Invariant Over-Current Protection Circuit Using Homogeneous Transistors
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Prior art current sense and current limit circuits experience wide variations in current limit trip value due to process, temperature, and over-drive voltage variations, making it challenging to maintain accurate over-load protection in power supply circuits.
Innovation Solution
The proposed current sense and current limit circuit employs a power transistor, a sense transistor, and current reference transistors, where all transistors are of the same type and size, ensuring consistent current limit trip values across varying conditions by using transistors with matching μ·COX and VOV parameters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional current sense and current limit circuits are used with resistors and transistors of different types and sizes, then the circuit can be designed with component variety, but the current limit trip value varies widely (over 60% variation) due to process, temperature, and over-drive voltage variations
Solution Approach 1:
The patent applies homogeneity by using transistors of the same type and size for all current sensing and reference functions. Specifically, the sense transistor and reference transistors are configured with identical width-to-length ratios, ensuring they have matching μ·COX parameters. This uniformity eliminates the wide variations (over 60%) in current limit trip value that occur with traditional mixed-component designs, achieving consistent over-load protection across process, temperature, and over-drive voltage variations.
2Device complexity
If resistors with high temperature coefficients are used for current sensing, then the circuit can be simplified, but the current limit trip value becomes highly sensitive to temperature variations
Solution Approach 1:
The patent changes the critical parameter from resistor temperature coefficients to transistor width-to-length ratios. By configuring the sense transistor and reference transistors with identical geometric parameters, the circuit achieves temperature-insensitive current limit detection. The matching transistor parameters compensate for temperature effects, maintaining accurate over-load protection without requiring complex temperature compensation circuits or specialized low-temperature-coefficient resistors.
3Power
If different sized transistors are used for power switching and current sensing, then the circuit can achieve proper current amplification, but the drain to source resistance variations cause significant drift in current limit trip value
Solution Approach 1:
The patent applies counterweight by introducing reference transistors with matching parameters to compensate for the drain to source resistance variations in the power transistor. The reference transistor configuration creates a balanced measurement system where the VDS variations in the power transistor are counterbalanced by corresponding variations in the reference transistors, eliminating the drift in current limit trip value while maintaining proper current amplification through the sense transistor.
Data Source
AI summary
A system and method are disclosed for providing a process, temperature and over-drive invariant over-current protection circuit. The over-current protection circuit comprises a power transistor, a sense transistor, at least one current sense transistor and at least one current reference transistor. The over-current protection circuit provides a current limit trip value that remains substantially constant over temperature variations, and over-drive voltage variations, and process variations.