Capacitive Current Sensing in Switched Mode Power Supplies
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Solution Overview
Problem
Conventional DC-DC converters face challenges with precision and complexity due to limited common mode range, significant offsets, and low input impedance in current sensing modes, which affect the accuracy of output current measurement.
Innovation Solution
The implementation of a current sensing stage with capacitive coupling for the comparator inputs, using additional capacitors and switches to manage common mode requirements and reduce offset, and the use of parasitic resistance of the output inductor for efficient current sensing without additional components, along with a direct current resistance mechanism to enhance efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional current sensing with shunt resistor and amplifier is used, then output current can be sensed, but measurement precision deteriorates due to limited common mode range, significant offsets, and low input impedance
Solution Approach 1:
The patent introduces a capacitor as an intermediary element between the shunt resistor and the amplifier input. This capacitor couples the voltage signal while blocking DC common mode voltages, allowing the amplifier to operate with a fixed common mode voltage level. The capacitor acts as a mediator that transfers only the AC voltage component caused by output current, eliminating common mode range limitations and offset issues.
Solution Approach 2:
The patent changes the electrical parameters at the amplifier input by introducing capacitive coupling. This transforms the DC coupled sensing approach into an AC coupled approach, where only voltage variations (dV/dt) are amplified. By changing from direct voltage sensing to derivative voltage sensing, the system achieves high input impedance and eliminates common mode voltage constraints.
2Measurement precision
If conventional current sensing with amplifier is used, then output current can be sensed, but device complexity increases due to additional components and circuit requirements
Solution Approach 1:
The patent extracts and utilizes the parasitic resistance inherent in the output inductor as the sensing element, eliminating the need for a separate shunt resistor. By taking out only the essential capacitive coupling component and leveraging existing parasitic elements, the design reduces overall component count while maintaining sensing functionality. The approach removes unnecessary components rather than adding complex sensing circuits.
3Measurement precision
If shunt resistor is coupled in series into output path, then current sensing is achieved, but manufacturing precision requirements increase due to impact on output current
Solution Approach 1:
The patent employs the parasitic resistance of the output inductor, which is an inherent property of the existing component, to perform the current sensing function. This self-service approach eliminates the need for an additional precision shunt resistor, as the inductor's own parasitic resistance serves dual purposes: energy storage and current sensing. The solution leverages existing component characteristics rather than requiring additional precision components.
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 provides higher precision and reduced complexity in DC-DC conversion by overcoming common mode issues and improving matching properties, allowing for accurate detection of small voltage differences and efficient current sensing without additional components.
Implementation Method 1
A first input of the comparator can be capacitively coupled to the first node and a second input can be capacitively coupled to the second node for determining a magnitude of the output current
Implementation Method 2
The output current causes a voltage drop across the sense (or shunt) resistor RS which is coupled in series to the output inductor L
Implementation Method 3
The voltage difference between pins SA1 and SA2 is sensed with an amplifier AMP. The amplifier AMP amplifies the voltage difference and provides an output signal
Data Source
AI summary
An electronic device for switched mode DC-DC conversion is provided that includes a stage for sensing an output current causing a voltage difference between a first and a second node. The current sensing stage includes a comparator being capacitively coupled with a first input to the first node and with a second input to the second node for determining a magnitude of the output current.