# NTC Inrush Current Limiting

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Inrush current limiting is an important consideration in electronic designs, especially in the case of switched-mode power supplies or similar devices which have large amounts of bulk storage capacitance.  The designer needs to manage the inrush current carefully in these circumstances for reasons that I hope will become obvious.

Let’s look at a typical example. Figure 1 shows the input circuitry from a simple 500W switched-mode power supply modelled in LTspice. The mains source impedance is modelled as a 0.4Ω resistance in series with 800µH inductor. The mains voltage – 240Vrms in my part of the world – is rectified and filtered by two 330µF 400V capacitors each with an ESR of 0.96Ω.

If we were to switch the power on at the peak of the mains voltage cycle, the capacitors will fully charge in the first half cycle, and we see peak inrush current of 180A as shown in the simulation result. If the mains were connected at the zero crossing, things are better, but the current will nevertheless top out at 80A. Since we can’t predict what part of the cycle the power will be applied, we have to design for the worst case.

As you can imagine, this level of peak current will not be good for the rectifier diodes and may even trip any upstream circuit breakers. Something has to be done.

The obvious solution is to add a current-limiting resistor as shown in Figure 2. With 22Ω in series with the mains, the capacitors are charged progressively over several cycles, with the peak current limited to a more reasonable 12A. This is great for inrush, but there is a problem when it comes to steady state operation. This design was intended for a 500W SMPS which will draw around 2.2A from the mains at full load.  The 22Ω resistor would dissipate over 100W at this current – assuming the supply can manage the 48Vrms dropped across the resistor. This is clearly not practical.

We could use a relay to short the 22Ω resistor after a couple of hundred milliseconds, but this adds cost and complexity. An alternative is to use a NTC inrush protection resistor in place of the 22Ω resistor. These have a relatively high resistance when cold, limiting the inrush, but their resistance drops as the they self-heat when conducting a “normal” level of current.

They are made from a variety of metal oxides in a polycrystalline structure and are typically available in disc form as shown in Figure 3. This figure also shows the typical normalised resistance temperature characteristic of these devices, extracted from an application note by TDK. You can see that their resistance typically drops by a couple of orders of magnitude between room temperature and around 150˚C.

For our application, we could use the Ametherm AS32 20010. This device has a nominal resistance of 20Ω at 25˚C – limiting our inrush current to about 13A peak. This device is specified to have a resistance of 0.06Ω at 10A and 0.13Ω at 5A through self-heating. Although not specified, at 2.2A we can expect the resistance to be under 0.5Ω, and so drop only a couple of volts at our maximum load while dissipating around 2W.

The main limitation you need to be aware of is that these devices take a little time to cool down and regain their inrush-limiting resistance. This makes them unsuitable for applications which have to re-start frequently onto fully discharged capacitors. If you can arrange it so that the capacitors discharge gradually over a similar period of time that the NTC takes to cool down (several tens of seconds) you can obviate this limitation.

References
Ametherm. “Ametherm AS32 20010 —20 Ohm / 10 Amp Inrush Current Limiter Data Sheet.” Accessed August 2, 2023. https://www.ametherm.com/datasheets/as3220010.

Ametherm. “PTC Thermistors For Inrush Current Limiting.” Accessed August 2, 2023. https://www.ametherm.com/inrush-current/ptc-thermistors-for-inrush-current-limiting.