Bootstrap Capacitor Refresh Circuit for Synchronous Converters
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Solution Overview
Problem
Conventional refresh techniques for bootstrap capacitors in synchronous switching DC/DC converters face challenges, particularly at light load conditions where insufficient turn-on time of the low-side MOSFET leads to negative inductor current, causing undesired input voltage increase and battery discharge, and require interruption of normal operation in 4-switch buck-boost converters for refresh.
Innovation Solution
A synchronous switching converter design incorporating a main refresh circuit and an auxiliary refresh circuit, with the latter using a charge pump and control signals to refresh the bootstrap capacitor during specific operating modes, such as discontinuous current mode or pulse-frequency modulation, without interrupting normal operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional refresh technique is used to charge bootstrap capacitor through diode during low-side MOSFET turn-on, then bootstrap capacitor can be refreshed, but at light load conditions insufficient turn-on time causes negative inductor current which increases input voltage and causes battery discharge
Solution Approach 1:
A current source circuit is introduced as an intermediary component to provide the refresh current for the bootstrap capacitor. This current source replaces the conventional diode-based refresh path and allows precise control of the refresh current independent of the main inductor current, thereby preventing negative inductor current while ensuring reliable bootstrap capacitor charging.
Solution Approach 2:
The invention changes the parameter of refresh current delivery by using a controlled current source instead of a passive diode. The current source can deliver a predetermined refresh current that is sufficient to charge the bootstrap capacitor within the available low-side MOSFET turn-on time, even at light load conditions, without causing the inductor current to go negative.
2Reliability
If low-side MOSFET turn-on time is extended to ensure sufficient bootstrap capacitor charging, then refresh reliability improves, but inductor current is forced into negative territory causing boost-back condition and input voltage increase
Solution Approach 1:
The current source circuit acts as an intermediary that decouples the bootstrap capacitor refresh function from the main power transfer path. By providing a dedicated refresh current path, the system can extend the low-side MOSFET turn-on time sufficiently for capacitor charging without forcing the main inductor current negative, thus avoiding the boost-back condition and input voltage stress.
3Reliability
If conventional refresh technique is used in 4-switch buck-boost converter, then bootstrap capacitor can be refreshed, but normal operation must be interrupted to perform refresh
Solution Approach 1:
The current source circuit serves as an intermediary refresh mechanism that operates independently of the main converter switching cycles. It can deliver refresh current during any operating mode (buck, boost, or transition) without requiring interruption of normal power conversion operation, thus maintaining both refresh reliability and operational continuity.
Solution Approach 2:
The invention enables continuous operation of the 4-switch buck-boost converter by implementing a refresh mechanism that works throughout all operating modes. The current source can refresh the bootstrap capacitor during buck operation, boost operation, and transition modes without interrupting the useful power conversion action, ensuring uninterrupted operation.
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
The solution ensures adequate charging of the bootstrap capacitor across various operating modes, preventing negative inductor current and maintaining efficient operation without interrupting normal converter operation, even at light loads.
Implementation Method 1
the auxiliary refresh circuit includes a charge pump circuit which is configured to receive a clock signal as input and generate a sufficiently high output voltage for refreshing the bootstrap capacitor
Implementation Method 2
a bootstrap capacitor coupled to the high-side switching MOSFET to provide turn-on voltage for the high-side switching MOSFET
Data Source
AI summary
The disclosed embodiments provide a synchronous switching converter that converts a DC input voltage into a DC output voltage. This synchronous switching converter includes a high-side switching MOSFET coupled between an input node and a first node. The converter also includes a low-side switching MOSFET coupled between the first node and a ground node and is in series with the high-side switching MOSFET. This converter additionally includes a bootstrap capacitor coupled to the high-side switching MOSFET to provide turn-on voltage for the high-side switching MOSFET. Furthermore, the converter includes a main refresh circuit coupled to the bootstrap capacitor and is configured to refresh the bootstrap capacitor during a first operating mode of the synchronous switching converter. Moreover, the converter includes an auxiliary refresh circuit coupled to the main refresh circuit and the bootstrap capacitor and is configured to refresh the bootstrap capacitor during a second operating mode of the converter.