Step-down DC/DC Converter Bootstrap Charging Skip Mode
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Solution Overview
Problem
In step-down DC/DC converters with N-channel MOSFETs, the bootstrap capacitor cannot be charged when the switching transistor is fixedly turned on, leading to a gate-source voltage lower than the threshold voltage, preventing the transistor from remaining in the on state, and limiting the input voltage range that can generate a desired output voltage.
Innovation Solution
A control circuit that includes a pulse generator capable of transitioning to a skip mode, where the pulse signal is maintained at an on level for a first period and then forcibly switched to an off level to charge the bootstrap capacitor, increasing the effective duty ratio beyond the conventional maximum, allowing the switching transistor to remain on and extending the input voltage range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If the switching transistor is fixedly turned on to achieve high duty ratio operation, then the output voltage can be maintained at target value, but the bootstrap capacitor cannot be charged, causing gate-source voltage to drop below threshold voltage
Solution Approach 1:
The patent implements periodic switching between normal mode and skip mode. In skip mode, the switching transistor is forcibly turned off for a predetermined period to enable bootstrap capacitor charging, then returns to normal mode. This periodic interruption ensures the bootstrap capacitor is regularly recharged while maintaining high overall duty ratio operation, resolving the contradiction between sustained on-state and capacitor charging capability.
2Adaptability or versatility
If the duty ratio is increased beyond conventional maximum to extend input voltage range, then the converter can operate in severe reduced voltage states, but the bootstrap capacitor charging is prevented
Solution Approach 1:
The patent dynamically switches between two operational modes based on input voltage conditions. The mode controller monitors the input voltage and transitions to skip mode when severe reduced voltage states are detected, allowing operation beyond conventional duty ratio limits. This dynamic adaptation enables extended input voltage range while ensuring bootstrap capacitor charging occurs periodically in skip mode.
3Loss of energy
If the switching transistor remains continuously on to improve efficiency, then power loss is reduced, but the bootstrap circuit cannot recharge the capacitor
Solution Approach 1:
The patent maintains continuous useful action by quickly transitioning between normal mode and skip mode. The skip mode interruption is brief and predetermined, minimizing energy loss while ensuring bootstrap capacitor charging occurs. After charging, the system returns to normal mode for continuous efficient operation, thus maintaining near-continuous useful action with minimal disruption.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enables an improved maximum duty ratio, allowing the step-down DC/DC converter to maintain the output voltage at a target value even in severe reduced voltage states, surpassing the conventional duty ratio limit of around 90%, thereby enhancing the converter's operational range and stability.
Implementation Method 1
a bootstrap capacitor C2 arranged in the form of an external component between an LX terminal and a bootstrap (BST) terminal
Implementation Method 2
a rectifier element 212 arranged between a BST terminal and an output of the bootstrap power supply circuit 220
Data Source
AI summary
A pulse generator includes a mode controller that detects a state in which there has been no transition of a pulse signal to the off level for a given cycle (period). When such a state is detected, the pulse generator transits to a skip mode. In the skip mode, the period is repeatedly switched between: (i) a first period in which the pulse signal is maintained at the on level over multiple cycles; and (ii) a second period in which the pulse signal is forcibly switched to the off level so as to charge the bootstrap capacitor by means of a bootstrap circuit.