How to Design Heating and Cooling Systems for HEVs/EVs

Internal combustion engines (ICEs) have powered cars and heating and cooling systems for decades. As the automotive industry electrifies and transitions to hybrid vehicles with small internal combustion engines or fully electric vehicles with no engines at all, how will heating, ventilation and air conditioning (HVAC) systems perform?

In this white paper, we will introduce new heating and cooling control modules in 48V, 400V, or 800V hybrid and electric vehicles. In it, you will learn about the unique subsystems in these modules through examples and system diagrams, and finally we will help you start planning your implementation by reviewing the functional solutions of these subsystems. How the Internal Combustion Engine Works in the HVAC System In a vehicle equipped with an ICE, the engine is the foundation of the heating and cooling system. Figure 1 illustrates this concept. When cooling, air from the blower enters the evaporator, where the refrigerant cools the air. The air conditioning compressor driven by the engine then compresses the refrigerant leaving the evaporator. Similarly, when heating the air, the heat generated by the engine is transferred to the coolant. This hot coolant enters the heater core, which heats the air that will be blown into the vehicle cabin. In this way, the engine plays a fundamental role in the heating and cooling of the vehicle cabin.

Figure 1 The engine plays a fundamental role in the heating and cooling system of an ICE vehicle.

Methods for heating and cooling hybrid and electric vehicles

In HEV/EV, due to size restrictions or the absence of an internal combustion engine, two additional components need to be introduced, which play a key role in the HVAC system, as shown in Figure 2:

1. A brushless DC (BLDC) motor is a DC motor that rotates the air conditioner compressor instead of the engine.

2. A positive temperature coefficient (PTC) heater or heat pump heats the coolant instead of the engine.

Apart from these components, the rest of the heating and cooling system infrastructure is the same as for vehicles with ICEs. As mentioned earlier, in the absence of an engine, a BLDC motor and a PTC heater or heat pump are required, which presents challenges in terms of power consumption, motor and resistive heater control, and overall HVAC control, respectively.

Figure 2 Heating and cooling systems in hybrid/electric vehicles

Electronics controlling BLDC motor and PTC heater

In high-voltage HEV/EV, both the BLDC motor and PTC heater use high-voltage power. The air conditioning compressor may require up to 10kW of power, while the PTC heater may consume up to 5kW of power. Figures 3 and 4 are block diagrams of the air conditioning compressor BLDC control module and the PTC heater control module, respectively. Both block diagrams show that the air conditioning compressor BLDC motor and PTC heater are powered by a high-voltage battery. In addition, these modules use insulated gate bipolar transistors (IGBTs) and corresponding gate drivers to control the power supply to the BLDC motor and PTC heater.

Figures 3 and 4 also illustrate the similarities between the remaining subsystems of the two control modules. Both systems include a power subsystem, a gate driver bias supply, microcontroller (MCU), communication interfaces, and temperature and current monitoring. Many of the subsystems used in these control modules, such as transceivers for communication and amplifiers for current measurement, are similar to those used in other heating and cooling control modules. However, the power subsystem and gate driver subsystem are unique to these control modules in the vehicle heating and cooling system. These subsystems interface with both the low-voltage and high-voltage domains.

Later in this white paper, we will discuss functional block diagrams of circuit topologies for these subsystems. Note that the choice of circuit topology must satisfy subsystem functionality as well as system design requirements such as efficiency, power density, and electromagnetic interference (EMI).

Figure 3 Block diagram of high voltage air conditioning compressor BLDC motor control module

Figure 4 Block diagram of high voltage PTC heater control module

Heat Pump

An alternative to using a high-power PTC heater to heat the cabin is to use the cooling circuit as a heat pump as shown in Figure 5. In this mode, the reversing valve reverses the flow of the refrigerant. In addition, there may be other valves in the system that regulate the refrigerant flow. For example, a stepper motor is used to control the valve in the heat pump.

In heating and cooling systems based on heat pumps, the following types of valves are used:

• Expansion valves, which control refrigerant flow. They help facilitate the transition from high-pressure liquid refrigerant in the condensing unit to low-pressure gaseous refrigerant in the evaporator. Electronic expansion valves generally benefit from faster, more accurate response to load changes and the ability to more precisely control refrigerant flow, especially when using a stepper motor to control the expansion valve.

