Bootstrap Capacitor Overcharge Prevention in Inverter Control Circuits
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Solution Overview
Problem
Existing control circuits for bootstrap capacitors in inverter systems face issues with overcharge due to freewheeling currents, requiring complex analog control and additional power supplies, and are costly and voluminous to maintain constant voltage.
Innovation Solution
A control circuit with a totem-pole configuration that includes detection of excessive currents, turning off MOSFETs to prevent overcharge by allowing freewheeling currents to flow through body diodes, thereby reducing the risk of overcharging the bootstrap capacitor with a simple configuration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a bootstrap capacitor clamp circuit is installed to prevent overcharge, then the bootstrap capacitor overcharge problem is solved, but a floating power supply is additionally required and the control becomes complicated
Solution Approach 1:
The patent extracts the overcharge prevention function from a separate clamp circuit and integrates it into the existing MOSFET switching control. By using the body diode of the low-side MOSFET as a natural clamp element and controlling the high-side MOSFET switching timing, the patent eliminates the need for additional clamp circuit components while maintaining overcharge prevention capability.
Solution Approach 2:
The high-side MOSFET is made to serve dual functions: normal switching operation and bootstrap capacitor overcharge prevention. By controlling the high-side MOSFET to turn off when the bootstrap capacitor voltage reaches a certain level (indicated by the low-side MOSFET drain-source voltage), the same component performs both power switching and voltage regulation functions.
2Reliability
If a bootstrap capacitor clamp circuit with analog control is used, then the bootstrap capacitor voltage can be maintained constant, but the control becomes complicated with feedback requirements
Solution Approach 1:
The patent employs a self-service mechanism where the bootstrap capacitor itself provides the control signal for overcharge prevention. When the capacitor voltage becomes excessive, it automatically causes the low-side MOSFET drain-source voltage to drop, which naturally triggers the high-side MOSFET gate driver to turn off the high-side MOSFET, thereby limiting further charging current without external feedback control.
Solution Approach 2:
The patent implements inherent feedback through the bootstrap capacitor voltage affecting the low-side MOSFET operation. The capacitor voltage directly influences the drain-source voltage of the low-side MOSFET, which in turn controls the charging current to the capacitor, creating a natural feedback loop that maintains voltage stability without complex external control circuitry.
3Reliability
If a Pch-MOSFET is located on the bootstrap charging path to limit charging current, then overcharge is prevented, but the charging current is limited or volume and cost increase to ensure energizing capability
Solution Approach 1:
The patent uses dynamic control of the high-side MOSFET switching to regulate bootstrap capacitor charging. Instead of a fixed current limitation through a Pch-MOSFET, the system dynamically adjusts the charging current by controlling the high-side MOSFET on/off timing based on the capacitor voltage state, allowing full charging current capability when needed while preventing overcharge.
Solution Approach 2:
The patent changes the control parameter from fixed current limitation (via Pch-MOSFET resistance) to voltage-based timing control. By monitoring the low-side MOSFET drain-source voltage as an indicator of bootstrap capacitor voltage state, the system adjusts the high-side MOSFET switching parameters to achieve both adequate charging current and overcharge prevention.
Data Source
AI summary
A control circuit includes an inverter circuit including a high-side MOSFET and a low-side MOSFET connected to form a totem-pole, a first gate driver configured to switch the low-side MOSFET, a second gate driver configured to switch the high-side MOSFET, a bootstrap circuit configured to supply a voltage to the second gate driver, and a detection section configured to issue an anomaly signal when a current larger than a predetermined value flows in the inverter circuit. In response to the issuing of the anomaly signal, the low-side MOSFET is turned off, and the high-side MOSFET is turned off. After that, in a state in which a freewheeling current is flowing through the low-side MOSFET, the low-side MOSFET is turned on to prevent a bootstrap capacitor of the bootstrap circuit from being overcharged.