A second simulation (Figure 6) shows the function of the PTC as a resettable fuse. A voltage variable in time V(mains) is applied to another Vishay PTCEL (surge arrester). The three panes of Figure 7 show that as soon as the current into the PTC goes above a defined value (non-trip current of 120 mA), the PTC heats up and switches to high resistance values within a time period dependent on its electrical resistance tolerance (Monte Carlo analysis). After 1300 s (arbitrary time), when the voltage comes back to 220 V, the PTC remains switched at a high resistance (middle pane) and the mains voltage needs to be tuned down to a low value (t = 1800 s) to allow the PTC to cool down and to return to its low initial resistance.
Now we get back to the applications mentioned at the beginning of this article: let's more closely examine a ceramic PTC as an inrush current limiting device for OBCs, plug-in batteries for hybrid or electric vehicles, as well as power supplies for motor drives.
In the example in Figure 8, the secondary winding of a three-phase transformer (ph1 to ph3) provides power to a load after voltage rectifying and smoothing by a DC-Link capacitor C1. For the purpose of this simulation, the load (normally connected in parallel to C1) is not represented.
The design problem is in defining how many PTCs you need to place in parallel (two in Figure 8) in order to ensure that no PTC switches in normal conditions. In the case of a PTC switch, the voltage of capacitor C1 will never reach a value near the maximal swing of the AC source. Figure 9 shows the simulation of the voltage on C1 charging for three ambient temperatures (0 oC, 25 oC, and 50 oC) with two (red curve), three (yellow curve), or four (white curves) PTCs in parallel. We see that there are problems with only two PTCs reaching the maximal voltage, especially when TEMP increases. Using three or even four will put the application on the safe side.
Using an electronic simulation allows for the hands-on testing of more combinations of PTCs in series and in parallel (as some suppliers recommend ). Figure 10 presents an example of a circuit with only one PTC (left side) in comparison with the same circuit with four PTCs (a network of two parallel branches of two PTCs in series) having the same average resistance but an enhanced thermal capacity. Figure 11 presents the simulation results of a circuit with one PTC (higher pane) and four PTCs (lower pane). We effectively see that the PTC network allows for the capacitor to charge at the top of the voltage swing while the single PTC tends to switch before this capacitor is charged.
Of course, because of the restricted number of PTC SPICE models available on the market at this moment, itís not guaranteed that youíll find the right component that meets all your expectations. In that case, don't worry, the good old methods will always be applicable.
The generalization of SPICE PTC modeling has only just begun, and of course, Rome wasnít built in a day, as the saying goes. Itís remarkable though that after more than 30 years of SPICE [R]evolution, one can still find component types without readily available accurate models. The PTC SPICE models presented in this article can be downloaded here.
The presented simulations or specific requests for PTC SPICE models can be obtained by emailing here.
- Theoretical Aspects of PTC Thermistors, Sang-Hee Cho, Journal of the Korean Ceramic Society, Vol. 43, No. 11, pp. 673-679, 2006.
- PTC Explanation of Terms, 2018, web
- PTC thermistors for overcurrent protection and as inrush current limiters, TDK, 2018, web