The role of power semiconductors in electric vehicle charging

Looking at the current market, high-power liquid-cooled charging piles using third-generation semiconductors can achieve a maximum power of 600kW, and 480kW high-power charging piles are already in the popularization stage. Theoretically, the charging time of 10%-80% of a battery with a range of 1,000 kilometers can be compressed to about 10 minutes.

Range anxiety is a major concern for electric car owners, but it often has nothing to do with battery capacity.

When a traditional car runs out of gas, it can be filled up in 5 minutes and then it can be on the road leisurely, but electric cars often have to occupy a charging station for about half an hour to complete temporary recharging. This also directly leads to a low rotation rate of high-speed charging stations, and "grabbing" charging stations in service areas has become a necessary skill for every long-distance tram owner.

Improving charging speed has become an important task for major car companies, and the factor that has the greatest impact on charging speed - power semiconductors has become the primary target. This article will take you to understand the working principle and development of power semiconductors, but before understanding power semiconductors, we must first know why electric vehicles charge so slowly.

Why do electric cars charge so slowly?

The battery charging process is actually a series of oxidation/reduction reactions inside the battery. Currently, electric vehicles are usually equipped with lithium-ion batteries, sodium-ion batteries, etc., and their structures include positive and negative electrode materials, diaphragms, and electrolytes. Electric current is essentially generated by the directional movement of electrons, and the magnitude of the current is closely related to the number of electrons and the speed of movement. We can adjust the ion concentration of the electrolyte to control the number of electrons, and the speed of electron movement becomes the key to the charging speed.

Theoretically, we can infinitely increase the external voltage or current to accelerate the movement of electrons in the battery. However, the diffusion of metal cations in the battery is limited by physical conditions. If the electrons move too fast and exceed the diffusion rate of cations, other byproducts will be generated next to the positive and negative electrodes, which will greatly affect the battery life. When the impurities are deposited to a certain amount, it will also cause the battery to short-circuit, increasing the risk of battery overheating and fire.

After the battery limits the charging speed ceiling, the external charging station becomes the key to limiting the charging rate.

Theoretically, the greater the power of the charging pile, the shorter the charging time of the car. There are two types of charging piles: AC charging piles and DC charging piles.

At present, AC charging piles are more universal and can be installed in the basement environment of ordinary families. They use household 220V electricity, but their charging speed is slow, usually referred to as "low-voltage slow charging". The main reason for the slowness is that the power of the on-board OBC to convert AC into DC is low.

In contrast, "high-voltage fast charging" usually refers to a DC charging pile, which can be connected to a dedicated power grid with a voltage of 380V or above, and can convert AC into DC through the built-in AC/DC converter of the charging pile, and directly charge the on-board battery. Since the converter is placed outside the car, there is also a larger space for arranging the heat dissipation system, and the power of the converter can also be designed to be larger.

It can be seen that the key to affecting the charging speed lies in the power of the AC/DC converter, and its core is the power semiconductor.

Power Semiconductors

The heart of car charging

In fact, we can give an example to illustrate the role of power semiconductors.

When we try to take out a continuous and small flow of water from a water pipe with a larger diameter, we can adopt two strategies: one is to use a transfer valve and open the valve in a position with a smaller flow rate. This is the working principle of the linear power supply of the transistor. However, the linear power supply will put a greater pressure on the regulating transistor (specifically, heat energy dissipation); the other method is to let the water flow from the large water pipe flow into a larger "bucket", and then use a small water pipe to connect to the bucket to take water. We only need to open the valve on the large water pipe intermittently to ensure that the water in the bucket will neither dry up nor overflow. This is the working principle of power semiconductors that can switch quickly, such as MOSFET and IGBT. Since the transistor is not in a normally open state, its loss is relatively small, the heat is lower, and the reliability is higher.

