When high-power devices and systems such as power supplies, frequency converters or onboard chargers are switched on, loads that are often many times the rated current can cause significant stress or damage. Although this extra load is only for a short period of time, it can damage the system, trip fuses or cause other issues with how the device operates. In order to protect the devices and circuitry, ceramic inrush current limiters (ICLs) may be used that are based on NTC and PTC thermistors (Figure 1). With NTC (negative temperature coefficient) thermistors, the resistance decreases with increasing temperature. In PTC (positive temperature coefficient) thermistors, resistance increases as temperature increases. When a specific temperature is exceeded, PTC thermistors show a sharp rise in resistance. High inrush currents come in two different types. First, inductive loads that occur in transformers and motors require very high currents to create the magnetic fields needed to operate properly. Second, high-capacitance capacitors in DC links cause high charging currents and cause significant stress to the capacitors and especially to the rectifiers at the moment of connection (Figure 2).
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