FET Gate Capacitor for Load-Dump Transient Protection
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Solution Overview
Problem
Existing electric circuits for protecting DC applications against load-dump transients and reversed polarity in alternator charging circuits are bulky and energy-consuming, leading to substantial power losses and potential damage to components.
Innovation Solution
An electric circuit utilizing a field effect transistor (FET) with a capacitor connected across its gate and source terminals, providing a time delay before switching off to prevent damage from load-dump transients while ensuring protection against reversed polarity, using a resistor and diode to control the time delay, resulting in a more compact and energy-efficient design.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If bi-directional diode, MOSFET or relay is used to protect against load-dump transients, then protection is provided, but the circuit becomes bulky and energy-consuming
Solution Approach 1:
The patent changes the operational parameters of the FET by controlling its gate-source voltage timing. The capacitor delays the voltage change at the gate, creating a time-based parameter change that allows the FET to remain conductive during transient events while still providing protection against reversed polarity. This temporal parameter modification enables the circuit to distinguish between harmful reversed polarity conditions and temporary load-dump transients.
2Reliability
If FET is used to protect against reversed polarity, then circuit interruption is achieved, but load-dump transients may cause avalanche damage
Solution Approach 1:
The capacitor is pre-charged during normal operation, storing energy that will be used to maintain the gate-source voltage during transient events. This preliminary action ensures that when a load-dump transient occurs, the FET gate already has the necessary voltage to remain conductive, preventing avalanche conditions before they can occur. The pre-charged capacitor acts as a buffer that maintains FET operation during temporary voltage fluctuations.
Solution Approach 2:
The capacitor provides a cushioning effect by absorbing voltage fluctuations and maintaining the gate-source voltage during transient events. This beforehand cushioning prevents the FET from experiencing sudden voltage changes that would cause avalanche breakdown. The capacitor acts as a protective buffer, smoothing out voltage variations and preventing harmful stress on the FET structure.
3Reliability
If FET switch opens immediately to protect against reversed polarity, then current flow is prevented, but transient protection is compromised
Solution Approach 1:
The capacitor acts as an intermediary element between the power source and the FET gate. It mediates the voltage transmission by filtering and timing the voltage changes that reach the FET gate. This intermediary function allows the circuit to respond to reversed polarity conditions while ignoring temporary transient events, as the capacitor smooths out short-term voltage fluctuations and only responds to sustained polarity reversals.
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 solution effectively protects DC applications from load-dump transients and reversed polarity without causing the FET to enter avalanche, reducing component damage and energy dissipation, while maintaining a compact and energy-conserving circuit.
Implementation Method 1
a capacitor connected across the gate terminal and the source terminal of the FET, the capacitor, in combination with an internal capacitance of the gate terminal of the FET, defining a capacitance, C
Data Source
AI summary
An electric circuit for supplying power to a DC application is disclosed. The electric circuit comprises a DC power source (1) connectable to an alternator charging circuit (2), an FET (T7), and a capacitor (8) connected across the gate terminal and the source terminal of the FET (T7). The drain terminal of the FET (T7) is connected to a negative terminal of the DC source (1). The FET (T7) protects the circuit against accidental connection of the alternator charging circuit (2) to the DC power source (1) with reversed polarity, by opening a switch (6) of the FET (T7) when this occurs. The capacitor (8) protects the FET (T7) from being pushed into avalanche in case a load-dump transient occurs. This is because the capacitor (8) in this case will charge and discharge, thereby introducing a time delay before the switch (6) is opened. Protection against load-dump transients is thereby obtained by means of a small component with low energy dissipation. Thereby the size of the circuit is reduced and energy is conserved.