Inrush Current Limiters (ICL) PTC and NTC thermistors limit turn-on currents
Inrush Current Limiters based upon PTC and NTC technology assist in limiting turn-on currents in capacitive and inductive loads so that line fuses do not operate.
By David Connett, Director IC Reference Design EPCOS AG, A Group Company of TDK-EPC Corporation
Turning on electrical devices generally cause high inrush currents which can damage electronic components and cause interruption of the line voltage if measures are not taken to minimize this switch-on current.
In order to avoid impermissibly high current peaks when charging capacitors or switching on inductive loads, as a rule it is necessary to limit the initial current by a resistor connected in series. This function can be implemented by either fixed resistor, NTC or PTC thermistors.
In conventional power supplies i.e. less than 100W rating, an effective way to reduce the inrush current, at low cost, is to use an Inrush Current Limiter (ICL) based upon the principles of NTC Thermistors. NTC Thermistor is a temperature dependent resistor with a Negative Temperature Coefficient (NTC), which means that the electrical resistance decreases with increasing temperature. Whereas NTCs are frequently used as a temperature sensor measuring temperature applied from external environments, ICLs use the self-heating effect due to electrical current flow through the component.
When the device is first turned on, the inrush current is limited by the high resistance of the ICL in its cold state (ambient temperature). During the initial transient sequence of switching on capacitive loads, which typically take a few milliseconds, the ICL will heat up in the range of approx. 10 K to 30 K.
Additional further warming may come from steady state current during normal operation of the load. The steepness of the R/T characteristic of ICLs results in low residual resistance during operation in this mode meaning that the component has practically no effect on the application. This feature is also advantageous since lower resistance results in lower losses in terms of energy and with careful selection can assist designers to achieve new operation and standby energy limits without relays.
However, in industrial applications fixed resistors have been traditionally used which rely upon a relay, to short circuit the resistor after the inrush sequence. The following image shows a simple power supply circuit with a short-circuit relay as an option.
However, failure of the bypassing mechanism can place excessive stresses on the fixed resistor potentially causing it to overheat. In such applications, the deployment of NTC or PTC based ICLs offers substantial safety related advantages.
In such a configuration, the life time of the device can be extended to in excess of 100.000 operations since the mechanical stresses placed on the device by the switching process of the high continuous currents can be eliminated. However, in the case that the relay closing fails, the ICL should be capable of carrying the maximum continuous current to ensure that the system remains functional. (see Notes on Scaling a NTC Inrush Current Limiter).
On the other hand it may be desirable that the system actually shuts down in such a situation since, in this case, a component has actually failed. Here an ICL based upon Positive Temperature Coefficient Thermistors could be advantageous.
In fault-free charging, these components act like a fixed ohmic resistor and limit the peak value of the charging current. In the case of a malfunction, the resistance of the PTC will increase due to the inherent rise in temperature effectively limiting the current into the load. In contrast, when a fixed resistor is used as a charging current limiter, these malfunctions would produce very high power dissipation in the resistor, thus requiring an uneconomic over-dimensioning of this component.
During charging of a capacitor the energy transferred to it will heat up the PTC Inrush Current Limiter in series. During normal operation state it should however be avoided that the PTC heats up to an undesirable extent and as a result becomes highly resistive as the capacitor would otherwise not be completely charged.
The selection of PTCs for this application will be covered under the section Notes on Scaling a PTC Inrush Current Limiter
Load Capacitance of Device to be Protected
The high inrush current of devices results from the higher energy required to turn on. In power supplies the energy requirement is primarily caused by load capacitors. The associated turn-on operation imposes a current pulse load on the inrush current limiter. So this energy must be known to select the right component. It can be converted into capacitance for a given voltage. This capacitance is used as a measure of the pulse handling capability (Ctest) of inrush current limiters.
