In any electrical system, current is a vital parameter. Both electric vehicle (EV) charging systems and solar systems require detection of current levels in order to control and monitor power conversion, charging, and discharging. Current sensors measure current by monitoring the voltage drop across a shunt resistor or the magnetic field created by the current in a conductor.
Metal-oxide-semiconductor field-effect transistor (MOSFET) control schemes use current information to control PV inverter operation or to sense current on or off the AC output to protect components from overcurrent or fault events. There are many different types of current sensors available, and each technology has its own advantages and disadvantages. The most appropriate current sensor type for a specific application depends on several factors, including the system's power level, expected accuracy, and cost. This article explores which devices are suitable for sensing current in electric vehicle chargers and photovoltaic inverters.
Current sensing in electric vehicle chargers
In electric vehicle chargers, current sensors are used to measure the current at locations such as the input AC power supply, DC/DC converter, and output power supply to confirm whether the charger is correctly delivering AC power to the electric vehicle's on-board charger system, or DC power is delivered directly to the battery. Today, 400V batteries are moving toward 800V and higher voltages for greater power and faster charging.
In Level 1 and Level 2 chargers, the charger delivers AC power to the EV's on-board charger, which in turn converts the incoming AC power to more appropriate voltage and current levels for charging the EV battery. In domestic Level 1 and Level 2 chargers, current sensing typically does not require very high accuracy because the user is not billed. However, current information gives users an overview of current and power consumption through an app or the user interface on the charger. Figure 1 shows two Level 2 EV chargers and two charging EVs in a parking lot.
In a Level 3 EV charger, the charging infrastructure converts AC power to DC power to quickly deliver DC power directly to the battery, bypassing traditional on-board chargers and enabling ultra-fast EV charging at the charging station. Increased power capacity in EV chargers and batteries helps meet the need for fast charging and increased driving range. Current sensing can help control the charging process to ensure optimal and safe charging of batteries, extending the life of electric vehicles and battery systems.
In a Level 3 charger, the frequency of the switching signal is 50kHz to 100kHz, so a current sensor of at least 250kHz is required to obtain appropriate measurement data. In addition, propagation delay is also very important because the current sensor needs to be able to respond quickly to changes in the signal switching. Devices such as Texas Instruments' TMCS1123 have a maximum error of ±1.75% over temperature and lifetime without calibration, which drops to ±1.00% over temperature and lifetime after a single point calibration.
Because the TMCS1123 has high accuracy and speed in current information, these accuracy and speed specifications enable system engineers to eliminate DC blocking capacitors from isolated DC/DC converters, thereby helping system engineers save costs when designing Level 3 chargers. .
Current sensing in photovoltaic inverters
In PV inverter systems, current sensors are used to measure current in a variety of configurations, such as the AC and DC inputs of the inverter, DC/DC boost, DC/DC converters, and grid output, thereby aiding in monitoring and control power conversion process. Current sensing is performed on individual power rails in residential PV inverters, where voltage levels on the rails may be as high as 1,000VDC, but the voltage at the PV input is typically around 500V to 600VDC, and the grid input and output are up to 400VAC. The current sensing function can help optimize the PV inverter system, ensuring that the power levels and frequencies delivered on the grid output are reliable and appropriate so that all loads are within their safe operating area (SOA).
The switching signals in photovoltaic inverters are similar to those in electric vehicle chargers, with frequencies between 50kHz and 100kHz. Additionally, current sensors can be used for diagnostic purposes, such as monitoring solar panels for faults that might indicate loose connections or damaged panels. The TMCS1123 offers an enhanced operating voltage of ±1,100VDC, making it ideal for use with most string inverters. Figure 2 shows several examples of current and voltage sensing used in single-phase string inverters with the corresponding circuit portions marked in red boxes.
Current Sensing Design Considerations
Let’s take a look at some of the main considerations when selecting current sensors for electric vehicle charging systems and photovoltaic inverter systems:
• rated power. The current sensor (whether magnet-based, shunt-based or other technology) must be able to handle the operating current and voltage levels of the system. Designers must select appropriate techniques based on the system's inputs to ensure that current can flow into the system uninterrupted throughout its lifetime.
• Accuracy. Current sensors must be accurate enough to provide the intended control and monitoring functions to ensure the system operates as expected within the SOA. The high accuracy helps maintain high efficiency levels while reducing component count and any harmonics that may be injected into the grid due to noisy switching systems.
• Bandwidth. In switching systems, speed is an important parameter. The TMCS1123 offers a signal bandwidth of 250kHz and a propagation delay of 600ns, which gives the system enough speed to make appropriate measurements. TI is also developing more high-speed devices with similar mechanical dimensions. We observe that in our devices, the propagation delay decreases as the bandwidth increases.
• cost. When selecting a current sensor, one must weigh the cost of the sensor against the advantages it offers. While one-piece packaged Hall-effect current sensors are often limited to detecting current within a specific range, shunt-based systems are more flexible because you can choose the shunt resistor value based on system parameters.
Shunt-based current sensing technology
In electric vehicle charging systems, photovoltaic inverter systems, and other systems that require current sensing, the most common current sensing technologies are Hall effect current sensors and shunt-based current sensors.
Shunt-based current sensors are generally more accurate over the entire current range than Hall effect current sensors. When using stable amplifier technology or analog-to-digital converters (ADCs) and precision shunt resistors, shunt-based current sensors can achieve less than 1% accuracy over the entire current measurement range, operating temperature range, and lifetime. A shunt-based solution can be as simple as an op amp, a specially designed current sense amplifier (such as TI's INA241A), an isolated amplifier for higher voltages (such as TI's AMCS1300B), or with a digital output Σ-Δ modulator (such as TI's AMCS1306). This type of amplifier is typically used to monitor the voltage drop across a shunt resistor and provide a proportional voltage output. Each solution differs in operating voltage, offset voltage, drift, bandwidth and ease of use. Much like all-in-one Hall effect solutions, shunt-based sensors are an intrusive technology that involves resistance, and power consumption is also a factor that needs to be considered in the overall design.
Hall Effect Current Detection Technology
One-piece packaged Hall effect current sensors are popular in high voltage systems because they provide reinforced isolation or double isolation. However, Hall-effect current sensors tend to drift over temperature and life, which gives them a low reputation. TI has dramatically reduced the drift error of the TMCS1123 to ±0.5%. The device features differential Hall effect sensing, which significantly reduces magnetic field interference or crosstalk, and provides additional features such as overcurrent detection, precision voltage reference, and sensor alarms; see Figure 3. When using an all-in-one package solution, current flows within the package through the leadframe, which introduces leadframe resistance and chip thermal limitations, which in turn limits the amount of current the device can handle. The TMCS1123 device product family is capable of measuring 75 Arms of current at 25°C.
Other solutions include ambient Hall effect sensors or fluxgate sensors (such as TI's DRV401), which may require different types of cores, shielding, or mechanical designs to function properly, as well as devices or circuit boards in the manufacturing or use process. Movement can cause displacement errors, potentially altering measurement accuracy.
There are several design challenges in high voltage applications that make systems more difficult and costly to design. With TI's product portfolio and resources, you can solve design problems quickly and affordably, making technological advances accessible to the masses and having a greater impact on our lives.