Electric Vehicle PTC Heaters
Positive temperature coefficient (PTC) heaters bring efficiency and safety to EV heating systems. The heating element inside a PTC heater has a positive temperature coefficient and its resistance increases with temperature. When power is first applied to the cold PTC heating element, it has a low resistance and draws a large amount of current. As it heats up, the resistance increases and the current draw decreases. This makes a PTC heater inherently both safe and efficient; the PTC heater will stop drawing current if it overheats and it only draws the current it needs to maintain temperature. A PTC heater also heats faster than a traditional element, as it draws maximum current when it is cold.
Why Isolation is Critical in an EV PTC Heating System
In an EV, the PTC heater runs off the high voltage battery to maximize the power delivered to the heater. An IGBT or other high current power device is used to turn the heater on and off, while voltage and current sensors monitor the input voltage and current draw. An MCU is used to manage the heater and communicate with the other systems in the vehicle. The diagram below depicts a PTC heater configuration with the MCU in the low voltage domain. Isolation is critical for enabling the MCU to monitor and control the PTC. An isolated IGBT gate driver, such as the Si828x, or FET drivers, such as the Si823x or Si826x, safely drives the main power device. The voltage and current measurements are made using isolated measurement devices such as Si892x. The entire system can be monitored and controlled via an automotive bus such as CAN or LIN which is isolated with digital isolators, including Si86xx and digital isolators with integrated dc-dc power converters such as Si88xx.