Introduction

As the automotive industry evolves towards manufacturing hybrid and plug-in electric vehicles, the introduction of highly efficient systems such as electric braking, electric steering, and motor control is becoming increasingly important to reduce battery consumption and energy costs. Current monitoring is extremely useful from a control and protection perspective in automotive applications such as battery chargers, LED lighting, and inverter drives, especially in hybrid and electric vehicles.

To improve efficiency in both AC and DC applications, it is necessary to properly monitor the current in each of these systems and perform feedback control. By selecting the right current sensing feedback device for your application, you can further improve system efficiency.

Current sense resistors are a cost-effective solution because they are easy to integrate into circuit designs and occupy little space. For example, Figure 1 shows a three-phase inverter configuration using current sense resistors to measure the motor winding current.

Figure 1: Three-phase inverter motor drive with current-sense resistors for measuring winding currents. (Courtesy of Bourns)

This article covers the following topics:
・Outline of various technologies used for current detection
・Location of current detection resistor in converter
・Thermal calculations required to select an appropriate resistor
A strategic approach to current sensing

Current Monitoring

There are various methods for monitoring electrical current, including intrusive and non-intrusive techniques. Intrusive current monitoring involves connecting a measuring device in series with the device being monitored, while non-intrusive measurements are made using inductive or Hall-effect devices placed near the wiring supplying the controlled device. See the table below for the advantages and disadvantages of various current sensing methods.

As shown in the table above, current-sensing resistors offer multiple benefits in power supply and inverter control designs. Sensors used in contactless technology are prone to drift, making it difficult to maintain accurate control over the wide operating temperature ranges typical in automotive applications. Designers are introducing new digital ICs that can distinguish very small signals, reducing the dynamic range to less than 30mV. This trend is driving demand for resistors with lower resistance values, helping to reduce power losses and improve system efficiency.

Location of current sensing resistor in converter

In a voltage regulator module (VRM), the current measurement of the power inductor is combined with the output voltage of the VRM controller. These two feedback signals form part of the power supply control loop and are essential for maintaining stable voltage regulation against load variations. The measurement circuit and inductor waveforms are shown in Figures 2 and 3. The resistor can be placed at the output, where the signal is stable and does not require special filtering by the controller. However, continuous current flow through the current-sensing resistor causes efficiency losses that may be unacceptable in some applications.

Another option is to place the resistor on the upper or lower FET (Field Effect Transistor) side. This side is difficult to measure because the signal is noisy and most of the current signal does not contain useful information for the controller. However, VRMs usually have a large step-down ratio, so the inefficient time caused by the resistor is shorter than on the output side. Another problem with placing the resistor on the FET side is that parasitic elements in the converter circuit can cause overshoot in the current signal, making it difficult for the controller to read.

Figure 2: Synchronous buck converter with current-sense resistors on the high and low sides (Courtesy of Bourns)

Figure 3: Current waveforms in the inductor and high-side resistor. (Image courtesy of Bourns)

Using the example of a synchronous buck converter, we can analyze the efficiency of different current sense resistor locations. In this example, we assume the following key parameters for the post-inductor resistor configuration:

・ ​_Iout = 30A _DC, Vout = 5V _DC
・Duty ratio = 20 %
・Controller input V _peak = 100mV
・ R _sense = 3mΩ (calculated value)
・Power consumption at room temperature = 2.7W (at full power)
This configuration results in an efficiency loss of approximately 1.8 %.

Under ideal conditions, the power dissipation is 2.7W. Let's assume we select a resistor rated at 4W (e.g., Bourns® CSS2H-2512). To ensure component reliability and increase MTBF, it is important to keep the device surface temperature as low as possible even under worst-case operating conditions (maximum load and maximum operating temperature). This is based on the Arrhenius law, which states that time to failure is a function of absolute temperature. In fact, a 10°C reduction in operating temperature can double the MTBF. If the current sense resistor is placed in series with the high-side FET, the following calculations can be performed:
・ I _average = 6 ADC
・ R _sense = 15mΩ​
・Power consumption = 0.5W (efficiency reduction of approximately 0.33 %)

Placing the resistor on the high side of the circuit has obvious efficiency benefits. To calculate the resistor's surface temperature, you first need to find its thermal resistance from the datasheet. Then calculate the thermal resistance of the external environment (e.g., solder pads) where the resistor will be mounted.

