What tools would you use to easily simulate surge protections ? I am looking for a tool to simumate transient in zener diodes, PTC fuse, Gas Discharge Tube and the like.

I am a beginner in circuit simulation, and I have no clue on how to simulate things like a PCT fuse or a gas discharge tube properly. Let's say I have a very basic overvoltage protection consisting of a zener diode and a PTC fuse. I would like to properly simulate the behaviour of this circuit when using PCT fuse A or B. In other word, given the usually available technical characyeristics of a component, is there a software that has a good library or easily modifiable predefined components targeted at circuit protection elements ?

I could find a lot of simulator targeted at switching power suppy simulation, and I wonder if there is a software that would be ideally suited to overvoltage, esd and other circuit protection.

  • \$\begingroup\$ I don't think my simulator will be any good (microcap) for the record. \$\endgroup\$
    – Andy aka
    Oct 15 '13 at 12:02
  • 2
    \$\begingroup\$ I use linear.com/designtools/software/#LTspice Linear give this away to simulate thier SMPS, however it is good for general use as well. Zeners it does ok , the others I couldn't find. Zeners I use a changing supply voltage to obser it's action. PTC I would use the datasheets to select, Gas Discharge is very high voltage under normal operation, so I would not expect them to operate at all until hit with a lightning strike, these are selected based on peak voltage protrection required. \$\endgroup\$
    – Spoon
    Oct 15 '13 at 12:34
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    \$\begingroup\$ I think you are on the wrong track. When you design with PTC and zener, you need to design to worst case DC conditions. In other words, the zener needs to survive indefinitely at the highest current the PTC can pass without "tripping." Transient voltage suppression diodes are rated differently, and used for a different purpose. They are designed to dissipate large ESD (usually) pulses, but cannot due so at a high duty cycle, nor can they prevent against high voltage applied as a DC Voltage. \$\endgroup\$
    – mkeith
    Sep 6 '15 at 18:22

To answer your specific question of if there is a software tool to simulate something using nothing but the generally available technical specifications (in other words, stuff in the datasheet): no there is not. It is not possible to simulate nonlinear components using the datasheet. They do not include the needed information to construct an accurate model.

That said, you're already using the industry standard tool. SPICE. Microcap is using SPICE as its backend, and SPICE can simulate whatever you have models for, and the accuracy will be limited by those models (and in some situations, trtol). There is nothing to switch to, you're already using the simulation engine that is as good as the word of God. Do you think we design 14nm FinFETs interacting on a breadboard? Of course not. We model them (along with no small amount of quantum mechanical effects which begin to dominate, like tunnel injection) using SPICE. Then those simulations have sufficient predictive power that when we scale up those interactions to literally billions of transistors in a single circuit, spend millions (or billions) tooling up a fab to produce a CPU, guess what? It works. There are generally 1-2 revisions of a new Intel CPU before it's released, and always to improve yield. They always work on the first try though. The cost of one lithography mask is so high that you're allowed to screw up zero times. That is why I say SPICE (or internal tools based on it) is as good as the word of God: it is. Math works. It can never not.

Engineers who are suspicious of it are blaming their mistakes on the least likely link in the chain. SPICE simulates exactly what it is told - if you tell it garbage, you get garbage. You have to tell it something that can physically exist using verified empirical models before it will have useful predictive power.

Fortunately, this means you already have the tool you need. With this in mind, I'll tell you how to simulate PTC fuses, zeners, TVS diodes, all under overload conditions or transient surges.

So how would you go about simulating PTC fuse A vs PTC fuse B? You go about it exactly like simulating anything else in SPICE: Using PTC fuse A and B's respective SPICE models. Same with the zener diode. Or discharge tube. Or TVS diode. You should always use the model for the specific part number you plan to use.

Most SPICE simulators will include a built-in library but this is rarely going to be adequate and is really only there as a convenience. NEVER use ideal/unmarked components. If the zener diode doesn't have a specific part number you can enter on digikey and get a small selection of the same part to buy, it is not suitable for simulation of a real circuit. You can spot a generic/ideal component easily: It will instead of a real part number have none, and will include some basic parameters like breakdown voltage etc., things you'd see in a datasheet.

So you shouldn't even be looking for some generic PTC fuse in the library, and should not be using generic zener diodes (if you are). You must always simulate actual parts on your BOM using vendor supplied models for each one.

Littlefuse, Bourns, Vishay all supply SPICE models for their PTC fuses. That's just off the top of my head, but I would say most reputable vendors do so as well. They are usually in PSpice format which can be used by Microcap. You can even just copy and paste the .SUBCKT directly into your simulation as a SPICE directive. Microcap has a walk through in the documentation for adding 3rd party models. I'd advise becoming familiar with this task, as it is a vital one and one you will need to do often if you are planning on your simulations being useful except as a theoretical learning tool.

Now you might ask, "what if the PTC fuse part I'm using doesn't have a SPICE model? How to I simulate it then?" The answer is: you can't. Pick a new part with a model, or accept the trade-off of using the model-less part in exchange for not being able to simulate (with any useful predictive power) the circuit protection part of your circuit. I know that's not what you wanted to hear, but it's the hard realty unfortunately.

It is vitally important to use part-specific models for every component (except perhaps resistors, but make sure they have a Tc value for when things get hot) in a simulation of this nature. It wouldn't hurt to guesstimate parasitics from a real PCB layout and put those in as well, but you can probably get by without going that far.

I am guessing you want to verify the response time etc to an overload transient. This will depend heavily on part-specific capacitances and other empirically measured values for a given part, values that will be missing in a generic model.


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