Research on the technology of small three-electric system (PDU+DC+OBC) for new energy vehicles

In recent years, with the support of national policies, the production and sales of new energy vehicles have maintained rapid growth. With the scale effect, the state has gradually reduced subsidies for new energy vehicles, and the new energy industry has also started a new round of survival of the fittest. For new energy vehicle companies, cost pressure is increasing. The development time of new energy vehicles is relatively short, and the quality of parts and software is also a difficulty in vehicle development.

In response to these problems, a technical solution for the small three-electric system was determined: the main positive and negative contactors are moved from the PDU to the inside of the power battery, and the PDU only distributes power to lower-power high-voltage electrical appliances such as PTC, EAC, and wireless charging; the power module and control module of the on-board charging and DCDC converter are separated, and the power module is designed and produced by a power supply manufacturer with rich experience, and the software and hardware of the control module are designed and produced by the vehicle manufacturer, which defines the interface between the control module and the power module.

This solution simplifies the complex on-board charger and DCDC that must meet both automotive industry specifications and power supply specifications into power modules and control modules. The power module only needs to meet power supply-related specifications, allowing more industrial power supply manufacturers to enter, which is conducive to scale and product quality is guaranteed. The functions of the control module are integrated in the VCU or domain controller, and vehicle manufacturers can use mature software development processes to design and solve complex charging system logic, thereby reducing communication costs with charger suppliers and greatly improving software reliability.

1 Technical analysis and enterprise research

Xinrui Technology, Weimaisi and Hangzhou Fute, three domestic automotive power supply companies, account for 68% of the domestic market share, and 32% of the market share is divided up by other automotive power supply companies. Tesla and domestic companies such as GAC, Geely, Weilai, and Chery have different technical solutions. Miniaturization, integration, and high power density are the direction of market demand. Companies that turn small three electrics into products that are more conducive to scale will win in the next round of competition.

1.1 Technical solutions for on-board charger and DCDC

The OBC of electric vehicles is mainly composed of a power circuit (PFC+phase-shifted full bridge/LLC) and a control circuit, which can be divided into unidirectional OBC and bidirectional OBC. The unidirectional OBC can only charge the power battery, while the bidirectional OBC can invert the DC power of the power battery into 220V household AC power. OBC+DCDC magnetic integration is shown in Figure 1. OBC, DC common control circuit, DC output full bridge circuit, and DC output filter circuit can reduce costs and size.

The use of OBC and DC assembly in the whole vehicle is difficult and labor-intensive. The peripheral circuit of the logic microcontroller needs to meet the requirements of ISO16750. The logic function needs to be adjusted and defined to match the whole vehicle, the state machine definition of the charger, the charging power-up and power-down timing, the electronic lock control and detection circuit and the driving mode and verification of the electronic lock of the whole vehicle. This makes the single-piece and software and hardware R&D costs high.

1.2 High-voltage distribution box solution analysis

High-voltage power distribution box is PDU. Since PDU is related to the electrical layout of the whole vehicle, the PDU of each vehicle model is different, so it is difficult to form a standard product. There are two mainstream methods in the market: one is to customize and develop PDU products for specific models; the other is to integrate PDU functions into other components, such as customizing and developing OBC+DC+PDU all-in-one products for specific models.

Tesla integrates PDU into the power battery, eliminating the PDU component in the entire vehicle, reducing development costs; BYD E5 integrates OBC, DCDC, motor controller, power battery relay, power distribution and other parts together; compared with BYD, Geely lacks a motor controller in its PDU.

Combining the PDU solutions of multiple vehicle manufacturers, Tesla has the best cost and reliability. The integration capabilities of domestic vehicle manufacturers cannot reach Tesla's level, but there is room for improvement.

1.3 Charging System Control Analysis

The National Standardization Administration issued GB/T18487.1-2015 Electric Vehicle Conductive Charging System Part 1: General Requirements in December 2015, defining the charging interface principles and timing, and issued GB/T34657.2-2017 Electric Vehicle Conductive Charging Interoperability Test Specification Part 2: Vehicle in October 2017.

The principle of the control power circuit recommended by the national standard is shown in Figure 2, which solves the charging compatibility of new energy vehicles and charging piles, but the solutions implemented inside the vehicle are different. The detection circuits of detection points 2 and 3 are placed inside the charger, BMS, and vehicle controller. This requires OBC parts manufacturers to design multiple circuits to match the needs of different vehicle manufacturers, and the software strategy also needs to be adjusted. From an industry perspective, it is not conducive to reducing development and unit costs.

1.4 Research and Analysis Summary

The core components of new energy vehicles can be divided into three parts: power battery, electric drive (motor controller, motor, reducer), and small three electrics (PDU+DC+OBC); the reason is that the technology of each part is relatively independent of other parts, and there is room for integration and improvement.

Different models have different combinations of small three electrics, different PDU principles, different connectors, different power requirements, and different control and guidance circuit implementation methods, which makes it impossible to reduce costs through scale. Therefore, how to organically decompose the small three electrics and facilitate scale is the focus and difficulty of industry research.

2 Small three electric technology solutions

2.1 High-voltage distribution box solution

Before designing the PDU solution for the small three-electric system, the first thing to consider is the high-voltage principle of the entire vehicle. As shown in Figure 3, the main positive and negative contactors are integrated inside the power battery. The advantage is safety. They are integrated inside the BDU and can be modularized.

The electric drive is directly connected without PDU. The advantage is that it avoids electromagnetic interference between the electric drive and other high-voltage devices. During driving, the electric drive will produce greater interference, which may affect the compressor controller, DCDC, etc. Other high-voltage electrical appliances are connected to PDU, so the cable between PDU and power battery only needs 6mm2, which greatly reduces the cost of high-voltage cables.

When the failure rate of the small three-electric system is low, it can be done inside the power battery like Tesla.

2.2 Hardware Solution of Small Three Electrics

The difference between vehicle power supply and civilian power supply lies in the different application environments and reference standards. OBC and DCDC require low-voltage controller standards, such as EMC must meet Class 3 of CISPR25. At the same time, OBC is a device connected to a low-voltage power supply system and must meet Class B of CISPR16. Parts suppliers must be proficient in the technology of both fields, and the design is very difficult, which is equivalent to setting a very high threshold. Only a few suppliers can do it well, which is not conducive to improving the industry's technical capabilities.

If it is feasible to separate the power module and control module of OBC, the electrical performance of the power module should refer to the low-voltage power supply system equipment, combined with the vibration test of the whole vehicle such as ISO16750-3 requirements; the control module also interacts with the low-voltage system of the whole vehicle and the power module, and the functions of the control module can be integrated in the VCU. The control module is used to control the power module, such as power module switch control, output power control, diagnostic function, protection function of the control module, thermal management, functional software for interaction with the whole vehicle, etc. The software of the control module is developed by the whole vehicle with rich experience in vehicle-mounted controller software development. If it can be realized in terms of function, then it will be possible to decompose the complex OBC and DCDC into power modules and control modules (integrated in VCU) with single functions.

According to the above ideas, we have modified the OBC part and VCU of the current small three-electric system, shielding the original OBC hardware and functions such as CC, CP circuits, electronic lock drive and detection, leaving only the following hardware interfaces, as shown in Table 1. The modified charger assembly is shown in Figure 4.

The VCU is an independent design, and has reserved functions such as CC and CP detection, electronic lock drive and status detection. The only thing that needs to be added is the power enable signal between the VCU and the power module. The VCU is shown in Figure 5.