Smart Current Sensing and monitoring technologies
Smart current sensing
The most common method of protecting devices against overcurrent involves the use of an inline fuse. By principle, fuses are resistors. They heat up and melt whenever current flow leading to an open circuit. However efficient the fuse seems, it is a very crude solution. Sometimes replacing it is time-consuming and sometimes very expensive mostly in remote locations. This calls for a more sophisticated solution that can monitor the current in real time so as to detect potential overcurrent situations before they happen and take preventive measures. This is called smart sensing.
Smart current sensing and monitoring techniques
There are numerous techniques for sensing and monitoring current. These techniques are based on the magnitude and type of current being measured. These techniques can predominantly be categorized into two:
- Direct measurement
- Indirect measurement
In direct measurement, the current being measured has to flow through the measuring device. The measuring device can either be a shunt resistor or a transistor. Due to this, direct measurement is a low-cost that is very accurate. There two ways of measuring current directly, they include;
This is the simplest approach to measuring current directly. It is simple and linear. The measurement of current is based on Ohms law (V=IR). There are numerous resistors of this nature, a good example is the WSBBS818L100JK.
There are also shunt resistor ICs that can be used to detect overcurrent. The INA300 is a good example, it is a product of Texas Instruments and it operates from 5 V supply. It cannot accommodate a common voltage of up to 36 V. Its voltage threshold is adjustable and can be set by a DAC or an external resistor. The chip has a response time is between 10μs and 100μs. These ICs are readily available off the shelf.
The tolerance of the shunt resistor directly affects the accuracy of the overcurrent detection. The larger the resistor value, the higher the magnitude of the signal and the more the heat and the need for heatsinks.
Current sharing MOSFETs are used in high current applications. A current sensing MOSFET is a low-loss method of current measuring. It is composed of thousands of identical transistor cells that are connected in parallel so as to minimize the overall resistance hence its low loss nature. The IXTN60N04T4 is a good example of a current sensing MOSFET. They are also readily available over the shelf.
Indirect current measurement provides galvanic isolation between the conductor carrying current and the device tasked with measuring the device. In indirect measurement actually current-related parameter like magnetic field induced by current passing through a conductor. This technique is highly accurate but is quite complex and expensive to implement hence is rarely used.
There are numerous solutions for short-circuit protection hence designers have numerous options to choose from. While picking the method to use, designers need to consider the BOM and TCO so as to establish if the cost is worthwhile over the lifetime of the product.
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