• Stop and reversing valves, which are used to change the direction or path of the refrigerant, allowing reverse circulation and bypassing of certain components in both heating and cooling modes. Stop and reversing valves can be controlled by either solenoid actuators or brushed DC motors.

As can be inferred from Figure 5, the heat pump system still uses an air conditioning compressor module, which has been discussed in the previous section. In addition, the heat pump system also uses a motor driver module to drive the valve. This adds an additional design challenge to drive the valve to control the refrigerant flow. Figure 6 shows a typical block diagram of a motor driver module for driving a valve. The block diagram shows a stepper motor driver. If the motor is a brushed DC motor, the brushed DC motor driver will replace the stepper motor driver in this block diagram. The design requirements of the motor driver module include power density and EMI.

Figure 5 Heat pump system

HVAC Control Modules

Figure 7 is a typical block diagram of an HVAC control module. The HVAC control module controls the high voltage contactors that connect and disconnect the high voltage battery to the BLDC motor and PTC heater. The block diagram also shows the damper motor controller, defrost heater, communication interface, and power subsystem.

Figure 6 Block diagram of a stepper motor driver

Notes on high voltage battery heating and cooling:

Depending on the ambient temperature, the high-voltage battery may need to be heated or cooled. This can be done using the same system that heats and cools the cabin. A separate heater can also be used to heat the coolant that flows into the battery. While this coolant is used to heat the battery in cold conditions, it can also absorb heat from the battery and transfer it to a heat exchanger to heat the air in the cabin. In such a system, stepper motors control additional valves that pass the coolant through pipes in the battery and heat exchanger.

Figure 7 HVAC control module

Typical functional block diagram of a unique HVAC subsystem

As mentioned earlier, other control modules in new HEV/EV heating and cooling systems include subsystems unique to these control modules - power supplies, gate drivers, and stepper motor valve drivers for controlling refrigerant flow. In this section, we will explore the typical functional block diagrams of the circuit topologies of these subsystems in high voltage air conditioning compressor and PTC heater control modules. These topologies must address unique challenges in HEV/EVs, including isolation barriers and EMI, which we will discuss in the following sections.

For HEV/EV, there are high power consumption heating and cooling subsystems, such as BLDC motors or PTC heaters. But the rest of the subsystems in the module are usually low power, such as MCU, gate drivers, temperature sensors, and other circuits. The typical approach is to power the high power consumption loads directly from the available higher voltage (800V, 400V, or 48V), and power the circuits on the board from the 12V rail, as shown in Figure 8.

In 48V systems, critical systems such as the starter/generator or traction inverter typically require an O-ring between the power provided by the 12V and 48V rails. Heating and cooling subsystems typically do not require this O-ring. Figure 8 also shows an isolation barrier. In systems with high voltages such as 800V and 400V, isolation is always required between the 12V side and the high voltage side.

In a 48V vehicle, however, the answer is less straightforward. Because of the low voltages, electrical isolation may not be required between the 12V and 48V systems in the vehicle. In practice, functional isolation (isolation that enables the system to function properly without necessarily serving as shock protection) is most likely to be used between the 12V and 48V domains.

The isolation barrier can be placed at the input or output of the system. Figure 8 shows the isolation barrier at the input of the system, where most of the system components are on the high voltage side. In this case, the 12V power supply and the communication interface require isolation components. Conversely, if the isolation barrier is to be placed at the output of the system, most of the circuit components should be on the low voltage side. In this case, the module will use an isolated gate driver to drive the transistor, as shown in Figure 9.

Figure 8 Powering the circuits in the control module via a 12V rail

Automotive High Voltage High Power Motor Driver Reference Design for HVAC Compressors

An example is shown using the LM5160-Q1 isolated Fly-BuckBoost converter, which provides 16V for the gate drivers and 3.3V (5.5V followed by a low-voltage buck) for the MCU, op amps, and all other logic. This approach is relatively simple and compact (using a single converter and transformer to generate both voltages) and has good performance.