Introduction

The ever-growing electric vehicle (EV) market presents many opportunities to implement semiconductor technology, benefiting the vehicle's performance while assuring consumer safety. These vehicles are integrating innovative electronic solutions in the form of EV charging routines such as on-board chargers (OBC) and DC fast charging. In the realm of the analog signal chain, the performance of the vehicle’s charging systems is measured through various methods, including voltage regulation, current sensing, and safety & control. Though the architecture found in these systems is comprised of more than a few components, this brief covers a particularly significant IC: the operational amplifier.

DC Fast Charging

DC fast charging represents the peak of power delivery to an electric vehicle by providing a high-voltage AC supply (up to 800V) to the vehicle's power stage which is first converted to 800VDC. This high-voltage direct current is used to charge the vehicle's batteries. A DC/DC converter is then used to lower the voltage to around 400V, bypassing the on-board charger to charge the battery of an EV.

To ensure efficiency of the system, both the input and output of the power stage is monitored. Operational amplifiers serve the role in the monitoring of the voltage/current within the AC/DC and DC/DC power stages of DC fast-charging applications to regulate the delivery of voltage to the vehicle’s battery. If there is an inconsistency observed between these two voltages, the system will course correct through the power digital processing unit. There are two key methods to monitoring such a system with op amps.

A resistor voltage divider can be implemented to step down the voltage of the high-side rail. An amplifier, in this case OPA4991-Q1, reads in a voltage at a suitable level relative to its input, which it can then relay to the power digital processing unit. Monitoring a voltage rail through this method is cost-effective, though this method tends to consume a higher amount of power due to the resistor divider.

On the other hand, an isolated amplifier can be used to directly monitor a high-voltage battery rail, such as TI’s AMC1301-Q1 precision isolated amplifier. Here, the isolated amplifier steps down the higher voltage, relative to the input, to a 3.3V differential-ended output. A low-voltage amplifier such as TLV9001-Q1 takes in the differential-ended signal and produces a single-ended signal to monitor the voltage. In this case, the amplifier acts as a signal conditioner before the Analog-to-Digital Converter (ADC). Interfacing a Differential-Output (Isolated) Amp to a Single-Ended Input ADC provides more information on interfacing a differential-output amplifier to a single-ended input ADC.

On-Board Charging Systems

The On-Board Charging System (OBC) in an electric vehicle is the intermediate system between the external charging source and the battery pack located inside the vehicle which drives the powertrain. The OBC provides many benefits including the ability to use an AC power source through an integrated converter and offers greater convenience by protecting battery life.

The OBC effectively converts the voltage and current from an AC source to DC power which charges the battery. Because the output of the AC/DC converter in the OBC system ranges from 250 to 900 VDC, non-isolated operational amplifiers may be used to directly sense the current of the system.

There are three different methods to sense current in OBCs: in-line, inverter leg current sensing and DC-link single shunt, each with their advantages and challenges as shown in Table 1.

In-line shunt current sensing allows you to directly monitor the phase of each leg. Because the common-mode voltage is equal to that of the DC bus voltage (generally greater than 100V), an amplifier with a high common-mode input voltage range is required; therefore, an isolated amplifier is generally preferred. As mentioned in the DC fast charging application, an operational amplifier (such as TLV9001-Q1) is typically needed for differential-to-single ended conversion when interfaced with a single-ended input ADC, in order to maintain its full dynamic range & offset any introduced error.

Monitoring each individual inverter leg uses a low-side current sensing topology that can be accomplished without the need of an isolated amplifier as the common-mode voltage of each leg is close to zero. There are three methods of accomplishing low-side current sensing; one-, two-, or three-shunt topologies. While single-shunt measurement techniques tend towards higher bandwidth requirements, three-shunt solutions call for lower speed, general-purpose amplifiers such as TLV9061-Q1, as you are able to monitor each leg individually. An important requirement for accurate current sensing in OBC systems is ensuring the settling time is as short as possible which is why the TLV9061-Q1, a 10-MHz unity gain-bandwidth amplifier (1 µs settling time), is recommended for this application to quickly respond to changes in current.

OBCs also have the ability to operate bidirectionally, meaning power can be sent to and from the vehicle. Bidirectional charging allows the on-board charger to convert stored DC power to operable AC power to be delivered to the grid, another vehicle, or different power electronics. Moreover, single-phase chargers have a low carrying capacity (as opposed to three-phase OBCs), a specification which affects charging time. This guide on low-side, bidirectional current sensing provides a reference on how to build your circuit using a low-voltage amplifier. TI's TLV906x-Q1 family of amplifiers can be implemented in two ways for low-side bidirectional current sensing. A dual-channel, space-optimized solution can be used, with one channel for reading in voltage and the other dedicated as a reference buffer. On the other hand, two single-channel amplifiers can be used in cost-optimized designs, where TLV9001-Q1 can be used as a reference buffer in conjunction with the faster TLV9061-Q1.