Thermal resistance of the component

We assume that the datasheet parameters are taken under ideal operating conditions, so the thermal resistance is:

Solder pad thermal resistance

Based on the recommended solder pad dimensions given in the datasheet, the thermal resistance of the pad is:

Figure 4: Illustration of solder pad dimensions used to calculate thermal resistance (Courtesy of Bourns)

Therefore, the thermal resistance of each solder pad is 20°C/W. There are two solder pads, each of which plays a role in dissipating heat. This is similar to the relationship between two electrical resistors connected in parallel to a current source. Therefore, the combined thermal resistance of the entire solder pad is halved to 10°C/W.

This results in a thermal resistance of 35°C/W for the entire system (resistor + ambient). The temperature rise at full power is 94.5°C, and the maximum ambient temperature the resistor can operate at full power is 75.5°C. Above this temperature, the resistor's surface temperature exceeds its design limit, causing a rapid decrease in MTBF according to Arrhenius law.

If your controller can distinguish between high switching voltages and the relatively small difference between low and peak currents, you can use a smaller resistor (e.g., a Bourns® CRF1206-FZ-R012ELF). According to the datasheet, this resistor has a thermal resistance of 100°C/W. The solder pads each have a thermal resistance of 33°C/W. Therefore, at full power (0.5 W), the temperature rise is 58°C, allowing for a maximum operating temperature of 112°C.

In this application, the inductance of the component is very important. The controller switches the input voltage across a resistor at a relatively high frequency. Models in the CRF1206 series have a parasitic inductance of less than 5nH. The voltage induced across the resistor at the beginning and end of each switching period is calculated as follows:

In this case, the induced voltage depends on the rise time of the current when the FET turns on. The controller must implement a delay to avoid reading the input voltage for a certain period of time. This is commonly called the "leading-edge blanking time." However, if the induced voltage is too high, it can damage the controller, so a very low parasitic inductance, such as 5nH, is important.

The buck topology shown in Figure 1 uses FET transistors for controlled startup to limit the inrush current flowing into the output capacitor after switch-on. However, inverter drive circuits, for example, use large DC-link bulk capacitors, which generate large current pulses depending on the capacitor's capacitance. Therefore, current sense resistors must be able to withstand short-term overloads. The metal CRE series current sense resistors offer excellent load tolerance, depending on the model rating. For example, the 3W/5mΩ CRE series can withstand a 100W pulse load for 20ms, while the 3W/1mΩ CRE series can handle a 10W pulse for up to 2s.

Current Sense Resistor Product Lineup Overview

The table below summarizes Bourns' portfolio of surface mount current sense resistors.

Bourns current sense resistors are manufactured using metal alloy, electron beam welded, and thick film technology and are available in a wide range of resistance values, from 0.2 mΩ and 9 W power ratings to 1 Ω and 0.25 W power ratings. Bourns' extensive product portfolio addresses a variety of applications, from high current automotive battery chargers to battery detection in low power consumer devices.

The resistor can be connected in series with the load, as shown in Figure 1. This configuration is known as "high-side sensing." However, there are other configurations for connecting the resistor, which are shown in the diagrams below.

Figure 5: Low-side current sensing *Courtesy of Bourns

Figure 6: Low-side current sensing – Kelvin configuration (Courtesy of Bourns)

Figure 7: High-side current sensing *Courtesy of Bourns

Current Sensing Strategies

■High-side current detection
This sensing method uses a Kelvin configuration to monitor current in applications such as single-phase and three-phase DC motors and high-power loads. The circuit can be placed in series with the load drive FET and can be monitored using any amplifier circuit. The additional circuitry can be easily integrated into the motor drive design. A two-terminal current sense resistor can be placed on the PCB or at the load side, allowing for remote current sensing applications.

■ Low-side current detection
High-power current sensing applications typically require a common-mode, zero-drift operational amplifier (op amp) along with a Kelvin-configured or four-terminal current sense resistor. These sensing circuits can be used to monitor loads with both negative and positive DC voltages. For low-power current sensing, a two-terminal device can be used in conjunction with a non-inverting operational amplifier (op amp). This type of circuit is used to monitor battery charge in small appliances such as alarm panels, power tools, and various uninterruptible power supplies (UPS).

Summary

While there are several excellent methods for sensing current, this article has shown that current-sensing resistors are an ideal solution for modern automotive power supplies, inverters, and battery charging systems. As mentioned above, when selecting these components, it is important that designers carefully consider the operating conditions and resistor placement.

Bourns continues to innovate its resistor product lineup, including the addition of electron beam welded metal alloy current sense resistors. These resistors can be used in high current applications such as automotive and industrial equipment. To provide the optimum current sense resistor for each application, Bourns' products range from low power, high resistance thick film chips to high power, ultra-low resistance metal alloy types.

Recommended related articles

• Thick film type resistor
• Metal alloy type/Electron beam metal alloy type resistor

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