Bus Driver Circuit Overvoltage Protection Without Swing Loss
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing bus drivers face challenges in maintaining high impedance states during bus contention and protecting against overvoltage conditions, particularly in industrial, avionic, and automotive environments, while also meeting the voltage requirements of standards like RS485 and RS422, without reducing the output voltage swing.
Innovation Solution
The use of DMOS transistors connected in a back-to-back configuration with voltage limiting components between the gate and source of each transistor to inhibit uncontrolled current flow and prevent damage from overvoltage events, along with a pre-driver circuit to manage gate voltage during overvoltage conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If protection diodes are added to prevent uncontrolled current flow during overvoltage conditions, then reliability is improved, but device complexity increases
Solution Approach 1:
The patent extracts the protection function from separate protection diodes and integrates it into the DMOS transistor structure itself. The body diode of the DMOS transistor is utilized as the protection mechanism, eliminating the need for additional discrete protection diodes and reducing circuit complexity while maintaining overvoltage protection capability
Solution Approach 2:
The DMOS transistor serves multiple functions: it acts as the main switching element for bus driving and simultaneously provides overvoltage protection through its intrinsic body diode. This multi-functionality eliminates the need for separate protection components, resolving the contradiction between reliability and device complexity
2Reliability
If the bus driver is protected against overvoltage conditions, then reliability is improved, but the output voltage swing is reduced
Solution Approach 1:
The patent converts the potentially harmful effect of the body diode (which could clamp overvoltage and reduce output swing) into a beneficial protection mechanism. By configuring the DMOS transistor in a specific manner and utilizing the body diode's natural characteristics, the circuit achieves overvoltage protection while maintaining full output voltage swing capability during normal operation
Solution Approach 2:
The patent changes the operational parameters and configuration of the DMOS transistor to ensure that the body diode only conducts during overvoltage conditions and does not interfere with the normal output voltage swing. This is achieved through proper biasing and circuit topology that keeps the diode reverse-biased during normal operation
3Reliability
If DMOS transistors are used instead of conventional transistors, then protection against overvoltage is improved, but manufacturing precision requirements increase
Solution Approach 1:
The patent utilizes the intrinsic body diode that is automatically formed during the standard CMOS fabrication process of DMOS transistors. No additional manufacturing steps or specialized processes are required beyond the conventional DMOS fabrication, as the body diode is a natural byproduct of the transistor structure itself
Data Source
AI summary
A bus driver is provided that can withstand over voltages being applied to its output terminal without the protection circuit detracting from the voltage swing that can be provided by the driver. The circuit arrangement also allows transistors having good on state resistance and large tolerance of drain-to-source voltages to be used.