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How do DC EV Charging dissipate heat?

DC charging pile integrates power electronics technology, embedded technology and IoT technology. The biggest difficulty is the power electronics technology. The power electronic technology in the field of charging pile is rectifier technology. The power size of rectifiers has progressed rapidly from 3kW in the 1990s to 20kW-60kW now, but the mainstream circuit topology is still three-phase Vienna (PFC), LLC, or phase-shifted full bridge (DC/DC). Rectifier technology in the 1990s, the main application is communication power supply, the output voltage range is very narrow, 42V-56V; applied to the DC charging pile, the output voltage range is very wide, 200V-1000V. through the transformer vice-side of the series or parallel way to achieve a wide range of voltages, there have been a number of innovations in this area over the years. Due to the harsh environment in which charging piles are used, the industry has also been seeking breakthroughs in heat dissipation methods. Four types of cooling methods, such as forced air cooling, independent air ducts, liquid cooling, and natural cooling, are used in different types of DC charging piles.

Forced Air Cooling
Forced air cooling refers to the cooling method of circulating air by means of a fan. The fan blows or extracts air directly to the “heat source devices” (such as MOS tubes, transformers, inductors, electrolytic capacitors, etc.), and takes away the heat in the way of strong exhaust air. Heat source devices like MOS tube, you need to MOS close to the heat sink, the heat of the MOS tube will be dispersed, because the volume of the MOS tube is very small, the heat is very concentrated, the accumulation of fast. With a larger heat sink, the fan speed can be smaller.

Forced air cooling is the most common cooling method for switching power supply products used indoors, such as server computers and desktop computer power supplies. Compared with natural cooling, forced air cooling is faster and more efficient. The disadvantages of forced air cooling are: low protection level and high noise.

In the forced air cooling method, a special design is to use the forced fan and heat pipe together to dissipate heat, this design is particularly common in high-power electronic products (such as laptops, high-performance desktop computers, etc.). The principle of heat pipes is that heat is transferred through the evaporation and condensation of a liquid inside the heat pipe. Specifically, the heat pipe has a low level and a high level inside, with the low level being close to the heat source and the high level being connected to a higher position on the fan. The high temperature on the heat source makes the liquid inside the heat pipe evaporate, while the vapour will flow from the low position to the high position, and after flowing to the high position, the vapour will condense into liquid quickly due to the help of the fan, and the heat transfer is completed. The use of heat pipes can solve the heat dissipation problem without adding too much space, and at the same time, it does not affect the performance of the product and the appearance of the aesthetics. However, this heat dissipation method has not gained application in the field of charging modules.

Forced air-cooled switching power supplies are concerned with the design of air ducts, as shown in Figure 1, which is the internal and overall photo of Energy Efficiency Electric’s 40kW charging module. Nowadays, charging modules are mostly made of two single boards buttoned together, sharing 80 fans, with three 80 fans taking about 1600W of heat away. Simulation alone cannot accurately know the temperature rise of each heat-generating component at different ambient temperatures. Before the temperature control strategy is determined, hundreds of temperature acquisition probes are needed to measure the temperature rise of various heat generating components; however, it is impossible to have so many temperature probes in the actual product. The actual product may use 4-8 temperature probes. According to the temperature measured by hundreds of instruments to generalise to the temperature acquisition results of several temperature probes, the temperature control algorithm is abstracted.

The protection level of the charging module itself is only IP32, however, the minimum protection level required for outdoor charging piles is IP54. Using the charging module to make a whole pile with IP54 requires structural design on the outside of the module. In order to achieve IP5X, there should be dustproof cotton at the inlet and outlet of the air duct; in order to achieve IPX4, there should be louvers at a 45 degree angle at the inlet and outlet of the air duct. This increases the resistance of the air ducts and requires additional fans to be added to the outer structure of the module to increase the differential pressure. It is not difficult to make an IP54 rating, what is difficult is the long term reliability to adapt to harsh outdoor environments.

