The application scope of high voltage DC contactors is extensive, ranging from the booming photovoltaic market of recent years to various fields in the new energy vehicle sector, including DC charging stations, water and air-cooled PTC charging devices, power batteries, high voltage cabinets, electric air conditioning, among others. These places involving high voltage components often utilize high voltage DC contactors, which are like best buddies with fuses. The following discussion primarily focuses on the application of these core electrical components within PDUs.

Firstly, let's take a look at the high voltage distribution box, abbreviated as PDU (Power Distribution Unit), which serves as the high voltage power distribution unit in new energy vehicle high voltage system solutions. By connecting high voltage components through busbars and harnesses, it provides functions such as charge and discharge control, power on/off control of high voltage components, circuit overload and short-circuit protection, high voltage sampling, low voltage control, etc., thus protecting and monitoring the operation of the high voltage system. PDUs can also integrate functions such as BMS control, charging modules, DC modules, PTC control modules, etc., compared to traditional PDUs, they include more vehicle functional modules, are more integrated in functionality, and structurally more complex, often featuring water or air-cooling structures. PDUs are flexible in configuration, allowing customization and development according to customer requirements to meet the needs of different customers and vehicle models. The BDU (Battery Disconnect Unit) is a battery pack isolation unit, designed specifically for internal battery pack use, and is also a type of high voltage distribution box.

Introduction to the Principle of High Voltage DC Contactors

Simply put, a high voltage DC contactor is an automatic switch that controls a large current with a smaller current. It plays roles in automatic regulation, safety protection, and circuit conversion within circuits. The primary purpose of controlling high voltage DC contactors is to ensure the normal operation of battery system power on/off processes, closing the high voltage DC contactor during vehicle startup and opening it during vehicle shutdown. Control of high voltage DC contactors belongs to the domain of automotive electronic electrical systems, and its failure modes may pose risks to the driver's life safety, thus development should adhere to the ISO26262 standard to meet functional safety requirements.

Monitoring the status of high voltage DC contactors is mainly done by checking the contact status of the high voltage DC contactor to determine if it meets control requirements, avoiding the application of electrical loads to the high voltage circuit when the high voltage DC contactor is open, and disconnecting the high voltage DC contactor when there is a significant electrical load on the high voltage circuit. Contact detection of high voltage DC contactors can accurately identify situations such as contact sticking, preventing damage to high voltage circuit components due to misoperation under contact sticking conditions, while also prolonging the life of the relay by operating the high voltage DC contactor (open/close) properly. Contact detection methods for high voltage DC contactors include using high voltage DC contactors with auxiliary contact detection or separately designing auxiliary contact detection circuits to determine the open/close status of high voltage DC contactor contacts by detecting the voltage across the two segments of the high voltage DC contactor.

Two Typical Technological Paths for High Voltage DC Contactors (Ceramic Sealed and Epoxy Sealed)

Ceramic sealing, represented by Panasonic, is one technological path; the other is epoxy sealing, represented by TE. In fact, there is also a solid-state type, which is currently less utilized. If there are any colleagues who know about it, feel free to add.

Why do high voltage DC relays need to be filled with gas? What is the arc extinguishing principle of gas? Why is hydrogen filled in ceramic-sealed ones (Panasonic), while nitrogen is filled in epoxy-sealed ones (TE)? The main difference between DC and AC lies in the fact that DC does not have zero crossing points. Once an arc is formed, it will not extinguish on its own and will continue to arc, which is fatal for contacts! What is the nature of the arc? It's the escape of electrons under the action of electric field force, which is an extreme discharge. Therefore, there are two methods of arc extinguishing. One method is to increase the contact gap (the essence of magnetic blowout arc extinguishing is to change the arc path using Lorentz force, which can be understood as increasing the contact gap. If only the physical contact gap is increased without using magnetic blowout arc extinguishing, the relay's magnetic gap also needs to be very large. In that case, under the same load capacity, the volume and weight would be unimaginable).

The other method is to have a substance between the contacts that can block the path of electrons. The best method one can think of is to fill with gas. Gas filling is crucial; firstly, the gas must be very active, making it easier for electrons escaping from the arc to collide with it, thus producing a blocking effect and carrying away heat more easily. Secondly, the gas itself must have a stable molecular structure, with its own electrons not prone to escape. Following these two requirements, hydrogen gas naturally comes to mind as the first choice for arc extinguishing. Among all gases, hydrogen has the smallest molecular weight, is the most active, and has a sufficiently stable molecular structure. Therefore, this is the technological path taken by Panasonic, Hongfa, etc., because they adopt ceramic sealing. Why does TE not fill with hydrogen but instead fill with nitrogen? Because they use epoxy sealing. Hydrogen is the best arc extinguishing gas, but its small molecular weight makes it difficult to contain. Epoxy resin cannot contain it, so nitrogen is chosen as an alternative.

The difference in arc extinguishing methods leads to differences in the dimensions, service life, price, and other aspects of their products. There is no absolute superiority or inferiority, only choices that suit oneself.

Initial Selection of High Voltage DC Contactors

1. Understand the customer's initial requirements: such as rated voltage, rated current, peak charge and discharge current, duration of peak current, seismic performance, environmental requirements, altitude, vehicle operation simulation conditions, etc.;

2. Common parameters of high voltage DC contactors: contact parameters, normally open contacts, normally closed contacts, performance parameters, insulation resistance, withstand voltage, operating current, operating voltage, installation hole size, installation hole spacing, etc.;

3. Initially select matching models based on rated voltage. Most high voltage DC contactors have a wide voltage range, suitable for various application scenarios, but this is just a reference;

4. Narrow down the selection based on rated current. Choose a high voltage DC contactor with a rated current slightly higher than the rated current, for example, if the rated current is 200A, choose a high voltage DC contactor with a rated current of 250A from a certain manufacturer for temporary selection, and then confirm further information. See Figure 1 on the next page for technical parameters of high voltage DC contactors;

5. Estimate the expected service life based on the peak current provided by the customer. See Figure 2 on the next page for the expected life of high voltage DC contactors;

6. Estimate the short-circuit current based on the electrical performance of the battery pack, calculate the short-circuit current based on the estimated short-circuit current, and estimate the failure time of the high voltage DC contactor based on the circuit current;

7. An important step is to compare the failure time of the high voltage DC contactor during short-circuit with the failure time of the fuse. The failure time of the relay during short-circuit is longer than that of the fuse. This comparison can be made by oneself or by asking the relay manufacturer to do it.