Compared to conventional cars with internal combustion engines, electric vehicles are much more energy efficient, but this also poses a problem: the waste heat from the electric motor is no longer sufficient to heat the interior of the car. To meet heating needs, some of the electrical energy stored in the battery must be converted into heat. To achieve adjustable heating power independent of operating temperature or battery voltage, power semiconductors are used in a new generation of high-voltage heaters to control the flow of energy from the battery to the heating element. The refrigerant is heated by the heating element, and delivered to the air conditioning system of the vehicle through the heat exchanger, and finally the warm air is delivered into the vehicle by the blower.

foreword

Compared to conventional cars with internal combustion engines, electric vehicles are much more energy efficient, but this also poses a problem: the waste heat from the electric motor is no longer sufficient to heat the interior of the car. To meet heating needs, some of the electrical energy stored in the battery must be converted into heat. To achieve adjustable heating power independent of operating temperature or battery voltage, power semiconductors are used in a new generation of high-voltage heaters to control the flow of energy from the battery to the heating element. The refrigerant is heated by the heating element, and delivered to the air conditioning system of the vehicle through the heat exchanger, and finally the warm air is delivered into the vehicle by the blower. See Figure 1 for the schematic.

IGBT-a key technology of electric vehicle air conditioner

Figure 1. The working mode of the high pressure heater in an electric vehicle

Ordinary electric vehicles require a heating power of 5kW~7kW to meet the heating needs. If the heating system of the car is heated only by the load resistance (heating element), this power range is reduced accordingly. However, there are also some heating systems that do not only rely on resistance to generate heat, but use the principle of heat pumps: the heat energy is transferred from the cold source (the environment) to the heat source (the interior of the vehicle) by means of external energy supply. The energy balance of the heat pump is better than the load resistance heating method and has less impact on the power range. However, with this heating system, the cost of building the vehicle increases and its usefulness depends on the ambient temperature. Such systems cannot generate enough heat in regions with very cold winters, where traditional resistance heaters are essential.

The heating system not only ensures occupant comfort, but also has important safety features. For example, defrosting the windows or dehumidifying the interior so the driver can clearly see the road outside. Batteries require a certain operating temperature, and the heater ensures that the battery is always within the normal operating temperature range. The heater can also act as a discharge resistor in the event of a high voltage peak; if the voltage of the car’s electrical system rises abnormally, the system can absorb the abnormal energy, thereby limiting the extent of the overvoltage. These features protect the battery and other systems connected to the vehicle’s electrical system.

A simple resistance heater is shown in Figure 2. The duty cycle of the switch is adjustable, enabling the power output to always match the setpoint. Several (usually two or three) heating branches are connected in parallel for better heat distribution. In order to be able to switch off the heating system safely in the event of a fault, a safety switch needs to be configured and ensured that the safety switch is always on during normal operation. In the event of a malfunction, these switches will turn off, disconnecting the heating element from the vehicle’s high-voltage electrical system.

IGBT-a key technology of electric vehicle air conditioner

Figure 2. Basic circuit of a high pressure heater with two heating elements

ROHM’s RGS IGBT product line is AEC-Q101 qualified

The circuit breakers used in this case are only IGBTs. This IGBT technology has very good conduction characteristics under high current conditions. Although switching losses are high compared to MOSFETs, they are largely negligible because switching frequencies are typically in the range of tens of hertz (double digits) to a few kilohertz. In addition, the product lineup includes two voltage levels, 650V and 1200V, both of which are commonly used in common heating systems. Table 1 lists the AEC-Q101 qualified discrete package RGS series IGBTs from ROHM that are ideal for this type of application. These IGBTs have very high reliability and can meet the typical requirements of heaters, which are described in further detail below.