Ctest figures typically refer to line voltages of 110 V and 230 V. If an inrush current limiter is operated at other voltages (e.g. the low voltages of electronic circuits), the appropriate Ctest figure is easily calculated:
The required Ctest determines the minimum size of the component.
Notes on Scaling a NTC Inrush Current Limiter ICL
A few items of data are needed to scale an inrush current limiter:
• The load capacitance value of the device to be protected (determination of minimum size of the component)
• The maximum steady state current and maximum ambient temperature (if the ICL is not shortened after inrush sequence)
• The required reduction of the inrush current (determination of the “cold resistance” at 25°C)
• The maximum supply voltage
Steady-State Current and Maximum Ambient Temperature
Select the component so that the steady-state current does not exceed the maximum admissible current (Imax) of the inrush current limiter. The maximum admissible current is produced from the figure for Imax and the derating with the maximum ambient temperature. When scaling a design, remember the possibility of line voltage fluctuations and different operating states (steady-state currents) of the device itself, and incorporate appropriate precautionary measures.
The following equations can be used to calculate the load derating according to the above mentioned curves.
Required Reduction of Inrush Current
The required Ctest figure alone will determine the component that is needed. Within this component model the maximum steady-state current then determines the highest possible cold resistance (R25) that can be used for an application.
The higher the cold resistance (R25) of the inrush current limiter, the more the inrush current is dampened. If the current limiting effect of a component is inadequate, choose a larger model.
Notes on Scaling a PTC Inrush Current Limiter
The energy that can be applied to a PTC Thermistor without driving it to a highly resistive state is to be calculated as the heat capacity multiplied with the maximum rise in temperature allowed. The maximum increase in temperature is the difference between Reference Temperature (Tref) of the PTC and the initial temperature before applying energy to the PTC (Ambient Temperature TA,max). Therefore the maximum energy that can be applied to a single PTC without getting into the high ohmic (selfprotecting) state is:
The number of required components (N) (connected in parallel and up to some extent in Series) can now be calculated as the quotient of Ec and EPTC:
- n the number of required J20X elements
- C the capacitance of the link-circuit capacitor in F
- V the maximum peak charging voltage of the capacitor in V
- Cth the heat capacity of a J20X charging resistor in J/K
- TRef the reference temperature of the PTC ceramic in °C
- TAmax the maximum ambient temperature at the insertion point of the charging resistor in °C
- EPTC the maximum energy to be applied to the PTC without turning into self protecting mode (high ohmic state)
- EC the energy needed to fully charge a capacitor
However, the use of NTC ICLs is not limited to the reduction of inrush currents in power supplies. They are also ideal for the protection of transformers and the soft starting of motors ( power tools, compressors, vacuum cleaners, conveyors belts).
In the next example we will consider the inrush current reduction in a transformer.
Transformer: 1.0 KVA
inrush current: 350 A
Line voltage / tolerance: 110 Vac ± 10% = 99 / 121Vac
Transformer efficiency: 70%
Maximum steady state current can be calculated through the KVA rating, the efficiency rating and minimum line voltage:
To calculate the maximum energy, we need to take into consideration the in inrush current (ISC) and the Inductive Reactance (Z).
We need to use the measured inrush current and the maximum supply voltage to determine the reactance of the transformer. Hence,
Form this, we can determine the inductivity of the transformer using the formula:
Where f is the given supply frequency in Hertz.
Since energy equates to E = 0.5*Z*I2 where I is the maximum inrush current, we can calculate that the Energy that must be absorbed by the ICL is
Hence, a suitable ICL for this application would be the B57364S2109A 2 as this has a Joule rating of 70J and a maximum steady state current (0 to 65°C) of 16.0A.
Inrush Current Limiters based upon PTC and NTC technology assist in limiting turn-on currents in capacitive and inductive loads so that line fuses do not operate. In the case of ICL bypassing, the difference technologies permit equipment to remain fully functional in the case of failure of the bypassing mechanism or to interrupt the operation of the equipment in high power applications.