Isolated MOSFET Gate Drive Using Pre-Charged Capacitor Switching
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Solution Overview
Problem
Existing isolated MOSFET switching circuits are slow to switch on newer MOSFETs with higher gate capacitance, leading to increased switching times due to reduced rate of voltage change, and previous solutions like gate isolation transformers or floating power supplies are costly and complex.
Innovation Solution
An isolated high speed MOSFET switching circuit that includes a larger capacitor in parallel with the MOSFET gate capacitances, constantly charged by an opto-battery, allowing for quick charging of the MOSFETs when the circuit is on, and a transistor for rapid discharge when off, reducing switching time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional opto-battery circuit is used to drive MOSFET gates, then isolation is provided, but switching speed becomes slow due to high gate capacitance
Solution Approach 1:
The circuit pre-charges a large capacitor (C1) to 6V through the opto-battery before switching is needed. When switching is required, this pre-charged capacitor rapidly discharges through the MOSFET gates, providing the necessary charge without requiring the opto-battery to deliver high current during the switching event itself.
Solution Approach 2:
The circuit uses periodic charging of the large capacitor C1 through the opto-battery, followed by rapid periodic discharge through the MOSFET gates. This separates the slow charging process (when isolation is maintained) from the fast discharge process (when switching occurs), achieving both isolation and fast switching.
2Loss of energy
If MOSFET size is increased to reduce on-resistance, then power loss decreases, but gate capacitance increases causing slower switching
Solution Approach 1:
The large capacitor C1 is continuously pre-charged to 6V through the opto-battery, storing energy in advance. When switching is needed, this pre-stored energy is rapidly discharged through the high-capacitance gates of larger MOSFETs, enabling fast switching despite the increased gate capacitance from larger device size.
Solution Approach 2:
The circuit changes the voltage parameter dynamically - maintaining a constant 6V charge on capacitor C1 through the opto-battery, then rapidly transitioning to a high-current discharge path through the MOSFET gates. This parameter change enables the circuit to handle both the high capacitance of large MOSFETs and achieve fast switching.
3Reliability
If the opto-battery continuously charges MOSFET gate capacitances, then MOSFETs remain on, but switching time increases due to gradual voltage rise
Solution Approach 1:
The opto-battery continuously maintains capacitor C1 charged to 6V, but does not continuously charge the MOSFET gates. Instead, the pre-charged capacitor C1 is rapidly discharged through the MOSFET gates only when switching is required, achieving both reliable conduction maintenance and fast switching.
Solution Approach 2:
The circuit segments the charging function - the opto-battery charges capacitor C1 continuously at a slow rate, while capacitor C1 then provides rapid discharge to MOSFET gates when needed. This segmentation separates the isolation-maintaining charging function from the fast-switching discharge function.
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
Enables faster switching of newer lower-on-resistance MOSFETs by increasing current output to the MOSFETs, reducing switching time and maintaining high voltage handling capability, while eliminating the need for costly transformer designs.
Implementation Method 1
The light from the LED causes a small current (e.g., 10-30 μA) to flow in the current source of the opto-battery 100
Implementation Method 2
a capacitor, an opto-battery in parallel with the capacitor
Data Source
AI summary
A circuit structured to drive an isolated high speed voltage metal-oxide-semiconductor field-effect transistor (MOSFET) switch, including a first MOSFET and a second MOSFET configured to operate as a switch, a capacitor, a charging component in parallel with the capacitor, a first switch in series with the charging component, and a second switch in parallel with the charging component and the capacitor. The stored voltage in the capacitor is sent to the gates of the first MOSFET and the second MOSFET when a second switch is open and a first switch is closed.