Dry information | Two current detection circuit design solutions

For most applications, current is measured by sensing the voltage drop across a resistor. Circuit detection circuits are commonly used in current detection scenarios such as high-voltage short-circuit protection, motor control, DC/DC converters, system power consumption management, secondary battery current management, and battery management.

Generally, a resistance value with a voltage drop of tens to hundreds of mV when the current passes through is used. A low resistor for current detection uses a smaller resistance value of several Ω or less. When detecting a large current of tens of A, a very small resistance value of several mΩ is required. Resistance value, therefore, metal plate type and metal foil type low resistors, which are known for their small resistance values, are more commonly used, while small currents are detected through larger resistance values ​​of hundreds of mΩ to several Ω.

When measuring current, resistors are usually placed in two places in the circuit. The first position is between the power supply and the load. This measurement method is called high-side sensing. A common second place to place a sense resistor is between the load and ground. This method of current sensing is called low-side current sensing.

The current sensing resistor of the low-side current sensing circuit is connected in series to the ground, while the sensing resistor of the high-end current sensing circuit is connected in series to the high voltage terminal. Both methods have their own characteristics: the low-end current-sensing method adds additional wirewound resistance in the ground loop, while the high-end current-sensing method has to deal with larger common-mode signals.

As shown in the figure, the low-end current-sense op amp uses the ground level as the reference level, and the current-sense resistor is connected to the positive-phase terminal. The main disadvantage of low-side current sensing is that the voltage drop across the sensing resistor will be different when using the power supply ground and the load and system ground. Problems may arise if other circuits are referenced to the power supply ground. To minimize this problem, all circuits that interact should be referenced to the same ground. Lowering the current sense resistor value will help minimize ground drift.

As shown in the figure above, if the GND pin of the op amp in the figure is based on the positive terminal of RSENSE, then its common mode input range must cover below zero, that is, GND- (RSENSE × ILOAD). Rsensor separates ground (GND).

Although the low-side current-sense circuit is relatively simple, there are several fault conditions that the low-side current-sense circuit cannot detect, which can put the load in a dangerous situation. These problems can be solved with the use of high-end current-sense circuits.

The high-end current detection circuit is directly connected to the power supply terminal and can detect any faults in the subsequent circuit and take corresponding protective measures. It is especially suitable for automatic control applications because the chassis is usually used as the reference ground in these application circuits. With the introduction of a large number of ICs containing high-precision amplifiers and precision matched resistors, the use of differential amplifiers in high-side current measurements has become very convenient. High-side detection has driven the development of current detection ICs, reducing problems such as parameter changes and too many devices caused by discrete devices. Integrated circuits make it easier for us to use.

There are many implementation solutions for traditional high-end/low-end current sensing methods, most of which are based on discrete or semi-discrete component circuits. High-end current-sensing circuits usually require a precision operational amplifier and some precision resistors and capacitors. The most commonly used high-end current-sensing circuit uses a differential operational amplifier for gain amplification and shifts the signal level from the high-end to the reference ground (as shown in the figure):

VO=IRS*RS;R1=R2=R3=R4 This scheme has been widely used in actual systems, but this circuit has three main shortcomings:

The input resistance is relatively low, equal to R1;

The input resistance at the input end generally has a large error value;

The matching of resistors is required to ensure acceptable CMRR. A 1% change in any resistor will reduce the CMRR to 46dB; a 0.1% change will bring the CMRR to 66dB, and a 0.01% change will bring the CMRR to 86dB. High-end current sensing requires high measurement skills, which promotes the development of high-end current sensing integrated circuits. There seems to be no corresponding progress in low-end current sensing technology.

Source: WeChat public account hardware engineer

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