There are various subsystems such as battery management, air conditioning and traction inverter inside the moving car. The working conditions are different, and the requirements for power devices and their driving are not the same. At the same time, the charging pile has the intersection of AC/DC, and the charging power/speed is also different. Optocouplers are widely used in these different subsystems and power systems for isolated communication, drive, and feedback. Optocouplers are very mature in traditional industrial applications. So what are the development trends and new performance requirements in the emerging applications of electric vehicles/chargers?
A few days ago, at the Tech Shanghai Automotive Electronics Design Forum co-organized by Aspencore's "EDN Electronic Technology Design", "EET Electronic Engineering Album" and "ESM International Electronic Business", Broadcom's Isolated Products Division (formerly Avago Light) Chen Honglei, Product Manager of Coupling Division, brings the topic of “Full-function optocoupler drive and protection of next-generation power devicesâ€. From the two parts of electric vehicles and charging piles, the new application opportunities of optocouplers in the electric vehicle market are discussed. (In addition, in this forum, Jin Yatuo senior product manager Zhu Zhixing and Kang Mouer (COMSOL) application engineer Shi Wei also brought "software security authorization in the automotive electronics industry" and "multiple physics in power battery research and development" The field simulation solution."
Optocoupler principle and classificationChen Honglei said that optocouplers are divided into three categories: digital optocouplers, gate drivers and isolation amplifiers. The basic function of these three types of optocouplers is to provide signal isolation. Digital optocouplers simply implement signal isolation. The gate driver adds current drive capability to drive the gates of power devices such as IGBTs and MOSFETs, including the latest SiC and GaN. The isolation amplifier is used for the feedback path, and returns the detected voltage, current and other signals to the MCU for processing.
The optocoupler consists of three parts: LED, isolation medium and photodiode, which form three parts: illuminating, transmitting and sensitizing. The principle of optocoupler is simple, but it is divided into categories according to the safety conditions such as creepage distance, clearance and isolation voltage.
Optocouplers are used in a wide range of applications, especially in the field of power electronics. Emerging industries such as electric vehicles and charging piles bring great opportunities for optocouplers.
Chen Honglei revealed to the author after the meeting that a trend in automotive electronics is to integrate the electronic part and the hub, and directly drive through the CAN bus or power line, so as to avoid the loss of the physical drive. Such components are often complex and costly. Thus, the semiconductor device needs to withstand high temperatures and withstand vibration. For silicon-based semiconductor materials such as optocouplers, there are still great challenges. The Grade 1 optocoupler is currently only 125 °C, so cooling is required for high-rotation (and high-temperature) motor drives.
Broadcom is also considering whether it is possible to apply high-temperature materials such as GaN and SiC to optocoupler devices such as gate drive and current detection. In addition, the light-emitting diode itself also needs a technological breakthrough to withstand high temperatures. For example, for optical attenuation characteristics, optocoupler devices have been carefully selected, and the design has also considered sufficient margin. However, the overall temperature rise of the system will still pose new challenges for LEDs in optocouplers. Broadcom is also working hard to resolve it.
What kind of optocoupler does an electric vehicle IGBT need?As shown in the figure below, in electric vehicles, optocouplers are widely used in traction inverters, CAN bus interfaces, signal sensing, heating and ventilation (HVAC) and power management systems. (The corresponding item numbers can be found on the right. These are all Broadcom's years, according to customer needs.)
Chen Honglei said that the requirements for different subsystems such as battery management, air conditioning system and traction inverter in electric vehicles are different. For example, the operating temperature of air conditioners is not too high; and traction inverters need to withstand higher temperatures due to the temperature in the engine. The battery management is at the back end and the temperature is not too high. Therefore, like a DC/DC charger, a MOSFET is generally used as a power device, and a MOSFET is used. In addition, its operating temperature is also different, and the power capacity/drive current is usually not too large. The traction inverter requires a large power, and has functions such as protection and feedback, so the drive is different. In addition, current monitoring can be done on the battery management bus, and the traction inverter will be more complicated.
The figure below shows the choice of gate driver models for car chargers, DC-DC converters and inverters. “ACPL-32JT is a product with high market acceptance. There is also ACPL-33JT. ACPL-3540JT is a product with emitter current detection.†Chen Honglei introduced.
The figure below shows an IGBT module using ACPL-32JT with integrated 6-channel IGBT. This development board is very small.
The ACFJ-3540T is a new device developed to address the evolution of power devices such as IGBTs in automobiles. Some new IGBT devices have a current branch at the emitter—proportional to the emitter current for IGBT current sensing. For this new device, Broadcom developed the ACFJ-3540T gate driver, which is capable of driving the IGBT gate and effectively monitoring current conditions.
