April 2012

NTC and PTC thermistors

Reliable limiting of current surges

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A wide range of ceramic EPCOS NTC and PTC thermistors is available to protect the inputs of power supplies from excessive inrush and surge currents. These components are characterized by their high reliability and the fact that they require only minimal additional design-in effort.

High-rated capacitors are used to smooth and stabilize the DC voltage in power supplies after rectification. If they discharge at start-up, they produce a short circuit affecting the rectifier and power line. The resulting extremely high currents can destroy the rectifier or trigger the power fuse. Extremely high inrush currents can also occur with inductive loads such as larger transformers or motors.

EPCOS NTC or PTC thermistors offer cost-effective and highly reliable solutions to such problems. NTC thermistors are used as inrush current limiters (ICL) especially for power supplies in the output class up to 100 W. Figure 1 shows a simplified circuit diagram of a power input with an ICL.

Figure 1: Power input with ICL
The EPCOS ICL limits the current at start-up and prevents triggering of the fuse or destruction of the rectifier.

NTC-based ceramic ICLs possess the typical characteristic of an NTC thermistor, i.e. a temperature-dependent resistor. As the temperature rises, their resistance drops. They exhibit a relatively high resistance (1 to 120 Ω, depending on the type) at start-up at 25 °C, allowing only a low current to flow through the load. This current flow then gradually heats them up so that the current increases further until it reaches its rated value as defined by the load. This behavior consequently assures gentle and safe start-up of the load (Figure 2). As a rule, the ICL heats up by 10 to 30 K in this phase with respect to the ambient temperature.

Figure 2: Current flows as a function of time
It can be clearly seen that the inrush current is significantly limited by using an EPCOS ICL (green).

EPCOS ICLs feature an extremely low resistance in their conducting state. Their power loss is correspondingly low – at least for equipment of the lower and middle power classes – and their effect on the overall efficiency of the equipment is negligible. For larger power supplies, the ICL may be bridged with a relay after the rated current of the equipment has been reached.

Selection criteria for ICLs
The following data is critical for selecting the correct ICL:

  • The load capacitance which determines the minimum rating of the ICL
  • The maximum continuous current and the maximum ambient temperature
    (until the ICL is bridged after the start-up phase)
  • The required reduction of the inrush current at 25 °C

When rating the component, it is crucial that the maximum continuous current never exceeds the maximum permissible current of the ICL (Imax). Figure 3 shows the derating characteristic of EPCOS ICLs referred to current and temperature.

Figure 3: Derating characteristic of EPCOS ICLs

The maximum currents can be calculated as a function of the temperature with the following equations for S153 to S464 ICLs as well as for the S237 type:

NTC ICLs for protecting inductive loads
Very high inrush currents with their negative consequences as outlined above also occur with inductive loads. These include high-rated transformers or motors with slow-starting loads such as compressors, pumps, larger vacuum cleaners and drives of conveyor belts. The following example of a transformer shows how an NTC ICL can be rated for this application:

Output: 1.0 kVA

Measured inrush current: 350 A

Voltage and tolerance: 110 V AC ±10 % (99 to 121 V AC) 

Frequency: 60 Hz

Efficiency (η): 70 %

The maximum continuous current results from the output, efficiency and lowest supply voltage:

To calculate the maximum energy occurring at start-up, the impedance (Z) and inductance (L) of the transformer must be calculated:

The equation E = 0.5 × Z ×I2 can then be used to calculate the maximum energy occurring at the NTC ICL:

The EPCOS B57364S2109A002 ICL satisfies the required specification: it can absorb 70 J and is designed to handle a continuous current of 16 A over a temperature range of 0 to 65 °C.

PTC thermistors protect against short circuits
Apart from inrush currents, another danger is represented by excessive continuous currents or short circuits inside equipment. As a rule, the risks come from defective link-circuit capacitors or power semiconductors. These danger sources are eliminated by EPCOS PTC thermistors connected in series (Figure 4). Unlike NTC ICLs, they have a positive temperature characteristic, i.e. they have low resistance at ambient temperature. Excessive currents then gradually heat up the PTC ICLs – which go over to a highly resistive state and thus limit the current.
These ceramic components are practically self-resetting fuses: as soon as the current surge subsides, they cool off and return to their low-resistance conductive state.

Figure 4: EPCOS PTC ICLs

Two types of EPCOS PTC thermistors used as ICLs for charging capacitors.

Rating PTC ICLs
The maximum energy that can be applied to the component is determined by the product of the heat capacity of a PTC ICL and the maximum permissible temperature rise below which it remains conductive. It can be calculated with the following formula:

If very large loads need to be protected, it may be necessary to connect the PTC ICLs in parallel and/or series. The number of required components is calculated as follows:

The following applies to both formulas:
C: Capacitance of the link-circuit capacitor in F
Cth: Heat capacity of the PTC thermistor in J/K
EPTC: Maximum energy supply to the PTC thermistor in Ws before it becomes highly resistive
N: Number of components required
TA, max: Maximum ambient temperature of the PTC thermistor in °C
Tref: Reference temperature of the PTC thermistor in °C
V: Peak value of the capacitor charging voltage

Thanks to their steep characteristics, EPCOS PTC thermistors are also suitable for limit-temperature measurement. They are offered in SMD case sizes 0805, 0603 and 0402 for this purpose. The types of the B59721A* series in case size 0805 have response temperatures of 70 to 130 °C in steps of 10 K. Their rated resistance is 680 Ω. The response temperatures of the B59641A* (0603) and B59421A* (0402) series are between 75 and 145 °C or 75 and 135 °C respectively – also in steps of 10 K. The rated resistance of these components is 470 Ω. The maximum permissible operating voltage is 32 V DC for all types.

Mounted at hot spots in power supplies, these components may be used in the fan controllers in order to avoid critical temperatures.



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