Current Sensing Measurements in Automotive Systems

Modern automotive electrical system design is currently undergoing one of the most dramatic changes in history. From revolutionary motor/generator hybrid electric propulsion and “fly-by-wire” electric transmissions to smart accessories for extended life and efficiency (such as beltless pumps and LED lighting), they are all being integrated into new vehicles. The increasing expectation of users to have automated on-board diagnostics and predictive maintenance features has also promoted the emergence of various new body and engine management system designs. An important piece of information feedback in many of these system redesign areas is the current used by a particular load. Current measurement is used to analyze whether the state is normal and provide a basis for fault protection and control law implementation. The fundamental change in this area is that smart and efficient “closed-loop” designs are replacing the traditional “open-loop” systems of the past.

Basic Current Sensing Topologies

Although non-contact current measurement is possible, this method generally requires high-cost instruments or expensive power supply units, so it is only used when cost and complexity allow. In the automotive field, low cost is a key factor, so the detection resistor measurement method is the most suitable. By connecting a small resistance detection resistor (milliohm level) in series to the load and measuring the voltage drop across the resistor when power is supplied to the load, the current value can be accurately calculated.

There are essentially six different topologies for the series connection of the switch, load, and sense resistor, as shown in Figures 1(a) through 1(f). These topologies can be categorized as high-side switching or low-side switching based on the position of the switch relative to the load; and as low-side sensing, "floating" sensing, or high-side sensing based on the position of the resistor relative to the power rail. Each may be the best solution for certain applications. Another consideration is when a fault occurs, which can vary depending on the characteristics of the load. As a rule of thumb, it is generally assumed that the most likely fault is a connection to the chassis (electrical ground), either caused by a wrench touching a live bare terminal or a frayed wire contacting a grounded metal part. In this case, low-side sensing has inherent disadvantages. In most applications, the configuration of Figure 1(c) is the preferred topology because it allows the switching and monitoring functions to be centralized while keeping the number of wires small.

Modern loads and intelligent switches

Since the introduction of power MOSFET devices, designers have been looking at them as potential replacements for relays. Modern N-MOSFET switches have on-resistance values ​​in the single-digit milliohm range, allowing the use of standard surface mount technology without cumbersome heat sink structures. Low-cost integrated circuit solutions have been developed that provide self-contained boost gate drive functions. These circuits also incorporate fast fault protection mechanisms so that the MOSFET is never at risk of failure. The LT1910 from Linear Technology is one such "smart switch" control IC that uses a low-resistance high-side current sense resistor (similar to Figure 1(c)) to detect circuit overloads and shut down the operating MOSFET before damage occurs. Once the IC detects an overload condition, it sets a warning flag and periodically attempts to restart the load until the fault is cleared. Although this IC is binary in nature, it is a good example of using current sensing to form a robust "closed loop" electronic relay solution as shown in Figure 2.

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Real-time current monitoring

In addition to providing intelligent switch protection, current sensing also allows the signal on the sense resistor to be digitized after amplification and conversion, and the digitized signal can be used as an "analog" feedback signal for the control loop. Current monitoring can reveal many operating characteristics of the load in real time. For example, the current consumed by the motor is proportional to its torque, so the trend of bearing friction resistance can be calculated, and the status of various starters can be detected without the need for additional sensors. Other loads (such as lighting) are often driven in parallel with a common power supply, so it is only a matter of accuracy to determine whether some parts of the load have failed to open the circuit at the end of their life.

A particularly simple integrated circuit solution to achieve the above function is a current sense amplifier, and Linear Technology's LTC6102 is an example of such an integrated circuit, which is optimized for accurate unidirectional high-voltage terminal automotive sensing. Figure 3 shows a typical circuit example of connecting a general-purpose current sense output to an analog-to-digital converter (ADC) input using the LTC6102. Note that the output of the LTC6102 is a current, so the reconstruction load (R2) can be placed at a distance from the integrated circuit without introducing ground loop errors. Because of the extremely high accuracy of the integrated circuit, even sub-milliohm RSENSE values ​​are practical, so heat and voltage losses are minimized. The added components D1 and R3 in this circuit provide protection against reverse supply transients. Table 1 lists some of the available sense amplifiers and their basic characteristics.

Factors to consider when using pulse modulated loads

For duty-cycle modulated loads that use high-frequency pulse-width modulation (PWM) techniques to produce variable performance levels, there are other factors to consider when designing current monitoring circuits. Chief among them is that the response time needs to be fast enough to respond to fault conditions during the on-portion of the waveform. Another point is that the switching action should not interfere too much with the fidelity of the current reading. In general, the configuration of Figure 1(c) again provides the best results because the impedance of this circuit is low and common-mode problems are minimized. In cases where the average load current (DC component) is desired, post-filtering used in the analog or digital signal processing (DSP) fields can be used to remove the frequency components related to PWM. It is expected that the average supply current value will be related to the load current, and this value provides a good indication of the subjective effect, whether it is the intensity of the lamp or the starting force.

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Monitors the H-bridge driver current

An H-bridge driver can be viewed as a pair of half-bridges operating with complementary signals to produce bidirectional differential outputs. Each half-bridge can be viewed as an extension of the unidirectional circuit of Figure 1(c) with the addition of a low-side switch in parallel with the load. Figure 4 shows a circuit using an LTC6103, with the two devices producing differential outputs suitable for driving an ADC directly. Circuits like this are suitable for motors in mechanisms such as window lifts, climate control, and wherever reverse action is required.

Note that for a load-to-ground fault, the low-side MOSFET is not overstressed, so monitoring each half-bridge on the high-side provides all the information needed. The load current can be determined from the difference in the unidirectional current readings of the two half-bridges. Also, because of signed magnitude control, no duty cycle correction is required to accurately measure the load current when one high-side switch is 100% on.

Conclusion

Electronic drive functionality is proliferating in modern automotive development. Economical control designs require ruggedness but add diagnostic functionality to monitor the load current in the system in a closed loop. Whether the driver is single-ended or H-bridge type, high-side current sensing is the most practical way to implement the monitor function. The LT6100 series offers a rich selection of current sense amplifiers, and this series of integrated circuits can meet the specific needs of multiple applications, such as component accuracy/efficiency, operating voltage, high temperature operating monitoring solutions, and economical and practical high-side monitoring solutions.