Microcontroller Average Current Measurement Circuit
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Solution Overview
Problem
In switch mode power supply (SMPS) applications, measuring average current is challenging due to irregular current waveforms caused by advanced soft-switching techniques, which are difficult for Analog-to-Digital Converters (ADCs) to follow, especially when the current waveform does not reach zero, requiring complex methods like integration under the curve with large resistors and negative power supply rails.
Innovation Solution
A microcontroller with an integrated average current measurement circuit that includes a voltage-to-current converter, a sample capacitor, an ADC, and a sample voltage reset switch, along with optional features like programmable gain amplifiers and operational transconductance amplifiers, allows for efficient conversion of voltage to current and subsequent digital representation of the average current, enabling accurate measurement of complex waveforms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional peak current sensing is used, then the measurement is simple, but it cannot accurately measure average current when the waveform does not reach zero
Solution Approach 1:
The patent introduces an integrator circuit as an intermediary component between the current sense amplifier and the ADC. This integrator accumulates the current waveform over time, transforming the complex measurement problem into a simpler voltage measurement that directly represents average current, thereby achieving accurate measurement without requiring complex digital processing
Solution Approach 2:
The patent replaces complex digital signal processing methods with an analog integration approach. Instead of using digital algorithms to calculate average current from sampled waveforms, the system uses an analog integrator that continuously accumulates the current signal, substituting mechanical/mathematical computation with a physical integration process
2Measurement precision
If integration under the curve method is used with operational amplifier, then average current can be measured, but large resistance values are required which increase physical size
Solution Approach 1:
The patent modifies the integrator circuit parameters by using a transimpedance amplifier configuration with a feedback capacitor instead of a large resistance value. This changes the integration mechanism from resistance-based to capacitance-based, allowing accurate integration without requiring large physical resistors, thereby reducing component size while maintaining measurement precision
Solution Approach 2:
The patent uses a small capacitance value in the integrator feedback path instead of large resistance values. Capacitors can be made much smaller than equivalent resistance values, providing the same integration function with significantly reduced physical footprint, effectively replacing bulky resistors with compact capacitive elements
3Measurement precision
If integration method is used, then average current measurement is effective, but negative power supply rail is required which increases system complexity
Solution Approach 1:
The patent inverts the traditional integrator configuration by using a transimpedance amplifier with the feedback capacitor connected to the virtual ground of the operational amplifier. This inversion allows the integrator to operate with single-sided power supplies, eliminating the need for negative voltage rails while maintaining accurate integration functionality for complex current waveforms
4Speed
If ADC samples fast signal transitions, then current waveform can be captured, but the transitions are too fast for ADC to follow
Solution Approach 1:
The patent applies preliminary integration action before the ADC sampling process. The integrator circuit continuously accumulates the current waveform in real-time, so by the time the ADC samples the output voltage, the integration is already complete. This preliminary integration smooths out fast transitions and produces a slowly varying voltage that the ADC can accurately capture, effectively preprocessing the signal to match ADC capabilities
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 solution provides a simple and effective method to measure average current values in SMPS systems, overcoming the limitations of traditional peak current sensing and enabling precise calculation of average current without the need for large resistors or negative power supply rails, thus improving measurement accuracy and efficiency.
Implementation Method 1
a voltage-to-current converter having an input coupled to the external connection and adapted to convert the voltage to a current
Implementation Method 2
an output of the voltage-to-current converter may be coupled to a sample capacitor and the current therefrom charges the sample capacitor to a sample voltage during a measurement time period
Data Source
Figure 1(a)~2
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AI summary
The average of a complex waveform measured over a time period may be determined by first converting the complex waveform to a voltage, then converting this voltage to a current and using this current to charge a capacitor. At the end of the measurement time period the voltage charge (sample voltage) on the capacitor may be sampled by a sample and hold circuit associated with an analog-to-digital converter (ADC). Then the voltage charge on the sample capacitor may be removed, e.g., capacitor plates shorted by a dump switch in preparation for the next average of the complex waveform sample measurement cycle. The ADC then converts this sampled voltage charge to a digital representation thereof and a true average of the complex waveform may be determined, e.g., calculated therefrom in combination with the measurement time period.