Low-loss solutions focused on higher power level applications The design of power supplies is increasingly focused on eliminating power loss as much as possible. Once the rated power exceeds 500W, the disadvantages of the passive circuit solution become very obvious. If the ICL is always in series with the load, the power loss will be very large. The higher the power rating of the equipment, the longer the typical operating time and the more significant the incidental power loss. Assuming that the power loss of NTC ICL accounts for 1% of the total power of the device and the efficiency of the power supply is 92%, about 12.5 of the total loss is caused by NTC.

Active inrush current limiting Therefore for higher power levels, it is standard practice to use a relay or thyristor to bypass the ICL once the inrush current peak has subsided. Depending on the application requirements, the active inrush current limiting circuit can use power resistors, NTC thermistors or PTC thermistors (Figure 3) as ICL components. For example, PTC thermistors are often used in plug-in on-board chargers (OBC) for hybrid or electric vehicles. The rated power of such chargers usually reaches several kilowatts. While the benefits of active inrush current limiting are most apparent for power ratings greater than 500W, this approach may also be necessary to improve performance for lower power level applications. Although the system cost of active inrush current limiting itself is somewhat higher, it can reduce power loss for lower power rating applications, and can use relatively inexpensive lower rating switches and semiconductor devices.

When to Use a PTC Thermistor as an ICL In some applications, the use of a PTC thermistor as an ICL can provide excellent performance. The resistance of the NTC ICL at power-on depends on the ambient temperature. At a lower ambient temperature, the resistance of the NTC thermistor will be higher, resulting in lower charging current and longer charging time. On the other hand, a higher ambient temperature limits the ability of the NTC ICL to suppress inrush current because the NTC thermistor is already in a low resistance state. This temperature dependence can be problematic for some applications, particularly those with a wide operating temperature range. For example, an outdoor power supply used in the northern winter may never rise hot enough to make the resistance drop low enough.

On the contrary, the hot water circulation pump may be very hot when it is started, which will make the NT thermistor unable to limit the inrush current. After the system is shut down, the cooling time of NTC thermistors usually varies from 30s to 120s, depending on the specific equipment, installation method and ambient temperature. The charging current can only be limited again when the NTC ICL has cooled down completely. In many cases, this cooling time is already fast enough; however, it is sometimes necessary to effectively limit the inrush current before the NTC has cooled sufficiently. This may occur in the rapid discharge of the DC link capacitor, which occurs in inverter-driven household appliances such as new washing machines and dryers. The necessary cooling time after a brief power outage is very critical. Therefore, the active inrush current limiting design must always take into account the occurrence of inrush current peaks when all possible NTC ICLs are still in the low resistance state. In both cases, SINOCHIP PTC thermistors can provide an effective inrush current limiting scheme.

Built-in self-protection function Under normal operating conditions, the PTC ICL is used as an ordinary resistor. When the power is turned on and the temperature of the component is the same as the ambient temperature, the resistance value of PTC ICL varies from 20 ohms to 500 ohms depending on the model. This is sufficient to limit the surge current peaks. Once the DC link capacitor is fully charged, the PTC ICL is bypassed.

If the charging circuit fails, the special function of the PTC thermistor can protect the circuit. When current passes through the element, the temperature of the PTC thermistor increases and the resistance increases significantly. Therefore, thanks to its self-protection function, PTC thermistors have inherent advantages in the following failure modes:

-Capacitor short circuit

-The current limiting element is not bypassed (switching element failure) when the DC link capacitor is charged.

All these failure modes have one thing in common: the current limiting element is thermally stressed. There are two ways to ensure that the ICL element will not be damaged in similar situations: use a power resistor with sufficient power rating or use a PTC thermistor. The design of the SINOCHIP PTC ICL allows it to operate when directly connected to the supply voltage of the maximum rated voltage without additional current limiting measures, because the PTC ICL has a self-protection function. In the event of an excessive current such as a short circuit, the temperature of the PTC increases, causing its resistance to rise significantly, so that the PTC thermistor itself can limit the current to a non-critical level (fig. 4).

SINOCHIP PTC thermistors have some key advantages as ICL components for active inrush current limiting in a number of applications:

-Its ICL function will not be affected by extreme operating temperatures.

-Once the load is turned off, effective inrush current limiting can be achieved, and cooling is already carried out during normal operation.

-Self-protection against current overload caused by circuit fault.

Thanks to the broad product portfolio of SINOCHIP ICL, you can reliably protect your power supply from high inrush currents and short circuits under harsh temperature conditions.