When switching on electrical equipment such as motors, transformers, drives, ballasts, and power supplies, peak inrush currents can be several times greater than the circuit’s steady- state operating current.
The effects of this inrush current can be devastating to the circuit components. Contact bouncing in switches and relays may cause the contacts to become pitted from the arcing between the points; this can also cause the welding together of switch contacts. High inrush current severely stresses converters, input rectifiers, and capacitors, and is the most common cause of nuisance fuse and circuit breaker failures.
Proper corrective devices must be put in the circuits to prevent this damage and failure. These are called inrush current limiters, and there are a few different options to consider when selecting one.
For very small power supplies (a few watts at most), adding a resistor in series with the line is a simple and practical solution to limit the inrush current.
However, the large resistance required to limit peak inrush current causes a great loss in efficiency and is not suitable for higher wattage devices.
A thermistor is an electronic component that exhibits a large change in resistance with a change in its body temperature; it is actually a contraction of the words “thermal resistor”. These are regularly used as current limiters.
There are two types of these
- NTC (Negative Temperature Coefficient)
- PTC (Positive Temperature Coefficient ).
They are rated according to their resistance at room temperature (25C).
When power is applied to a circuit with an NTC, the NTC’s resistance starts off high. After the inrush current has passed and the converter is running at steady state, the NTC heats up and its resistance drops.
NTC’s limit the inrush current only when they are cold. When power is turned off and quickly on again, the NTC does not have time to cool down and will not limit the inrush current.
For inrush current limiting, the NTC must be connected in series with the load circuit. Several limiters can also be connected in series for higher damping. Inrush current limiters must not be connected in parallel.
PTC thermistors are also used instead of conventional fuses to protect from overcurrent. They not only respond to inadmissibly high currents but also if a preset temperature is exceeded. They limit the power dissipation of the overall circuit by increasing their resistance and thus reducing the current.
The voltage-current characteristic curve of the PTC makes it an ideal candidate as a short circuit or over-current protective device. When connected in series with the load the PTC remains in its low resistance state resulting in negligible attenuation to current flow. When a short circuit or over current condition occurs, the PTC will switch into its high resistance state thereby limiting the current flow in the circuit well below the normal operating level. When the fault condition is removed, the PTC will return to its low resistance state allowing current flow to recover to its normal level.
Although several PTC’s may be connected in series for temperature sensing applications, do NOT connect them in series to obtain higher voltage ratings. Since no two devices are exactly the same, one would tend to heat faster than the other, thereby limiting the current flow through the other device and resulting in the entire voltage available being dropped across the single device.
When the PTC thermistor is in series with a load, it goes high-resistance to respond and has to handle the entire supply voltage. So the Vmax should be chosen sufficiently high. Possible supply voltage fluctuations must also be allowed for.
The PTC has to be rated for sufficient current. This is the current at which the thermistor will under no circumstances turn off.
It is necessary to examine whether there will be conditions in which the maximum permissible switching current will be exceeded. Overloading the PTC by too high of switching current must be avoided. If there is a possible risk, it can be countered by connecting a resistor in series with the thermistor.
Take into consideration that inrush current limiters need time to return to their cold state in which they can provide adequate inrush current limiting due to their high resistance. Cool-down time varies according to the particular device, its mounting method and the ambient temperature. The typical cool-down time is roughly one minute.
There is a design technique engineers can employ to eliminate the problems posed by the cool-down/recovery time required for inrush current limiters to return to their initial level of resistance. Essentially, this involves designing inrush current protection to drop the inrush current limiters out of the circuit after they have performed their function. By removing them from the circuit once the initial surge has passed, the thermistors have an opportunity to cool down, so they are ready to respond to a subsequent surge after a power drop-out occurs.
This technique requires the addition of either a relay or a triac in parallel with the inrush current limiter, plus the circuits necessary to control it. All the components of the protection circuit would be in series with the input to the line. Once the inrush current has been absorbed by the thermistor, then either the triac starts firing or the relay closes.
These current limiter devices are available in a wide variety of configurations and protective coatings to suit almost any application. In general, bead-type thermistors offer high stability and reliability, fast response time, and operation at high temperatures. Disk and chip types are generally larger than the bead and so they exhibit response times that are comparatively slower. However, they usually have higher dissipation constants and thus better able to handle power in measurement, control, and compensations applications. They usually are a lower cost and more readily available with interchangeable characteristics.
When selecting an inrush current limiter for an application some things to consider are the energy rating, the max inrush current and the resistance. The following formulas will help with the selection process.
Selecting an Inrush Current Limiter
Use the maximum allowable surge current and Ohm’s Law (V/I=R) to determine the minimum resistance required for your application.
Peak Voltage/inrush current max = minimum resistance required.
- Vp/(inrush)max = R
- Determine the maximum steady state current that will flow through the Inrush Current Limiter.
- SSI = Output Power/(input V x Efficiency)
- Confirm that the device will be operating in the temperature range specified. Typically around 25ºC, but some data sheets provide the Resistance Temperature Curve for additional temp requirements if necessary.
- Check data sheets to confirm that the chemicals or use of potting and sealing compounds are ones that can be used with your thermistor. The reduction of the titanate ceramic caused by certain chemicals will result in formation of low-resistance conducting paths or altered thermal properties and can lead to overheating and failures.
As you can see, there are many advantages of using a Thermistor for in-rush current.
- Lower component costs.
- Simpler design.
- Significantly less board space
- High Inrush Current can severely damage circuit components.
- Proper corrective devices must be put in the circuits to prevent these damages and failures.
- Inrush current limiters are electronic components that exhibits a large change in resistance with a change in its body temperature and are used to control the inrush current.
- Resistors can be used in applications with very small wattage.
- Thermistors, PTC and NTC are used to control larger inrush current applications.
- When power is applied to a circuit, the NTC’s resistance starts off high: After the inrush current has passed and the converter is running at steady state, the NTC heats up and its resistance drops.
- PTC remains in its low resistance state resulting in negligible attenuation to current flow. When a short circuit or over current condition occurs, the PTC will switch into its high resistance state thereby limiting the current flow in the circuit.
- Inrush current limiters need time to return to their cold state.
- Selection criteria is, Reference Temperature, Minimum Resistance, Maximum SSI (Steady State Current) Rated Current, and Maximum permissible Switching Current.
- To select the best Surge Limiter for an application choose the smallest one available that is rated for the correct current.