The PTC load is the element that heats up once current is supplied to it. The more current flowing through the PTC load, the hotter the load will get. This trend continues to a certain magnitude of current, then the resistance of the PTC load will significantly increase, limiting the current flowing through the load. In their article “How does a PTC heater work?”, DBK USA explained the resistive characteristics of a PTC heater in detail. Figure 3-16 is a graph from the article mentioned above showing that trend. The heat is either captured by the coolant of the HVAC system or blown directly throughout the cabin. The power rating of the PTC load can vary from about 5Kw to well over 10kW.

A. Traction inverter: 15kW to 400kW

B. EV charger: 3.3kW to 22kW

C. PV booster and inverter: 5kW to 1MW

Figure 3-16 PTC Load Resistance Based on Temperature

When designing the control module and algorithm, it is critical the understand the resistive properties of the specific PTC load being used. A specific current magnitude flowing through the PTC load will result in a specific load temperature being achieved. So, it is highly recommended that the current, voltage and temperature that the PTC is experiencing is measured and monitored. This way, the current and voltage can be cross checked with the temperature to ensure that the PTC load is achieving the desired heat. Guidance for measuring temperature can be found in the “Selection of the temperature sensing” section. Measuring current may not be enough on its own due to possible variations in DC Bus voltage or resistive properties of the PTC load. Monitoring the current and heat can also help ensure that the PTC loads’ power rating is not exceeded, avoiding damage to the module, vehicle, and user. Guidance on monitoring load current can be found in Section 3.10.