI summary
A DC-DC converter has a control circuit for controlling a high-side power transistor and a low-side power transistor connected in series between supply terminals to which an input supply voltage is applied. The converter has a switching node at the interconnection of the power transistors for connection of an inductor to which a load is connected. The control circuit has a feedback loop that provides a pulse width modulated control signal, logic circuitry to which the pulse width modulated control signal is applied and gate drivers with inputs connected to outputs of the logic circuitry and outputs applying gate drive signals to the gates of the power transistors. A digital signal is obtained which is indicative of whether the converter switching node is at a potential above or below a zero reference at the time of the turn-off edge of the low-side gate drive signal. The turn-off edge of the low-side gate drive signal is advanced or delayed by a predetermined amount in response to the value of the digital signal.