Digital Input Receiver Current Limiting via Bandgap Reference
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Solution Overview
Problem
Industrial digital input receivers in industrial controllers face challenges with unregulated input current, leading to excessive power draw and heat dissipation, as most existing solutions lack effective current limiting without relying on field-side power or suffer from poor control due to variations in JFET threshold voltage.
Innovation Solution
A digital input circuit design that uses a bandgap voltage reference in quasi-parallel with a resistor to limit input current, splitting it between the isolation barrier and a sink transistor, allowing for precise current control without field-side power.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If no input current regulation is used in opto-coupler based digital input receivers, then device complexity is reduced, but power draw and heat dissipation grow very large
Solution Approach 1:
The patent extracts the current limiting function from complex regulation circuitry and implements it through a simple resistor placed in series with the input signal path. This resistor directly limits the input current without requiring additional active regulation components, thereby reducing device complexity while controlling power draw and heat dissipation.
Solution Approach 2:
The patent uses a simple resistor, which is a passive, inexpensive component with no moving parts or complex internal structure. This resistor serves as a disposable-like element that reliably limits current through its fixed resistance value, eliminating the need for complex, expensive regulation circuitry while effectively controlling power consumption.
2Loss of energy
If JFETs are used external to digital input receivers for current limiting, then power draw is reduced, but current control becomes poor due to variations in JFET threshold voltage
Solution Approach 1:
The patent replaces JFETs with simple resistors that provide stable, predictable current limiting. Resistors have fixed resistance values that do not vary with temperature or operating conditions like JFET threshold voltages do, ensuring precise and consistent current control without the complexity and variability of active devices.
Solution Approach 2:
The patent changes the approach from using active devices (JFETs) with variable parameters (threshold voltage) to passive devices (resistors) with fixed parameters (resistance value). This parameter change eliminates the variability issue and provides stable, precise current limiting across all operating conditions.
3Measurement precision
If field side power is used for current limiting in digital input receivers, then current control is achieved, but reliability decreases due to problematic field-side power requirements
Solution Approach 1:
The patent extracts the current limiting function from the field-side power domain and implements it using a resistor on the controller side that works with the incoming field signal. This eliminates dependency on field-side power for current limiting, as the resistor naturally limits current based on its resistance value and the applied voltage, improving reliability.
Solution Approach 2:
The resistor performs current limiting automatically based on its fixed resistance property, without requiring external control signals or field-side power. The current limiting is self-regulating through Ohm's law (I=V/R), making the system more reliable and independent of field-side power conditions.
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 approach enables accurate and constant input current limiting, reducing power consumption and heat dissipation while maintaining reliable isolation, applicable across various digital input types.
Implementation Method 1
A bandgap voltage reference is provided in quasi-parallel with a resistor in the input path of a digital input circuit. Because of the quasi-parallel nature, the current used by the digital input circuit is limited to an amount based on the value of the external resistor.
Data Source
AI summary
A bandgap voltage reference is provided in quasi-parallel with a resistor in the input path of a digital input circuit. Because of the quasi-parallel nature, the current used by the digital input circuit is limited to an amount based on the value of the external resistor. The input current is split between circuitry used to provide the logic signal across the selected isolation barrier and a sink transistor so that the current remains constant. This allows the digital input circuit to accurately limit input current without needing field-side power.