Inrush Current Limiters in Action

Circuit Guardians At the moment a high-power system is switched on, high loads can result in serious damage—even when the extra load is only for short time. Inrush current limiters (ICLs) can help prevent these issues. In this article, TDK Electronics’ Matt Reynolds examines ICLs based on NTC and PTC thermistors, discussing the underlying technology and the device options. When high-power devices and systems such as power supplies, frequency converters or on-board 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). FIGURE 1NTC inrush current limiter and PTC thermistors in housed and standard disk designs 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). FIGURE 2Current flow in a rectifier with and without inrush current limiters (ICLs) The most traditional way to limit inrush currents is by using low-ohmic power resistor to reduce the current. However, once the inrush is over, the resistor continues to cause a power loss that affects the entire