Create models graphically

Today's automotive power networks may have 50 or more fuses to prevent damage to wiring and auto electronics in case of faults. It's important to get the design right, as poorly designed fusing can cause a fire in the wiring harness.

To ensure that a design will properly protect a vehicle's wiring system and electronic modules, most manufacturers use simulation. Simulating the electrical system enables designers to safely test concepts, making it possible to analyze different system configurations and find the best solution. To perform a meaningful simulation, you need a fuse model that accurately recreates the behavior of the real component.

In general, you can use models provided by the simulation software vendor, but if models aren't available for all the components in your design, you'll have to create your own.

You can create these models with a Hardware Description Language (HDL), such as VHDL-AMS or MAST. HDLs let you create models on different abstraction levels, such as a physical or behavioral level. You need both programming and modeling skills, as well as specific knowledge of the applied physics. Creating and testing models is usually time consuming and not trivial. Fortunately, some vendors now offers graphical tools that make it easier to develop your own fuse models.

To be useful, a fuse model must accurately characterize the fuse's dynamic thermal behavior. When working with a graphical development tool, you'll most likely start by inputting data about the fuse, such as the minimum current, maximum current, and fuse melting temperature.

The second step is to import the fuse curves. There are three curves that describe this behavior:

• fuse blow time as a function of fuse current,
• current as a function of voltage, and
• transient temperature.

Graphical tools will let you input this data in various ways. The tool that our company makes lets you import an ASCII data file or scan a graph on a data sheet.

Graphical tools can help you develop simulation models faster by allowing you to graphically manipulate functions that describe the behavior of a componenet or system.

Once the tool has generated the curve, you may need to fine-tune it if the supplied curves don't match curves you measured yourself or if you want to make the model specs tighter or looser than the manufacturer's specs. To make this job easier, some graphical tools can display both the datasheet curve and the measured data curve so you can compare the two sets of data and make the appropriate changes.

When you're satisfied with the fuse curves, you can generate the fuse model and add it to the library or place it directly into a simulation. Some graphical development tools can create VHDL-AMS models (including some C routines) or MAST models.

To illustrate how you can use a model to learn whether a fuse is adequate for an application, we will present an example of how we simulated a fuse that must protect a fan-control system in a wiring harness. The power supply for the DC motor of the fan drive is the vehicle battery; a 30-A fuse (Littelfuse, ATO Fuse, Fast-Acting-Type) protects the drive. A controllable resistance network sets the fan speed. Stopping the motor suddenly will cause the current through the motor and the fuse to increase significantly. The fuse should blow and protect the fan system from over-current.

Our first step in simulating the fuse was to create a fuse model. The manufacturer's data sheet provided the following information:

R_nom: 19 m

I_min: 40 A

I_high: 400 A

T_dhigh: 0.01 s

T₀: 27°C

T_melt: 429°C

After creating the model, we compared the behavior predicted by the model with the parameters on the datasheet. Perhaps the most important characteristic for this simulation was the fuse blow time. The simulation returned a blow time of 0.22 s for a continuous current of 100 A, which matches the datasheet value. This gave us confidence that our model was accurate.

We then moved on to the system simulation. In the simulation, the fan starts up in four steps. At the last step, the fan reaches maximum speed and draws the most current.

The voltage across the motor increases proportionally to the fan speed up to the fourth stage and then drops immediately. The current, however, continues to increase until it reaches 30 A.

At that point, the fuse blows and the current quickly drops. In this scenario, the fuse behaves correctly and the simulation shows that the sudden stop of the motor doesn't compromise the safety of the wire-harness.

Using graphical tools helped us create our fuse model quickly, without any specific physical knowledge of the fuse. Nor did we need a lot of programming experience to create the model. The information provided on a datasheet is sufficient for the creation of a model.