MOSFET Gate Discharge Circuit for Faster Turn-Off
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Solution Overview
Problem
High power MOSFETs used in Time of Photonic Flight systems face challenges with rapid turn-off due to large gate-to-source capacitance, leading to slow discharge and steep falling edges, which complicates high-resolution distance measurement in augmented reality applications.
Innovation Solution
A gate charging and discharging circuit is implemented using a bipolar junction transistor (BJT) and an inductive circuit, where the charging voltage is applied with a polarity that induces a current to flow away from the gate, and the discontinuation of this voltage induces a second current to quickly discharge the gate, facilitating faster turn-off of the MOSFET.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a high power MOSFET is used in Time of Photonic Flight systems, then the power handling capability is improved, but the gate-to-source capacitance increases causing slow discharge and preventing rapid turn-off
Solution Approach 1:
An inductive circuit is introduced as an intermediary component between the MOSFET gate and the charging voltage source. This inductor mediates the charging process by storing energy in its magnetic field during voltage application, then rapidly releasing this energy to force a fast discharge current through the gate-to-source capacitance when the charging voltage is removed, enabling rapid turn-off of high power MOSFETs
Solution Approach 2:
The patent changes the electrical parameters of the gate charging process by using an inductive circuit that transforms the voltage application into a controlled current flow pattern. The inductor's impedance characteristics change with frequency, allowing fast current discharge at high frequencies while limiting steady-state current, thereby achieving rapid gate discharge without continuous high current flow
2Loss of time
If the gate discharge time is reduced for faster turn-off, then the timing resolution is improved, but the gate-to-source capacitance creates a natural delay that opposes this reduction
Solution Approach 1:
The inductive circuit serves as a mediator that decouples the gate discharge time from the gate-to-source capacitance value. By introducing the inductor, the discharge process is governed by the LC time constant rather than just the RC time constant, allowing independent optimization of discharge speed and timing characteristics for high-resolution measurements
3Ease of operation
If the charging voltage is applied continuously to maintain gate charge, then the MOSFET remains on, but rapid turn-off becomes impossible
Solution Approach 1:
The charging voltage is applied in periodic pulses rather than continuously. During each pulse, the inductor charges the gate capacitance; when the pulse ends, the inductor's collapsing magnetic field creates a rapid discharge current that quickly removes the gate charge. This periodic application allows the MOSFET to be turned on when needed while enabling fast turn-off when the voltage is removed
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
This solution enables faster turn-off of the MOSFET and steeper falling edges of laser light pulses, improving the precision and signal-to-noise ratio in Time of Photonic Flight systems for high-resolution distance measurements.
Implementation Method 1
applying the charging voltage to an inductive circuit having an inductance, the inductive circuit being coupled to the gate, the applying of the charging voltage to the inductive circuit being with a polarity that induces a first current to flow through the inductance in a direction corresponding to charge moving away from the gate
Data Source
AI summary
A large-power insulated gate switching device (e.g., MOSFET) is used for driving relatively large surges of pulsed power through a load. The switching device has a relatively large gate capacitance which is difficult to quickly discharge. A gate charging and discharging circuit is provided having a bipolar junction transistor (BJT) configured to apply a charging voltage to charge the gate of the switching device where the BJT is configured to also discontinue the application of the charging voltage. An inductive circuit having an inductor is also provided. The inductive circuit is coupled to the gate of the switching device and further coupled to receive the charging voltage such that application of the charging voltage to the inductive circuit is with a polarity that induces a first current to flow through the inductor in a direction corresponding to charge moving away from the gate and such that discontinuation of the application of the charging voltage to the inductive circuit induces a second current flowing through the inductor in the direction corresponding to charge moving away from the gate such that the second current discharges the gate of the switching device. Faster turn off of the switching device is thus made possible and is synchronized to the discontinuation of the charging voltage.