Buck Converter Bootstrap Charging in DCM With Resonance Control
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Solution Overview
Problem
Operating buck converters in discontinuous conduction mode (DCM) presents challenges such as bootstrap capacitor charging issues and resonance problems, which affect efficiency and stability, especially at low load conditions.
Innovation Solution
The proposed buck converter topology includes additional FETs and resistors to manage the gate voltages of key MOSFETs, ensuring proper charging of the bootstrap capacitor and mitigating resonance issues by redirecting the resonant waveform away from the high-side MOSFET.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the buck converter operates in discontinuous conduction mode (DCM), then the converter can achieve wider operating range and better light-load efficiency, but bootstrap capacitor charging issues and resonance problems occur affecting stability
Solution Approach 1:
The patent introduces a P-channel FET (Q3) as an intermediary component between the high-side N-FET (Q1) and the bootstrap capacitor. This P-FET acts as a mediator that controls the charging path of the bootstrap capacitor, ensuring it charges properly during DCM operation when the low-side FET is off. The P-FET's source is connected to the high-side N-FET's source, its drain to the high-side N-FET's gate, and its gate controlled by a resistor network, creating a controlled intermediary path that resolves the charging issue without affecting the overall converter performance.
2Loss of energy
If the buck converter operates in DCM, then light-load efficiency is improved, but resonance problems occur that affect stability
Solution Approach 1:
The patent converts the harmful resonance effect into a beneficial charging mechanism. During DCM operation, when the low-side FET turns off, the inductor current continues flowing through the high-side N-FET's body diode, creating a negative voltage spike at the high-side source. The patent harnesses this normally harmful resonance voltage to charge the bootstrap capacitor through the P-FET, transforming the harmful resonance into a useful charging path that ensures proper bootstrap capacitor voltage while maintaining light-load efficiency.
3Reliability
If additional FETs and resistors are added to manage gate voltages in DCM, then proper bootstrap capacitor charging and resonance mitigation are achieved, but device complexity increases
Solution Approach 1:
The P-channel FET (Q3) serves multiple functions simultaneously: it acts as a switch to control the bootstrap capacitor charging path, provides a high-impedance off-state to prevent unwanted current paths, and enables the resonance voltage to be utilized for charging. This multi-functionality reduces the need for separate dedicated components for each function, thereby minimizing the increase in circuit complexity while achieving reliable DCM operation.
Data Source
AI summary
A buck converter includes a high-side N-FET, a low-side N-FET, a P-FET, a between a gate terminal and a source terminal of the P-FET, aa capacitor, and a FET driver. The FET driver operates in a selectable one of a continuous current mode and a discontinuous current mode. In a first phase of the discontinuous current mode, a gate voltage on the gate terminal the N-FET equalizes to a source voltage on the source terminal of the N-FET to turn on the first N-FET. A high output voltage on a high-side output of the FET driver is high enough to overcome a threshold voltage of a body diode of the first P-FET to provide the high output voltage minus a threshold voltage to the gate terminal of the high-side P-FET to turn on the high-side P-FET.