Bootstrap Capacitor Pre-Charging Switch Circuit
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Solution Overview
Problem
In load switching applications, existing methods fail to pre-charge bootstrap capacitors effectively when the load is reduced or interrupted, leading to delays in startup and insufficient output current demand in NMOS transistors.
Innovation Solution
A switch circuit that can connect the bootstrap capacitor to a switch node when the load is connected or to ground when the load is not connected, allowing for alternate pre-charging paths to maintain sufficient charge for the NMOS transistor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a bootstrap capacitor is used to provide gate overdrive voltage to the NMOS transistor, then the desired gate overdrive voltage is achieved, but the capacitor cannot be pre-charged when the load is reduced or interrupted
Solution Approach 1:
The patent implements a pre-charge circuit that proactively charges the bootstrap capacitor before the NMOS transistor needs to switch. This preliminary action ensures the capacitor is fully charged regardless of subsequent load conditions, preventing startup delays and ensuring reliable operation under varying load scenarios.
Solution Approach 2:
The patent introduces an intermediary pre-charge circuit between the power supply and the bootstrap capacitor. This intermediary circuit includes a pre-charge switch and resistor that provide a dedicated charging path independent of the load switch, allowing the capacitor to be charged even when the load is reduced or interrupted.
2Reliability
If the load switch is interrupted or reduced, then load protection is achieved, but the bootstrap capacitor cannot be pre-charged leading to startup delays
Solution Approach 1:
The pre-charge circuit activates before the main load switch operation to charge the bootstrap capacitor in advance. This preliminary charging action eliminates startup delays by ensuring the capacitor is ready immediately when needed, regardless of whether the load is subsequently connected or interrupted.
Solution Approach 2:
The patent segments the charging function into two independent parts: a pre-charge path for initial capacitor charging and a main load path for normal operation. This segmentation allows the bootstrap capacitor to be charged independently of the load switch state, preventing startup delays caused by load interruptions.
3Device complexity
If the bootstrap capacitor is not pre-charged, then circuit simplicity is maintained, but output current demand becomes insufficient
Solution Approach 1:
The pre-charge circuit provides preliminary charging action to ensure the bootstrap capacitor reaches full voltage before the NMOS transistor operates. This preliminary action guarantees sufficient output current demand by ensuring adequate gate overdrive voltage is available, without requiring complex additional circuitry during normal operation.
Data Source
AI summary
A power conversion circuit and method of formation is provided, which in one embodiment includes a transistor, a driver circuit having an output connected to a control electrode of the transistor and having a bootstrap port configured to be connected to a first terminal of a capacitor; a switch circuit having a first port connected to a current carrying electrode of the transistor and having a ground port connected to a ground, a capacitor port configured to be connected to a second of the capacitor, a first switch configuration in which the capacitor port is connected to the first port, and a second switch configuration in which the capacitor port is connected to the ground port.