Thermistors are usually the preferred solution for detecting temperature limits. For NTC (negative temperature coefficient) thermistors, the resistance decreases with increasing temperature. In a PTC (Positive Temperature Coefficient) thermistor, the resistance increases with increasing temperature. When the temperature exceeds a certain temperature, the resistance of the PTC thermistor will rise sharply, so it is suitable for use as a temperature limit sensor. On the other hand, NTC thermistors exhibit higher linearity and are therefore suitable for temperature measurement.

The inrush current needs to be limited to prevent the current from exceeding a critical level and blowing the fuse or damaging the rectifier.

Therefore, NTC thermistors are not always used as an ideal solution for inrush current limiters (ICL)—especially in power supplies. Due to its characteristic resistance curve, PTC thermistors have multiple uses in thermal monitoring and current limiters.

When there are harsh temperature and power conditions, PTC thermistors can provide more consistent and reliable surge current surge and short circuit protection, while providing accurate temperature control and measurement.

When electrical equipment, inverters, or power supplies are activated and turned on, high currents are encountered and current spikes may occur. Disproportionate surge currents can damage power components, including rectifiers, fuses, and other components that receive current from the power supply. Therefore, protective measures are needed to prevent such problems.

In active inrush current limiting, ohmic resistors, NTC thermistors or PTC thermistors can be used as ICL components.

Most engineers usually use one of two methods to limit this problem. First, they may use inrush current limiting protection devices (passive ICL circuits). Secondly, they can use an active bypass circuit, which activates after any inrush current peaks have diminished (active ICL circuit). Deciding which current-limiting method should be used depends on various variables, including the rated power, the frequency at which the device may be exposed to inrush current, the operating temperature range, and system cost requirements.

Passive ICL has several limitations—especially in very small power supplies with a power rating of only a few watts. In this case, the simplest solution is to add an ohmic resistor in series with the load, but the power loss of the fixed resistor will significantly weaken the overall efficiency of the power supply with higher rated power. Therefore, NTC thermistor has become a standard ICL solution, which requires a passive current limit of up to about 500 watts.

Since the NTC thermistor is high ohmic when it is cold and low ohmic when it is hot, the high initial resistance of the NTC ICL will absorb the peak surge current when it is cold. However, the resistance of the ICL will then drop to a few percent of its value at room temperature, thereby reducing the power consumption of the inrush current limiter in continuous operation. This is why the NTC ICL may remain in the circuit after the capacitor is fully charged.

Once the rated power approaches 500 watts, the efficiency of passive circuit solutions will decrease and their weaknesses will become more obvious. Considering that power supply design is increasingly focusing on eliminating power losses as much as possible, passive ICL is not an ideal solution for these applications. This is because they are always connected in series with the load, which can cause excessive power loss. The higher the power rating of the device and the longer its typical operating time, the greater the parasitic power loss that occurs.

For example, NTC ICL may generate a power loss of one percent of the total power of the device. If the rated efficiency of the power supply is 92%, then the 12.5% ​​power loss is a direct result of NTC ICL.

In applications where power levels exceed 500 watts, once the peak of the inrush current decays, using a relay or triac to bypass the ICL is the standard method. In such applications, the active inrush current limiting circuit can use power resistors according to different requirements, such as NTC thermistors or PTC thermistors as ICL components. For example, PTC thermistors are usually used in on-board chargers for plug-in vehicles (pure electric and hybrid vehicles) with a rated power of only a few kilowatts.

Although the benefits of active inrush current limiting are most beneficial in applications with power ratings greater than 500 watts, active inrush current limiting may be required to help improve the performance of low-power applications. The most important consideration is whether the cost is worth the result. For example, although the overall system cost of active inrush current limiting may be slightly higher than without, the reduced power loss may allow the use of cheaper switches or other components due to the lower rated power.

Use PTC thermistor to limit inrush current

As mentioned earlier, PTC thermistors can perform better like ICL in many applications. For example, the resistance of NTC ICL depends on the ambient temperature when it starts to receive power. When the ambient temperature is lower, the resistance of the NTC thermistor is higher, and the charging time is longer due to the lower charging current. On the contrary, a higher ambient temperature will cause the NTC ICL to be in a low ohmic state, which will limit its ability to suppress inrush current. Due to this temperature dependence, the use of NTC ICL in applications with a wide operating temperature range due to the sun, friction, or other factors that affect the ambient temperature may cause problems.

In addition, the NTC ICL must be completely cooled to limit the charging current again. The typical cooling time of an NTC thermistor is between 30 and 120 seconds after it stops energizing, but it is also affected by the ambient temperature and installation method. In many applications, the cooling cycle is fast enough, but the inrush current needs to be limited before the NTC is cooled enough to work. This is the case in applications where DC bus capacitors can be quickly and actively discharged, for example, in inverter-driven household appliances such as modern washing machines and dryers. After a short power outage, the necessary cooling time is also critical.

Therefore, the active surge current limit design must consider all possible surge conditions, and the NTC ICL is in a low ohmic state. Alternatively, PTC thermistors provide an effective inrush current limiting solution. Under typical operating conditions, PTC ICL is used as an ohmic resistor.

When the system is powered on and the temperature of the components is the same as the ambient temperature, the PTC ICL usually has a resistance of 20 to 500 ohms. This is sufficient to limit the inrush current surge. Once the DC bus capacitor is charged satisfactorily, the PTC ICL will be bypassed.

In addition, if the charging circuit fails, the PTC thermistor can protect the circuit. When current flows through the PTC thermistor, it heats up and increases its resistance, making it self-protective and providing significant advantages for other forms of inrush current limiting. Due to these characteristics, PTC thermistors are more suitable for applications where a capacitor short-circuit or a switching element failure causes the current-limiting element to be bypassed after the DC link capacitor is charged. However, both of these failure modes will cause thermal stress on the current limiting device.

During such incidents, there are two ways to protect ICL components from damage. The most ideal is to use a PTC thermistor, and the second is to implement another power resistor with sufficient power rating to account for these surges. Although this second one may not always be practical, PTC ICLs are designed to be directly connected to the supply voltage even at their maximum rated voltage, which makes them suitable for almost all applications that may have problems.

If the short circuit causes the current to be too large, the temperature of the PTC will increase, which will cause the resistance of the device to increase significantly. Therefore, the PTC thermistor itself limits the current to a non-critical level.

Moreover, because these PTC ICLs have self-protection functions, no additional current limit is required, thereby reducing system costs.

TDK's EPCOS PTC thermistor product portfolio is particularly suitable for inrush current limiting. Compared to PTC thermistors, they use a more homogeneous ceramic material, which improves reliability while allowing processing by reflow soldering. Because of these characteristics, even in the most rugged electronic applications, PTC can meet stringent requirements.

When used as an ICL component for active inrush current limiting, PTC thermistors have the following main advantages:

* Their ICL function is not affected by extreme operating temperatures. * Effectively limit the inrush current immediately after the load is turned off, and it has been cooled during normal operation. * They have a self-protection function to prevent current overload caused by circuit failure.

The PTC thermistor is designed to have a very low resistance in the range of only a few ohms at its rated temperature. If there is a surge of current, their power consumption will increase, and the thermistor will heat up, causing the resistance to increase, thereby limiting the current. Only when the component cools, it will return to a low resistance state. This behavior makes PTC thermistors an ideal choice for current limiters in most applications.

Under severe temperature conditions, PTC ICL can reliably protect most power supplies from high surge currents and short circuits.

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