Bootstrap High-Side MOSFET Control for Stable Capacitor Charging
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Solution Overview
Problem
Existing switching power supply devices using N-channel MOSFETs as high-side switches face challenges in stably charging the boost capacitor, particularly when the input voltage is reduced, leading to a half-on state of the high-side FET due to insufficient gate drive voltage.
Innovation Solution
A switching power supply device with a bootstrap circuit, off-time generation unit, on-time calculation unit, and high-level priority unit that prioritizes maintaining the gate voltage high to ensure stable charging of the boost capacitor, even when the input voltage is low, by adjusting the gate voltage settings based on input voltage and output voltage comparisons.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the high-side FET is operated at a constant high duty ratio to minimize output voltage reduction, then the output voltage is maintained, but the boost capacitor charging is compromised
Solution Approach 1:
The control unit implements periodic forcing operation that overrides the constant high duty ratio operation at specific intervals to force the gate voltage low, enabling boost capacitor charging. This periodic intervention allows the system to maintain high duty ratio operation for output voltage stability while periodically enabling boost capacitor recharge, resolving the conflict between output voltage maintenance and energy storage.
2Ease of operation
If the gate drive voltage is generated by a bootstrap circuit, then the high-side FET can be driven, but the boost capacitor must be periodically charged by turning off the high-side FET which conflicts with maintaining high duty ratio
Solution Approach 1:
The invention uses periodic forcing operation to periodically turn off the high-side FET by forcing its gate voltage to a low level. This periodic turn-off enables the bootstrap circuit to recharge the boost capacitor while minimizing disruption to the overall high duty ratio operation. The periodic nature of this forcing operation ensures that the boost capacitor is charged at appropriate intervals without significantly affecting the average duty ratio and output voltage.
Data Source
AI summary
A switching power supply device, for generating an output voltage by switching an input voltage using an N-channel MOSFET as a high side switch, includes: a bootstrap circuit that generates a drive power supply of the high side switch; an off time generation unit that sets a gate voltage of the high side switch to be high when a division voltage signal of the output voltage falls below a target voltage; an on time calculation unit that sets the gate voltage to be low according to comparison between a terminal voltage of a capacitor to be charged by the constant current and a voltage obtained by multiplying the output voltage by a gain; and a high level priority unit that prioritizes the gate voltage to be high when the off time generation unit is outputting a signal for setting the gate voltage to be high.