Bootstrap Switch Circuit to Prevent DC/DC Converter Overcharging
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Solution Overview
Problem
The existing switching circuits experience overcharging of the bootstrap capacitor during dead time due to current flow through diodes or body diodes, leading to voltage instability and potential overvoltage, which can cause erroneous operations and electromagnetic interference.
Innovation Solution
A switching circuit configuration that includes a bootstrap switch with two P-channel MOS transistors connected in anti-series, controlled by a driver circuit to turn on during low-side transistor on periods and off during low-side transistor off periods, preventing current flow and maintaining the bootstrap capacitor voltage at a constant level.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a diode or bootstrap switch with body diode is used to charge the bootstrap capacitor, then the bootstrap capacitor can be charged during dead time, but the bootstrap capacitor becomes overcharged due to negative voltage at the switching terminal
Solution Approach 1:
The bootstrap switch is segmented into two P-channel MOS transistors connected in anti-series configuration. This segmentation allows independent control of current flow in opposite directions, preventing the body diode effect that causes overcharging while maintaining the ability to charge the bootstrap capacitor during low-side transistor on periods.
Solution Approach 2:
The anti-series MOS transistor configuration acts as an intermediary between the constant voltage line and the bootstrap terminal. This intermediary structure controls the charging process by allowing current flow only when properly biased, blocking the harmful current path during dead time when negative voltage appears at the switching terminal.
2Duration of action of moving object
If the bootstrap capacitor is overcharged during dead time, then the voltage across the bootstrap capacitor increases to VDD+VNEG, but this causes erroneous operations and electromagnetic interference
Solution Approach 1:
The anti-series MOS transistor configuration provides preliminary protection by blocking the charging current path before the negative voltage can cause overcharging during dead time. The transistors are biased to remain off during dead time, preventing the harmful charging effect before it occurs.
Solution Approach 2:
The driver circuit controls the bootstrap switch based on the state of the low-side transistor. When the low-side transistor is off (dead time period), the driver ensures the bootstrap switch remains off, providing feedback-based control that prevents overcharging and the associated electromagnetic interference.
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 configuration effectively suppresses overcharging of the bootstrap capacitor, stabilizes the voltage across it, and prevents substrate potential variations, thereby reducing the risk of erroneous operations and electromagnetic interference.
Implementation Method 1
a bootstrap capacitor CBST connected between the switching terminal and the bootstrap terminal
Implementation Method 2
The bootstrap switch includes two P-channel MOS (PMOS) transistors connected in anti-series with each other
Data Source
AI summary
Disclosed is a switching circuit including an input terminal, a switching terminal, a grounding terminal, a bootstrap terminal, a high-side transistor connected between the input terminal and the switching terminal, a low-side transistor connected between the switching terminal and the grounding terminal, a bootstrap capacitor connected between the switching terminal and the bootstrap terminal, a bootstrap switch connected between a constant voltage line and the bootstrap terminal, and a driver circuit configured to turn on the bootstrap switch in a period of time in which the low-side transistor is on and to turn off the bootstrap switch in a period of time in which the low-side transistor is off. The bootstrap switch includes two P-channel metal oxide semiconductor transistors connected in anti-series with each other between the constant voltage line and the bootstrap terminal.