Switch Current Measurement Using AC Superposition
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Solution Overview
Problem
Measuring current through a switch with unknown internal resistance is challenging due to dependence on temperature and scatter, especially in parallel connections, where correlation between measured temperature and actual switch temperature is poor, leading to inaccurate compensation.
Innovation Solution
An AC source with known amplitude and frequency is connected in parallel to the switch, superimposing an alternating current on the measured current, allowing for the determination of AC and DC voltage components, which enables the calculation of current independently of switch resistance, using a measuring circuit with filters and a gate signal generator to modulate switch resistance and determine internal resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If temperature compensation using calibration tables is used, then measurement accuracy is improved, but the correlation between measured temperature and actual switch temperature is poor, leading to inaccurate compensation
Solution Approach 1:
An AC test current is introduced as an intermediary signal to indirectly determine the switch's internal resistance. By measuring the AC voltage component generated by this test current and calculating R_sw = U_ac / I_ac, the method obtains an accurate representation of the switch resistance without relying on temperature sensors or calibration tables, thus resolving the poor temperature correlation issue.
Solution Approach 2:
The patent replaces the mechanical/thermal measurement approach (temperature sensors and calibration tables) with an electrical measurement approach. Instead of measuring temperature and looking up compensation values, the system directly measures the electrical property (internal resistance) through AC voltage measurement, eliminating the intermediate temperature correlation step.
2Adaptability or versatility
If the internal resistance of the switch is unknown, then the switch can be used in various applications, but the measurement results have significant dependence on the internal resistance
Solution Approach 1:
The switch itself provides the means to determine its own internal resistance. By applying an AC test current through the switch and measuring the resulting AC voltage component, the system enables the switch to self-characterize its resistance value (R_sw = U_ac / I_ac), eliminating the need for external calibration or known resistance values.
Solution Approach 2:
The patent changes the measurement parameter from DC voltage (which depends on unknown resistance) to AC voltage component at a specific frequency. By measuring the AC voltage component U_ac generated by the AC test current I_ac, the system can calculate the internal resistance and use it to accurately determine the DC current, thus resolving the measurement precision issue.
3Power
If multiple switches are connected in parallel, then the current capacity is increased, but the correlation between measured temperature and actual switch temperature becomes even poorer, making compensation more difficult
Solution Approach 1:
The patent applies segmentation by treating each parallel-connected switch as an independent measurement unit. The AC test current is applied individually to each switch, and the AC voltage component is measured separately for each switch. This allows each switch's internal resistance to be determined independently, eliminating the temperature correlation problems that arise in parallel configurations.
4Measurement precision
If the measuring circuit measures voltage difference at the switch, then the current can be determined, but the measurement results vary due to scatter and environment dependent parameters
Solution Approach 1:
The patent employs periodic action by superimposing an AC test current with a specific frequency on the DC current through the switch. By measuring the AC voltage component at this known frequency, the system can distinguish the switch's resistance contribution from other environmental variations and scatter effects, thereby stabilizing the measurement against external disturbances.
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 method reduces the influence of temperature and scatter on measurement results, providing accurate current measurements by implicitly determining switch resistance and compensating for changing parameters, suitable for various applications including battery charging and discharge currents.
Implementation Method 1
the internal resistance of the switch can be determined on the basis of the amplitude of the AC voltage component, assuming a linear switch resistance
Implementation Method 2
the measuring circuit has, for this purpose, at least one low pass filter for determining a DC voltage component of the voltage present at the switch
Data Source
AI summary
A method and a device for measuring a current flowing through a switch which has an unknown inner resistance and two connections, a voltage difference being measured at the switch. During operation, the current provided by an AC voltage source, which is part of an AC voltage circuit connected in parallel with the switch, is superimposed on the current to be measured, the current flowing through the switch, by way of the AC voltage source. Both the amplitude and the frequency of the current provided by the AC voltage source are known. An AC voltage component of the voltage difference and the amplitude of the component are ascertained, and the current between the connections is ascertained and output proportionally to the amplitude of the current of the AC voltage source.