IGBT-a key technology of electric vehicle air conditioner

Table 1 Product Lineup of ROHM IGBT RGS Series

Most systems designed for 400V batteries typically use 650V IGBTs. However, in recent years, in order to improve the withstand voltage of the heater, more and more tend to use 1200V solutions. If power from the battery to the heater is suddenly interrupted, the lines in the car’s electrical system can develop significant overvoltages and can even damage the switches. Therefore, the heater can be prevented from being damaged by taking advantage of the high breakdown voltage of power semiconductors. The 1200V IGBT supports 800V battery system and can be connected in series to increase the overvoltage load capacity.

Another feature of this application is the switching speed (dVCE/dt, dIC/dt). Switching speed depends on the system. Contrary to most other applications that aim to achieve high switching frequencies, in this application case the switching speed is usually limited to a low level. The reason is that, on the one hand, it is subject to the limitations of EMC (electromagnetic compatibility), and on the other hand, it is due to the design idea of ​​not using filters or reducing filters as much as possible to save costs. A simple method is to slow down the speed of the IGBT during switching to reduce the higher harmonic components of the rising and falling edges of the switch. Although this solution leads to increased losses during switching of the IGBT, it does not require any additional components. The increased losses can be compensated for by reducing the switching frequency. Switching times are in the range of a few microseconds (single digits). In rare cases, it can be on the order of tens of microseconds (the lower two digits). Figure 3 is an example of an IGBT turn-on process with gate resistance in the kiloohm range. Since it is a resistive load and not the usual inductive load, the voltage and current curves cross during switching.

IGBT-a key technology of electric vehicle air conditioner

Figure 3. Turn-on process of IGBT (RGS80TSX2DHR) with resistive load and RG = 1.1kΩ

Although this way of handling IGBTs may seem uncommon to experienced designers, it is not entirely impossible. Don’t drop the switching time too slowly, though. Excessive temperature spikes of the IGBT during each switching should be avoided to avoid compromising the power cycling capability. In addition, extremely slow switching times can also be a risk for IGBTs, which operate at lower gate voltages during switching. In ROHM’s experience, slowing down the switching speed appropriately does not cause problems. With the valuable experience gained and accumulated from various projects, ROHM is able to provide effective advice for customers’ evaluation to help customers find better solutions.

Another feature of IGBTs that cannot be ignored is short-circuit withstand capability (ensures shutdown in the event of a fault). Typically, short-circuit detection takes a few microseconds to react. In ROHM RGS series IGBTs, the short-circuit withstand time of 650V-class products is 8μs, and the short-circuit withstand time of 1200V-class products is 10μs. Excellent short-circuit withstand capability contributes to the successful implementation of fault handling countermeasures.

Another reason to choose power semiconductors is packaging. In this application case, components using through-hole technology (THT) are mainly used. Connect them to an external radiator for easy cooling. However, through-hole technology has its drawbacks in the production process due to the extra steps required. And components using surface mount technology (SMT) (such as common TO-263 package products) can be directly soldered with other components, which is more cost-effective. Although the technology must dissipate heat through the PCB, which has higher requirements for heat dissipation, this does not prevent some manufacturers today from considering this technology. ROHM has been working on relevant research in order to respond in a timely manner. At present, products using SMT technology are being developed, and the product lineup of RGS series IGBTs is continuously expanded. Figure 4 shows the different package specifications used by ROHM RGS series IGBTs.

IGBT-a key technology of electric vehicle air conditioner

Figure 4. Package lineup of RGS series IGBTs

Of course, in addition to IGBTs, there are many other products in ROHM’s product group that are also suitable for high-voltage heaters. These include gate driver ICs, shunt resistors, comparators, op amps and voltage regulators. In the field of IGBT, ROHM is able to provide a full range of IGBT products that meet the AEC-Q101 standard. These products are packaged in TO-247, rated current is 30~50A, divided into yes/no built-in diode. In addition, SMD products are also planned to be added to the RGS series in 2020: 650V voltage level, TO-263-3L package, 15~40A, yes/no built-in diode IGBT; TO-247 package will also be added to the product lineup , Products with larger rated current, namely 650V voltage level, the rated current of 50A is increased to 75A. The rich product lineup will give customers a wider range of choices, and customers can choose better products according to the working conditions of the heater.

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