Synchronous MOSFET Rectifier Without Large Gate-Drive Capacitors
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Solution Overview
Problem
Existing rectifier circuits using MOSFETs for synchronous rectification require large capacitors to store energy for gate-source voltage generation, hindering downsizing and cost reduction while also suffering from inefficient loss reduction due to slow gate-source voltage transitions.
Innovation Solution
A rectifier circuit design that utilizes control circuits to generate gate-source voltages from drain-source voltages during non-rectification periods, eliminating the need for capacitors and ensuring quick voltage transitions, thereby maintaining low loss and enabling downsizing and cost reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a capacitor is used to store energy for generating gate-source voltage in synchronous rectification, then the rectifier can maintain low loss, but the capacitor volume increases and hinders downsizing
Solution Approach 1:
The patent extracts the energy storage function from a dedicated capacitor by utilizing the inherent capacitance of the rectifying MOSFET itself and the body diode capacitance. The control circuit generates gate-source voltage by capacitive coupling during the MOSFET off-time, eliminating the need for a separate large-volume capacitor while maintaining the energy storage needed for synchronous rectification.
Solution Approach 2:
The rectifying MOSFET serves multiple functions: it acts as the main switching element for rectification, provides energy storage through its output capacitance during off-time, and its body diode provides freewheeling path. This multi-functionality eliminates the need for separate dedicated capacitor components, reducing overall circuit volume while maintaining low loss synchronous rectification performance.
2Volume of stationary object
If voltage division through resistances is used to generate gate-source voltage, then the circuit can operate without a capacitor, but the gate-source voltage transitions slowly deteriorating the loss reduction effect
Solution Approach 1:
The patent implements dynamic voltage generation by using the rectifying MOSFET's output capacitance to charge during off-time and discharge during on-time to generate the gate-source voltage. This dynamic capacitive coupling approach produces fast voltage transitions that match the switching speed requirements, unlike static resistance division which produces slow transitions. The fast transitions ensure the MOSFET switches quickly, maintaining low on-resistance and reducing conduction losses.
3Reliability
If a large capacitor is used to hold voltage during non-charging periods, then the gate-source voltage can be maintained, but the power supply capacitor volume increases
Solution Approach 1:
The rectifying MOSFET serves itself by using its own output capacitance to generate the gate-source voltage needed for its operation. During the MOSFET off-time, the voltage across its drain-source terminals charges its output capacitance, which then discharges during on-time to provide the gate drive. This self-service mechanism eliminates the need for external capacitors while maintaining reliable gate-source voltage throughout the switching cycle.
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 proposed solution achieves efficient synchronous rectification with reduced capacitor size, enhancing efficiency and reducing costs by minimizing capacitor requirements and improving gate-source voltage transitions.
Implementation Method 1
during at least a part of a time period within which a voltage between a drain and a source of the second MOSFET is inputted as a first input voltage to the first control circuit
Data Source
AI summary
A first MOSFET, a second MOSFET, a first control circuit, and a second control circuit are provided; the second MOSFET is in a non-rectification period when the first MOSFET is in a rectification period; the first MOSFET is in a non-rectification period when the second MOSFET is in a rectification period; the first control circuit outputs a voltage generated on the basis of a first input voltage as a first output voltage between the gate and the source of the first MOSFET in at least a portion of a period in which a voltage between the drain and the source of the second MOSFET is inputted as the first input voltage and a negative voltage is applied between the drain and the source of the first MOSFET; the second control circuit outputs a voltage generated on the basis of a second input voltage as a second output voltage between the gate and the source of the second MOSFET in at least a portion of a period in which a voltage between the drain and the source of the first MOSFET is inputted as the second input voltage and a negative voltage is applied between the drain and the source of the second MOSFET.