DC-DC Converter Current Sensing Using Switch Resistance
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Solution Overview
Problem
Conventional methods for determining average output current in inductive DC-DC converters are inefficient, often requiring large current sensing resistors that occupy space and cause power loss, and fail to accurately limit or disable the converter to preserve battery life, especially in applications like USB-ON-THE-GO where external device influences can lead to fault conditions.
Innovation Solution
A switching circuit that uses the resistance of the power switching element to determine inductor current, coupled with an average and hold circuit, a voltage-to-current converter, and a current modulator to produce a signal proportional to the average output current, eliminating the need for a current sensing resistor and allowing for accurate current representation without significant space or power loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a current sensing resistor is used to directly sense output current, then current measurement is achieved, but the resistor occupies substantial space and causes significant power loss
Solution Approach 1:
The patent extracts the current sensing function from a dedicated current sensing resistor and relocates it to the power switching element itself. By utilizing the inherent resistance of the power switch, the design eliminates the need for a separate sensing resistor, thereby removing the associated power loss and space occupation while maintaining current measurement capability.
Solution Approach 2:
The power switching element is made multi-functional by using it both for power switching and for current sensing. The resistance of the power switch, which would normally just cause power loss, is repurposed as a sensing element to determine inductor current, thereby serving dual purposes and eliminating the need for a dedicated sensing resistor.
2Measurement precision
If a current sensing resistor is used to directly sense output current, then current measurement is achieved, but the resistor occupies substantial space on the circuit board
Solution Approach 1:
The patent extracts the current sensing function from a dedicated current sensing resistor and relocates it to the power switching element itself. By utilizing the inherent resistance of the power switch, the design eliminates the need for a separate sensing resistor, thereby removing the associated power loss and space occupation while maintaining current measurement capability.
Solution Approach 2:
The power switching element is made multi-functional by using it both for power switching and for current sensing. The resistance of the power switch, which would normally just cause power loss, is repurposed as a sensing element to determine inductor current, thereby serving dual purposes and eliminating the need for a dedicated sensing resistor.
3Productivity
If conventional current sensing techniques are used, then output current can be measured, but the converter may deliver current significantly higher than necessary due to measurement inaccuracy
Solution Approach 1:
The patent implements a feedback mechanism where the voltage across the power switching element is continuously monitored and used to determine the inductor current. This feedback loop allows the converter to accurately sense the actual current being delivered and adjust its operation accordingly, ensuring it delivers only the necessary current and avoiding excessive current delivery due to measurement inaccuracies.
Solution Approach 2:
The patent replaces the mechanical/resistive current sensing approach with an electronic measurement approach. Instead of using a physical sensing resistor to drop voltage and measure current, the system uses electronic circuitry to measure the voltage across the power switch and convert it to a proportional current signal, providing more accurate and controllable current measurement.
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 precise determination of average output current, reducing the need for bulky resistors and minimizing power loss, while allowing for safe current limiting and disabling of the converter to preserve battery life, thus improving efficiency and cost-effectiveness.
Implementation Method 1
an inductor-based DC-DC converter
Implementation Method 2
resistance of the power switching element, instead of a current sensing resistor, may be used to determine the inductor current
Data Source
AI summary
A switching circuit for converting an input voltage into an output voltage has an input terminal for receiving the input voltage. A power switching element is coupled to the input terminal and has duty cycle d controllable to adjust the output voltage with respect to a desired level using inductor current representing current in an inductor element connectable to the power switching element. An average and hold circuit is responsive to a voltage at an output of the power switching element to produce an average switch voltage over an ON phase of a switching cycle of the power switching element. A voltage-to-current converter is responsive to the average switch voltage for producing representation of an average inductor current over one or more switching cycles. A current modulator having a duty cycle equal to 1-d modulates the representation of the average inductor current to produce a signal proportional to an average output current of the switching circuit over one or more switching cycles.