Whatever your applications design(s) — digital multimeter, oscilloscope, on-board charger (OBC), plug-in battery for hybrid or electric vehicles, power supply for a motor drives, etc. — it needs to include overcurrent protection. To that end, you might want to introduce a component that protects your circuit when the smoothing capacitor C of the SMPS in Figure 1 pumps a very high current at the device’s initial switch on, or when a faulty voltage input is applied for a long time at the ohmic measurement input of the multimeter in Figure 2. In this last circuit, the Zener diode will clamp a too high input voltage, but the current produced could be damaging if applied for a too long time.
Figure 1
Figure 2
PTC ceramic disc components are universally used for overcurrent circuit protection. The principle of their function is relatively simple. Let’s look at the electrical resistance characteristic as a function of temperature (Figure 3).
Figure 3
At a low temperature (low voltage), the component’s electrical resistance R is low, and in the case of a current surge or external temperature increase, the component heats up. The electrical resistance value increases drastically once the component reaches a temperature known as the switch temperature (Ts). Ts is defined by the material used to manufacture the component and can be adjusted at will by a proper mix of oxides Ba(Sr,Pb)TiO3.
Thus, at a low ambient temperature the PTC lets the normal current flow into the circuit, and above the Ts point it acts as an open circuit. However, in practice the phenomena underlying a PTC’s behavior are extremely complex and the equations describing them are seldom decipherable.
If you need to use a PTC in your application, there are two classic ways to explore. The first is the highly scientific way. For example, you might read articles about the Heywang model [1] and immerse yourself in the interesting principles of the grain boundary’s resistivity changes, coupled with the Curie temperature effect.
If you don’t have the time to dedicate to studying, the second approach is to go directly to a component’s datasheets. Be sure to look at Vishay’s document “PTC Explanation of Terms” [2], which describes the numerous PTC parameters, from Ts to the voltage-dependent resistance (VDR) effect and trip and non-trip currents. All these characteristics will have to be taken into account for a high performance and reliable design.
These two classic approaches are intellectually enriching and they will challenge your engineering skills. Be sure that you have enough time though, because designing-in a PTC that works properly in the application is a tricky job (we haven't talked about analyzing the influence of all the aforementioned parameters’ tolerances, and of course about wide potential variations of the ambient temperature).
So, one might wonder if there’s another method allowing for direct trial and error, a kind of plug-and-play approach that’s free, if that’s not too much to ask.
Enter LTspice
This might sound like a song’s chorus, but really, it’s worth repeating: in order to produce useful results with simulators like LTspice, you will first need good PTC models. And good models must in fact give a faithful image of the specifications. If modelling is done with this in mind, then the user will be able to combine all the modeled specification points at the same time in one simulation, showing all the interactions between them. And I’m speaking about the voltage-dependent effect, the thermal mass, dissipation factor, ambient temperature effect, resistance variation as function of the temperature, trip and non-trip current — all the tolerances on all of these can be tested at the same time. There will be no need for complex explanations anymore. Simply select a component according to a common-sense guess and you can visualize the complex conduction phenomena during the simulations.
After researching popular simulation software, some PTC models from outdated part numbers were found. These were unfortunately useless. The time was thus ripe to update overvoltage PTC modeling and for practical reasons we chose a widely available simulation software: LTspice from ANALOG DEVICES. It was applied to the PTCTL (overcurrent protection), PTCCL (current limiter), and PTCEL (surge arrester) from Vishay.
For the results, let’s look at the simulation in Figure 4. This small circuit illustrates the case of a PTC switching when an AC overvoltage of 800 V at 50 Hz is applied to a Vishay PTCCL05H100SBE (R25 = 1600 Ω) in series with a load having a resistance of 1000 Ω. The simulation is repeated at three ambient temperatures. The current decreases in time as the PTC heats up, showing a shorter switching time when the ambient temperature increases (Figure 5). Look at the flattening of the current form when it approaches 0; this is the VDR effect. At low voltage, the apparent resistance of the PTC is higher.
Figure 4
Figure 5
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