Power semiconductor devices, also known as power electronic devices, are high-power electronic devices mainly used for power conversion and control circuits of power equipment, including MOSFET (metal oxide semiconductor field effect transistor), IGBT (insulated gate bipolar transistor), BJT (full range bipolar junction transistor, also known as triode), thyristor, GTO (gate turn-off thyristor) and other types. Currently, the most widely used are switching mode power supply devices such as MOSFET, IGBT and BJT.

Let's talk about MOSFET first. The power MOSFET is different from the traditional lateral MOSFET in that its structure is vertical. In the planar structure, the current and rated breakdown voltage are related to the channel size (length and width), while in the vertical structure, these two parameters are related to the doping thickness, so the vertical structure can better utilize the chip area.

A vertically diffused MOSFET

Image source: Wikipedia

Compared with other power semiconductors (BJT, GTO, etc.), the advantages of power MOSFET are its fast switching speed and high efficiency at low voltage, which is used in most power supplies, DC/DC converters, low voltage motor controllers and many other applications.

However, the switching speed of power MOSFET is still limited, which is the internal capacitance in MOSFET. When MOSFET switches, the internal capacitance needs to be charged and discharged, and the external drive circuit will limit the charging and discharging speed of the capacitance, so the speed is slow, and the drive circuit will also directly affect the switching speed of MOSFET. However, the limit of MOSFET can only withstand a voltage of about 200V, which is not enough in today's electric vehicle charging speed anxiety. The high-voltage resistant IGBT is obviously more suitable as a choice for chargers or charging piles on the current market.

IGBT is a composite fully controlled voltage-driven power semiconductor device composed of BJT and MOSFET, which has the advantages of high input impedance of MOSFET and low on-state voltage drop of BJT. BJT has low saturation voltage drop and high current density, but large driving current; MOSFET has low driving power and fast switching speed, but large on-state voltage drop and low current density. Therefore, IGBT components are widely used in motor drive modules and AC/DC modules of new energy vehicles.

IGBT Source: Internet

IGBT accounts for about half of the cost of the motor drive system, and the motor drive system accounts for 15-20% of the cost of the entire vehicle. That is to say, IGBT accounts for 7-10% of the cost of the entire vehicle. It is the second most expensive component of an electric vehicle after the battery, and also determines the energy efficiency of the entire vehicle.

In summary, IGBT modules have become the mainstream choice for power semiconductors in the new energy vehicle field.

One thing to mention here is that power semiconductors actually consist of two parts: power semiconductor devices and power ICs. Power semiconductor devices designed for charging and power conversion are IGBTs, power MOSFETs, etc., while devices that control vehicle variable frequency charging, such as stopping fast charging when the power reaches 90%, are power ICs. However, power ICs belong to the category of analog chips.

Electric vehicle charging

Is it possible to be as fast as refueling?

In fact, the charging rate of electric vehicles has been steadily increasing over the years. At present, the discussion of charging piles supported by the third-generation semiconductors is very hot, and the charging speed of electric vehicles has also changed from the previous "charging all night" to the current "charging in a few hours". The third-generation semiconductors are actually semiconductors that use new materials. However, before power semiconductors changed from silicon-based to other materials, they had gone through a long period of development. Here we will briefly talk about its development history.

In 1957, General Electric Company of the United States invented the world's first thyristor, marking the birth of power electronics technology. However, since the first generation of power electronic devices can only control its conduction through its gate, but not its shutdown, it can only be a semi-controlled device. In the late 1970s, fully controlled devices represented by GTO, BJT, and MOSFET developed rapidly, and the operating frequency reached the megahertz level, marking the birth of the second generation of power electronic devices. In the late 1980s, IGBT, which combines the advantages of MOSFET and BJT, was born, and the development of power semiconductor modules has taken a step further towards high power, high frequency, and high efficiency. The development of all the above power semiconductors is based on Si, and these semiconductor devices with Si and Ge as substrates are collectively referred to as the first generation of semiconductors.