An isolation amplifier is used to detect the signal and provide feedback. Broadcom's newest devices are the ACPL-C799T and ACPL-C797T, which are suitable for high current drive market demand. At present, there is a demand for precision resistors on the market, and it is necessary to design a small resistance and a high power. The ACPL-C799T can be made into a small modular product that can be directly connected to the copper bar of a precision resistor; the current is sampled and connected to a digital filter.
Digital optocouplers are relatively simple and widely used; digital signals are transmitted from the primary side to the secondary side. The figure below is the choice, divided by input, with 100kBd, 100kBd-1MBd and 1MBd-15MBd. The other is divided by the operating voltage; automotive applications have lower operating voltages than industrial applications.
Advantages and Disadvantages of AC/DC Charging Piles and Drive Requirements
In the charging pile part, from the perspective of the industrial chain, it is necessary to transfer the electrical energy produced by the factory to the user's car for storage and use. The charging pile can be simply divided into two types: AC charging and DC charging. AC charging is done by charging the car to a DC through a car charger. This charging method has two disadvantages of a car charger volume limitation and a limited charging power (long charging time). DC charging is the direct charging of the battery with DC, which usually occurs in a fixed form, thus allowing fast charging at high power.
“The charging pile has AC and DC points, and the corresponding car has AC charging and DC charging. Each has its advantages and disadvantages. AC charging uses the on-board AC/DC converter to convert the AC into DC and charge the battery. DC charging is generally large. Power charging, there is a fixed charger outside the car, high power, high charging efficiency, fast charging. Super charging station is nothing more than greater power, can shorten the charging time. Extend the cruising range, and secondly, optimize the charging station distance.
“In addition, whether it is AC charging or DC charging, the battery can receive DC power. AC charging requires the car to have a car charger, the volume/power capacity is limited (long charging time), and secondly, the vehicle regulations must be met. The DC charging power can be very large (converted to the outside), the charging time can be greatly shortened. However, the disadvantage is that the charger can not run with the car, only to the place where there is a charging pile to charge, and can not be inserted at any time Charging on the electric network." Chen Honglei pointed out.
For DC charging, the industry already has primary and secondary solutions, and is also studying the three-level solution, charging the car battery to 80% or even full in 5-10 minutes. This will enable electric vehicles to have sufficient mileage coverage and further promotion.
According to HIS research institutes, by 2020, there will be more than 12 million electric vehicle charging stations.
The DC fast charging station includes AC input, power factor correction (FPC) inductance, and DC/DC conversion (including battery docking). In the process of power conversion and transmission, it is necessary to perform current detection and voltage detection on the gate of the transistor and perform signal communication and the like. These are potential applications for optocouplers. The right side of the figure below is a detailed list of optocoupler selections.
The ACPL-352J is a newer optocoupler for electric vehicle charging stations. It is suitable for voltage-driven devices such as IGBT/MOSFET (current load - current charging of the gate is required, and it is turned on when a certain voltage is applied).
In addition to driving the IGBT gate, the ACPL-352J also enables IGBT gate detection, thus providing effective information on automotive safety.
For SiC MOSFET/GaN devices, they have different driving characteristics, such as operating voltage, switching speed, and so on. At the same time, for these new power devices, Broadcom has also developed a targeted reference design.
Over the years, Broadcom/Avago's optocoupler devices have continued to evolve (including drive currents, drive speeds, and signal delays, as well as new features such as functional safety).
ASSR-601J can be used for leakage detection of automotive batteries or charging posts, which is equivalent to the characteristics of solid state relays, but can withstand high voltage, so it can provide higher reliability than traditional relays.
For the above various optocouplers, Broadcom has provided the corresponding development board.
In addition, Chen Honglei told the author that the advantages of new power devices such as GaN and SiC are that higher switching frequencies can be tolerated and junction temperature tolerance is high. Therefore, corresponding to the requirements of the optocoupler device, the operating frequency is fast and the driving current is large. In addition, the operating voltage and protection characteristics between them (such as undervoltage lockout, desaturation) are also different. Therefore, it is necessary to select different optocoupler devices to drive.
The challenges in optocouplers are high integration (number of channels; limited by insulation requirements) and second, breakthroughs in LEDs (developing LEDs that activate the layer to illuminate down and down, thereby reducing volume).
Finally, he emphasized that optocouplers, although relatively light-degraded with their lifetime, have a margin for design (with decades of service life), and there is no need to worry about this issue.
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