Electric vehicles are much more efficient than conventional vehicles with combustion engines.
One consequence of this is that the waste heat from the motor no longer suffices to heat the interior of the vehicle. In this case, part of the energy stored in the battery must be converted into heat. In order to enable an adjustable heating power without dependence on the operating temperature or battery voltage, power semiconductors are used in the new generation of high-voltage heaters. They control the energy flow from the battery to the heating element. The heating element heats the coolant, which is connected to the vehicle’s air conditioning system via a heat exchanger. A blower transports the warm air into the interior.
A normal electric vehicle needs a heating power between 5 and 7 kW to cover the heat demand. If the car is heated exclusively by a resistive load (heating element), the range decreases accordingly. Alternatively, there are also systems that do not rely solely on resistors to generate heat. They use the heat pump concept: thermal energy is transferred from a cold source (environment) to a warm source (interior) by means of externally supplied energy. The energy balance of a heat pump is better than heat generation by an ohmic load and the range is affected less. However, with this system, the costs of the vehicle increase and its availability is determined by the ambient temperature. In regions with a very cold winter, these systems cannot generate enough heat. Classic resistive heaters are indispensable there. Heating systems not only ensure the comfort of car occupants, but also have important safety functions: for example, they defrost windows or dehumidify the interior to give the driver a clear view of the outside world. The battery requires a certain operating temperature. The heater ensures that the battery is always in the green temperature range. The heater can also act as a discharge resistor in the event of high voltage peaks. If there is an unwanted increase in the voltage of the vehicle’s electrical system, the device is able to absorb this energy and thus limit the amount of overvoltage. This protects the battery and other systems connected to the vehicle electrical system.
The simplest form of a resistive heater is shown in Fig. 2: the switch is operated with an adjustable duty cycle so that the power output always matches the setpoint. To distribute the heat better, several branches are connected in parallel, usually two or three. In order to be able to switch off the heating system safely in the event of a fault, safety switches which are switched on permanently during normal operation are required. Should a fault occur, these switches switch off and thus disconnect the heating elements from the high-voltage vehicle electrical system.
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