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NTC thermistor chat with common sense

Article Source:Jocol Group Popularity:153 Published:2017-10-17 11:34:36 [SmallInBig]
Once the NTC thermistor is heated, its initial high resistance will be reduced to a negligible level, this feature makes the NTC ICL rated power up to about 500W power supply to become a standard ICL ...
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Once the NTC thermistor is heated, its initial high resistance will be reduced to a negligible level, this feature makes the NTC ICL rated power up to about 500W power supply to become a standard ICL solution. NTC thermistors have a higher resistance at lower temperatures and lower resistance at higher temperatures. In the lower temperature state, the higher initial resistance of the NTC ICL can effectively absorb the peak inrush current. Due to the effect of the current load and the consequent self-heating effect, the ICL resistance will then decrease to a few percent of its room temperature resistance. This feature reduces the power consumption of the ICL during continuous operation, so the NTC ICL can remain in the circuit after the capacitor is fully charged. Finally, the cost of using NTC ICL is low and the solution is easy to implement.

High-loss solutions focused on higher power levels The design of power supplies is increasingly focused on eliminating power losses as much as possible. Once the rated power exceeds 500W, the shortcomings of the passive circuit solution become very noticeable. If the ICL is always connected in series with the load, the power loss will be very large. The higher the rated power of the equipment, the longer the typical working time, the more obvious the incident power loss. Assuming NTC ICL power loss accounts for 1% of the total power of the device, the power efficiency of 92%, then about 12.5% of the total loss is caused by the NTC.

Active Inrush Current Limit So for higher power levels, the standard practice is to use relay or thyristor bypass ICL once the surge current peak has subsided. Depending on the application requirements, the active surge current limiting circuit can use power resistors, NTC thermistors or PTC thermistors as ICL components. For example, PTC thermistors are often used in hybrid or electric vehicle plug-in car charger (OBC), the rated power of such chargers usually reach several kilowatts. While the benefits of active inrush current limiting are most pronounced for rated power greater than 500W, this approach may also be necessary to improve performance at lower power levels. While active inrush current limits its system cost slightly higher, it can reduce power losses for lower rated power applications and can use relatively inexpensive rated switches and semiconductor devices.

When to use PTC thermistors as ICLs In some applications, the use of PTC thermistors as ICL can provide excellent performance. The resistance of the NTC ICL when the power is turned on depends on the ambient temperature. In the lower ambient temperature NTC thermistor resistance will be relatively high, resulting in lower charging current, charging time is longer. On the other hand, a higher ambient temperature limits the NTC ICL's ability to suppress inrush current because the NTC thermistor is already in a low-impedance state. This temperature dependence can cause problems for some applications, especially for applications where the operating temperature range is wide. For example, the use of outdoor power in the northern winter may never be enough to rise enough to make the resistance drop low enough. On the contrary, the hot water circulation pump may have been hot at the start, which makes the NTC thermistor can not limit the inrush current. After the system is turned off, the cooling time of the NTC thermistor usually varies from 30S to 120S depending on the particular equipment, installation method, and ambient temperature. The charge current can only be restricted again when the NTC ICL is completely cooled. In many cases, the cooling time is fast enough; however, it is sometimes necessary to effectively limit the inrush current before the NTC is fully cooled. This may occur in the rapid discharge of DC bus capacitors, which can occur in inverter-driven household appliances such as new washing machines and dryers. The necessary cooling time after a brief power outage is critical. Therefore, the active surge current limit design must always take into account the fact that all possible NTC ICLs are still present in the low resistance state when the surge current peaks occur. In both cases, the PTC thermistor can provide an effective inrush current limiting scheme.

Built-in self-protection function

Under normal operating conditions, PTC ICL is used as a common resistor. When the power is turned on and the component temperature is the same as the ambient temperature, the PTC ICL varies between 20 ohms and 500 ohms depending on the model. This is sufficient to limit the surge current peak. Once the DC bus capacitor is fully charged, the PTC ICL is bypassed,

If the charging circuit fails, the PTC thermistor's special function can play a role in protecting the circuit. When the current through the components, PTC thermistor temperature will rise, the resistance will be significantly increased. Therefore, thanks to its self-protection function, PTC thermistor in the following failure mode has a congenital advantage:

- Capacitor short circuit

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

All of these failure modes have one thing in common: the current limiting element is subject to thermal stress. There are two ways to ensure that ICL components are not damaged in a similar situation: use a power resistor with sufficient rated power or use a PTC thermistor. The design makes it work even when it is connected directly to the supply voltage of the maximum rated voltage, and does not require additional current limiting measures because the PTC ICL has a self-protection function. In the event of a large current such as a short circuit, the PTC temperature will rise, resulting in a significant increase in its resistance, so PTC thermistor itself can limit the current to non-critical level.

If a capacitor short circuit occurs, the current through the PTC ceramic resistor drops rapidly to a noncritical value (blue). However, if a common resistor is used, the current will remain at a high constant value (red).

PTC thermistors in some applications as active inrush current limiting ICL components have some key advantages:

- The ICL function is not affected by extreme operating temperatures.
- Once the load is turned off, an effective inrush current limit can be achieved and cooling has been performed during normal operation.
- Self-protection for current overload caused by circuit faults.
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