MOSFET Comparator Input Protection for High-Voltage Comparison
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Solution Overview
Problem
Conventional comparators with CMOS structure face limitations in comparing high input voltages due to the risk of gate oxide film breakdown in MOSFETs with thin gate oxide films, and the use of shunt resistors to mitigate this issue increases circuit complexity and degrades accuracy.
Innovation Solution
Incorporating Zener diodes in parallel between the gate and source of input MOSFETs to clamp the gate-source voltage, along with a feedback MOSFET for negative feedback control and semiconductor rectifying elements to interrupt current when necessary, effectively restricting the gate-source voltage and preventing oxide film breakdown.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a MOSFET with a thin gate oxide film is used to achieve high integration and fast switching, then the switching speed and integration density are improved, but the gate oxide film breaks down when subjected to high gate-source voltage
Solution Approach 1:
A Zener diode is introduced as an intermediary protective element connected in parallel between the gate and source of the input MOSFET. The Zener diode clamps the gate-source voltage to a safe level below the breakdown voltage of the thin gate oxide film, allowing the MOSFET to operate at high speeds while preventing voltage-induced damage to the gate oxide film.
Solution Approach 2:
The Zener diode provides beforehand cushioning by preemptively limiting the gate-source voltage to a safe level before it can reach the breakdown threshold of the thin gate oxide film. This protective measure is in place before any potential overvoltage event occurs, ensuring the MOSFET can consistently operate at high speeds without risking gate oxide film breakdown.
2Reliability
If shunt resistors are added to protect the gate oxide film, then the gate-source voltage is limited, but the circuit complexity increases and measurement accuracy degrades
Solution Approach 1:
The Zener diode serves as a simple, inexpensive protective element that can be easily integrated into the comparator circuit. Unlike shunt resistors that require precise matching and contribute to circuit complexity, the Zener diode provides robust voltage clamping with a single component, reducing overall circuit complexity while maintaining gate oxide film protection.
3Reliability
If shunt resistors are added to limit the gate-source voltage, then the gate oxide film is protected, but the measurement accuracy and signal-to-noise ratio deteriorate
Solution Approach 1:
The Zener diode acts as an ideal intermediary protective element because it exhibits high impedance in the reverse breakdown region, drawing minimal current and introducing negligible voltage drop and noise into the signal path. This preserves the accuracy of the voltage comparison operation while providing effective gate oxide film protection, unlike shunt resistors that directly load the input signal.
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 direct comparison of high input voltages without gate oxide film breakdown, maintaining accuracy and simplicity by protecting the MOSFETs with thin gate oxide films, avoiding the disadvantages of thick oxide films and shunt resistors.
Implementation Method 1
Zener diodes exhibiting a breakdown voltage lower than a withstand voltage of a gate oxide film of the input MOSFETs and connected between the gate and the source of each of the input MOSFETs in parallel in a forward direction
Data Source
AI summary
A comparator comprises a differential amplifier type including input MOSFETs receiving differential input of a reference voltage and an input voltage, load MOSFETs for the input MOSFETs, and a constant current source to supply the sources of the input MOSFETs. The comparator comprises a Zener diode that is connected between the gate and source of the input MOSFETs and exhibits a breakdown voltage lower than the withstand voltage of the gate oxide film of the input MOSFET. Another comparator further comprises a feedback MOSFET that performs negative feedback of an output voltage of a main body comparator to the gates of the load MOSFETs to restrict the amplitude of the output voltage. Still another comparator further comprises a semiconductor rectifying element that exhibits a reverse-blocking characteristic higher than the power supply voltage and is interposed between the constant current source and the source of each of the input MOSFETs.