This post was co-authored by Ryan Small.
The rapid increase of semiconductor content in automotive systems has prompted a need to manage key voltages and currents in each subsystem. Supervising supply voltages, load currents or other important system functions can help indicate fault conditions, prevent catastrophic failures and protect end users from potential harm.
Hybrid electric vehicles (HEVs) and electric vehicles (EVs) have some distinct challenges compared to conventional combustion-powered vehicles. HEV and EV vehicles feature numerous systems that require monitoring voltages and currents, including the on-board charger (OBC), battery-management system (BMS), DC/DC converters and inverters. In this post, we’ll discuss the three fundamental circuits for monitoring current and voltage in HEV/EV systems: low-side current sensing with a difference amplifier (DA), in-line isolated current sensing and high-voltage sensing using an attenuating DA.
Figure 1 is a simplified HEV/EV block diagram showing common systems that require voltage and current monitoring.
Figure 1: Typical HEV/EV system
While we won’t cover the operation of the subsystems in this post, designers sometimes use discrete DAs for low-side current sensing (as shown in Figure 2), particularly when the load current is bidirectional or they need a small reference voltage to pedestal the output to a known value greater than the amplifier’s minimum output swing. For the printed circuit board (PCB) layout, you should Kelvin-connect the shunt resistor. It is important to use a device with a common-mode voltage range that includes ground, such as the LM2904-Q1 or TLV313-Q1.
Figure 2: Difference amplifier for low-side current sensing
Equation 1 gives the transfer function for this circuit, assuming R4 = R2 and R3 = R1. The amplifier for this application usually has a common-mode range that includes ground. Depending on the situation, however, the common-mode voltage may not be near ground because it actually depends on the voltage divider created by voltage across the shunt resistor (ILoad*Rsh), Vref and the ratio of R3 to R4. Make sure that the gain (R2/R1) is such that the output-voltage swing does not saturate.
Figure 3 depicts a high-side, in-line isolated current-sensing solution. The AMC1200 is a differential-in, differential-out isolated amplifier. Isolated amplifiers help protect low-voltage circuitry from high common-mode voltages. The DA converts the differential signal to single-ended, which an analog-to-digital converter (ADC) can then digitize.
Figure 3: Difference amplifier used for differential to single-ended conversion, isolated in-line sensing
Equation 2 gives the transfer function for the circuit shown in Figure 3, assuming R4 = R2 and R3 = R1. The gain of the AMC1200 is 8V/V. Vref typically connects to V2/2 to bias the output to mid supply to support bidirectional current. A rail-to-rail input/output (RRIO) amplifier such as the TLV313-Q1 is a good fit for this application.
Figures 4 and 5 depict an attenuating DA and the corresponding TINA-TI™ simulation. In this circuit, the DA resistors divide the input voltage (±100V) down. Since the input voltage is bipolar, a reference voltage set to mid supply biases the output. When selecting the amplifier, be sure to comply with the device’s input common-mode and output-voltage swing specifications. This example utilizes the OPA314-Q1 because of its wide common-mode range.
Figure 4: Attenuating difference amplifier
Figure 5: TINA-TI software simulation of attenuating difference amplifier
Equation 3 gives the transfer function for the circuit shown in Figure 4, assuming R4 = R2 and R3 = R1. Notice that the gain is less than unity.
In this blog post, I presented three common circuits that appear in automotive subsystems such as the OBC, BMS, DC/DC converters and inverters. Additionally, I highlighted that HEV/EV applications use voltage and current sensing to monitor critical system functions, helping to maximize vehicle lifetimes. To learn more about the key operational amplifier specifications mentioned in this blog (for example, common-mode voltage) check out TI Precision Labs.
Who uses VCRs anymore? Voltage Coefficient of Resistance (VCR) is a rarely thought-about parameter of resistance. Depending on the resistor used, the attenuating difference amplifier can be off by 3% if the same 20M resistors are used that I use in one application versus the 100k used in Figure 4. Measuring high voltages means encountering VCR. Measuring 100V with 100k resistors means burning 1/10W continuously; perhaps not a problem for HEV/EV applications but the technique can't be extended to lower current +/-100V monitoring without increasing the resistance. Higher resistance may lead to higher VCR. But since the parameter is rarely discussed, what do we really know about it?
Thanks for your comment!
With respect to voltage controlled resistors (which is what I think you mean when you ask "Does anyone use VCRs anymore?"), I'm assume people do. However, I can't recall seeing one used in conjunction with the applications and devices that I primarly support. Perhaps they're less common due to digital potentiometers, but I can't say for sure. I do believe that some people use FETs as VCRs. However, I assume one must ensure that the Rds(on) is linear and accurate. I also would be concerned with changes over temperature, if applicable. Sorry I can't be of much more help on this topic.
Concerning the parameter VCR with respect to fixed value resistors...you have a good point. It is rarely discussed. Our general rule of thumb is to use lower value resistors, but for other reasons like noise, op amp stability, and interaction with input bias current. Perhaps we can add VCR to the list.
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