Separate Ducts
Separate air ducts may not be an academic term, nor was it ever an agreed upon term in the industry, but due to the use of energy-efficient electrics, “separate air ducts” have been written into the SORs of automotive products as an agreed upon method of heat dissipation.
The separate air duct means that the PCBA is completely sealed, and the heat generated by the heat source device is conducted to the teeth of the heat sink by conduction, and the fan only blows or extracts air to the teeth of the heat sink to take away the heat generated by the sealed part. With this heat dissipation method, the heat is evenly dispersed by the large area of the heat sink, and at room temperature, the fan can remove the heat at a very low speed (4000 rpm), so the noise can be very small. The noise at room temperature can be controlled within 45 dB. As the PCBA is sealed, IP6X “dust-tight” is guaranteed. Low noise and high protection are the two advantages of the independent air duct design.
The charging module PCBA, charge controller, auxiliary power supply, metering unit, insulation detection unit, etc. inside the DC charging pile based on independent air ducts can be made into board-level, highly integrated design as a whole, that is to say, the part inside the die-cast radiator is not only the charging module. Figure 2 shows the “charging module” inside a 20kW standalone air duct charging pile from Energy Efficiency Electric, which is well known in the industry. The base case of the charging module is die-cast aluminium, with grooves around the base case, a sealing ring inside the grooves, and a sheet-metal locking screw on the top to achieve the sealing. This sealing method is the same as the car products, theoretically can be made to IP67 protection level, however, due to the volume is too large, the face cover to lock 14 screws, to make IP67 on the production of consistency requirements are very high. Therefore, we say that this is a design based on automotive grade.

20kW energy-efficient, all-in-one air duct-based charging module with built-in auxiliary power supply, metering unit, and insulation detection unit

Although the all-in-one charging module based on separate air ducts already integrates a number of charging station functional units, if it is to be made into a whole station, the fan assembly, the light language and the emergency stop button, which include electronic circuits, have to be placed outside of the all-in-one charging module. The fan assembly, the light language assembly and the emergency stop button should all be IP65 or higher protection level components, and the structural design should also ensure that they are IP65 or higher, and at the same time, the termination part of the connecting harness between the charging module and these functional units should also use waterproof terminals. As shown on the right in Figure 2, there are three sets of wiring harnesses with waterproof terminals connecting the fan assembly, the light language assembly and the emergency stop button outside the charging module.
Energy Efficiency Electric produces a series of low power DC products based on independent air duct technology. The independent air duct technology has become the hallmark of high-end DC charging products.

Liquid Cooling
The PCBA process of liquid cooling is similar to that of independent air ducts, in that the PCBA is also completely sealed, and the entire die-casting body is also sealed by means of grooves and seals. the difference is that in liquid cooling, the heat is conducted to the underside of the die-casting body, and then taken away by the flow of liquid inside the water pipe. the PCBA has a water channel buried underneath it, and there is a water inlet and an outlet. the heat is taken away by the flow of liquid, and is then removed to the underside of the die-casting body. Heat is taken away by the flow of liquid, but ultimately how to dissipate it? The on-board chargers, motor controllers and other components in electric vehicles are mostly liquid-cooled, and these components are made very small, and the heat treatment part is shared with other parts of the vehicle. The heat is ultimately dispersed by the radiator and taken away by the pressure difference of the fan.

Figure 4 shows a liquid-cooled 6.6kW OBC module. It appears to be much smaller in size when compared to a separate duct based DC charging post of the same power. As mentioned earlier, this OBC assembly does not include a heat treatment section, meaning that it does not have the complete cooling racks and fans of a separate air duct based charging module. A complete liquid cooling system includes a pump, tank, fluid, radiator, fan, etc. in addition to the components shown. In our opinion, for low power DC products, 20kW is a better solution with separate air ducts.

Natural Cooling
Natural cooling is a cooling method that uses the high thermal conductivity of metal materials to carry away heat and emit it into the air, using natural convection between the air outlet and air inlet. Natural convection in the absence of specific air speed requirements, the heat sinks used are copper and aluminium sheets, aluminium extrusions, heat pipes, machined or alloy castings. Since natural cooling mainly utilises natural airflow, the area and layout of the heat sink needs to be planned well in advance in the product design, otherwise it will lead to insufficient or uneven heat dissipation.
There is a limit to the amount of heat that can be removed by natural cooling. In order to take away more heat, the volume of the radiator has to be larger, and accordingly, the weight is heavier. For a 7kW low power DC charging post for home use, the ultimate solution after going OBC is natural cooling.

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