The purpose of the power switches is to supply and regulate current to the PTC load. Current being cut off from the PTC load can either be due to the user turning off the vehicle heating system, a short circuit fault on the PTC load, a fault in the switch driver, or a fault in one of the switches themselves.

High-voltage automotive applications tend to use at least one of the three power switch types: Silicon metal-oxide field effect transistor (Si MOSFET), insulated-gate bipolar transistor (IGBT) and Silicon Carbide metal-oxide field effect transistor (SiC MOSFET). Gallium Nitride (GaN) is also emerging in some automotive applications depending on the battery voltage used. Since PTC heaters are typically rated for at least 5kW of output power, they exceed the limitations for traditional Si MOSFETs. So, there are really two choices to choose from: IGBT and SiC. The breakdown of the power switch type in regard to the power level is suitable for can be seen in Figure 3-15. SiC and GaN are great for applications that switch at high frequencies. However, switching losses are not critical to mitigate in PTC heaters. In addition, fast switching can introduce more EMI in the system, which is a much more important factor to mitigate in the PTC heating systems. SiC and GaN are also significantly more expensive than IGBTs, which are currently the most suitable solution for PTC heaters.

IGBTs are popular for high-power applications that operate at switching frequencies ranging from 5kHz to 20kHz, so they are compatible in typical PTC heater control module designs. IGBTs tend to have very low ON-resistances, enabling low-conduction losses, thus good efficiency.

The current going through the power switches depends on the impedance of the PTC load at that time and the high-voltage battery level. For the high-side switch(es), the designer must select a power switch that is rated for the high voltage battery level. It is recommended that power switches are rated higher than the maximum current that is expected to go through their respective PTC loads. This can be determined by dividing the battery voltage by the minimum resistance possible of the PTC load under normal operation. To understand when a typical PTC load is expected to be at minimum impedance, see Section 3.9.

A major decision the designer must make when designing the power switch into the application is how much drive strength is needed to be delivered from the switch drivers. There are multiple factors involved in this: power switch turn-on and turn-off time, efficiency and voltage overshoot risk mitigation. Having a higher peak drive strength will turn on the power switch faster since the gate threshold of the power switch will be reached sooner, also resulting in lower switching losses. The designer may want to consider the implications of increasing the drive strength, though. Too large of a change in drain-source voltage over time can put the power switch at risk of voltage overshoot induced by the parasitic inductances in the system. It is recommended that the drive strength is at a level at which risk of this condition is mitigated. Lowering the drive strength reduces the risk of V_DS overshoot, as well as reduces ringing of the gate-source voltage (V_GS) and radiated noise of the power switch. However, the rise time for the power switches in the system is mitigated by the resistance of the PTC load, so some degree of voltage overshoot is inherently mitigated by the application. Lower drive strength, on the other hand, can result in higher switching losses. To reach the appropriate drive strength needed for the power switch, the designer must do some testing to get the right balance between system efficiency, timing and risk mitigation.

In addition, the designer must consider the power needed to drive the gate of the switch, power dissipation ratings of the switch driver and switching frequency. For guidance on picking sufficient gate drive strength, Fundamentals of MOSFET and IGBT Gate Driver Circuits would be a great resource.

The initial gate resistance can be changed until the desired drive strength is achieved. For guidance on selecting gate resistance, the External Gate Resistor Design Guide for Gate Drivers can be a good resource.