Current sensing is used to perform two basic circuit functions. First, it is measuring how much "current" flows in the circuit. This information can be used for power management in a DC/DC power supply to determine the basic peripheral load to achieve energy savings. The second function is to make a judgment when the current is "too big" or a fault occurs. If the current exceeds the safety limit and the software or hardware interlock condition is met, a signal is sent to turn the device off, such as a motor stall or a short circuit in the battery. It is therefore necessary to choose a technique that can withstand the robust design of extreme conditions in the fault process. Performing measurement functions with appropriate components not only provides accurate voltage signals, but also prevents damage to the printed circuit board.
Measurement methods
There are a variety of different measurement methods that can generate a signal indicating "how big" or "too big", as follows:
Resistive (direct)
Current-sense resistor.
Magnetic (indirect)
Current Transformer;
Rogowski coil
Hall effect device.
Transistor (direct)
RDS(ON);
Ratio type.
Each method has its advantages, is an effective or acceptable current measurement method, but it also has its own advantages and disadvantages, which is critical to the reliability of the application. These measurement methods can be divided into two categories: direct, or indirect. The direct method means that it is directly connected to the circuit under test. The measuring component is affected by the line voltage. The measuring component of the indirect method is isolated from the line voltage. It is necessary to use an indirect method when the safety of the product is required.
Resistive
Current-sense resistor
Measuring current with a resistor is a straightforward method with the advantages of simplicity and good linearity. The current-sense resistor and the measured current are placed in a circuit, and the current flowing through the resistor converts a small portion of the electrical energy into heat. This energy conversion process produces a voltage signal. In addition to its ease of use and good linearity, the current-sense resistor is also cost-effective, with a stable temperature coefficient (TCR) of 100 ppm/°C or less, or 0.01%/°C, without potential avalanche multiplication or thermal runaway. Impact. Also, low-resistance (less than 1mΩ) metal alloy current-sense resistors have excellent surge immunity and provide reliable protection in the event of short-circuit and overcurrent conditions.
magnetic
Current Transformer
The current transformer (Figure 1) has three distinct advantages: it is isolated from the line voltage, non-destructively measuring current, and the large signal voltage is well protected against noise. This method of indirectly measuring current requires the use of varying currents, such as alternating current, transient current or switched direct current, to produce a varying magnetic field that is magnetically coupled into the secondary winding. The secondary measurement voltage can be scaled according to the turns ratio between the primary and secondary windings. This measurement method is considered to be "lossless" because the resistance loss of the circuit current through the copper winding is very small. However, as shown in Figure 2, the loss of the transformer can result in the loss of a small fraction of energy due to load resistance, core loss, and the presence of primary and secondary DC resistance.
Figure 1. Ideal current transformer circuit 
Figure 2, composition of current transformer losses
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