Selecting the most appropriate circuit protection solution can sometimes prove time-consuming and expensive, a system of trial-and-error, with the emphasis on error that is to say, time-consuming to do properly, and potentially expensive if you don't.
Fundamentally, there are a significant number of choices out there, and with thousands of protection options to begin with, it is difficult to begin with all the possible solutions, much less narrow down to the best choice through trial-and-error.
To help make an informed decision and eliminating guesswork, excellent application resources like Littelfuse’s Fuseology Application Guide or ESD Protection Guide with TVS Diode Arrays exist, and prove to be excellent summaries for the world’s best protection solutions . Even at the best component websites, searching for the right fuse or transient suppression device could still yield hundreds of results, again leaving the choice of either guessing or spending hours poring over detailed specifications and data sheets to find out if any of the options were actually an ideal fit for the product they were being designed into.
Littelfuse, the global leader in circuit protection, understands these challenges and set out to develop a way for engineers to quickly select and validate an appropriate protection solution through an easy to use, online tool. IDesign provides a guided selection process that allows circuit designers, procurement managers or sustaining engineers to quickly find the best device for their application. Compared to parametric searches,iDesign offers advantages in several areas.
First, the tool guides the user through a sequence of questions about their application instead of asking for all parameters of the fuse or diode arrays simultaneously. The primary weakness of parametric searches is that the wide variety of interdependent specifications entered into the search database can hide relevant results if the user makes even a single “wrong” entry. Littelfuse’s tool avoids this by stepping through the parameters one at a time in a sequence designed to capture the most important parameters first before moving on to secondary considerations. Searches conducted using iDesign protocol help drive choices from broad to specific through the selections or actions of the user.
Second, the iDesign tool incorporates the principles and theories in Littelfuse’s Fuseology Application Guide and ESD Protection Guide with TVS Diode Arrays to provide the “behind the scenes” logic and calculations during the selection process. This allows the tool to narrow in on the proper fuse or diode arrays choices for the application instead of requiring the user to perform the research and calculations to ensure that the fuse or diode arrays protection device will operate as intended given varied parameters such as nominal current, ambient temperature, fault current, and inrush current spikes or other regularly encountered transients. This is of significant benefit to the user, as a failure due to the interaction of these variables might not be caught until the product is on the market, exposing the users company to warranty return risk or a reputation for poor quality. Littelfuse also helps the user avoid last second regulatory compliance issues that can arise if the selected fuse does not have the necessary safety certifications. iDesign provides a very comprehensive list of international regulatory marks to meet American, European, and Asian safety requirements, and the results page further eases safety certifications by providing links to the safety certificate documents the user will need to supply during approvals. Finally, the tool performs a PASS/FAIL Time-Current analysis based on the nominal current, fault current, ambient temperature, recurring transients, and other parameters to ensure that the fuse will not open during normal operation, but during a fault will open in sufficient time to prevent damage to the circuit the device is intended to protect. When applicable, it also shows the same calculations for the next smaller part in the selected series to provide the designer with another option in case their device needs more sensitive protection. These advanced features are simply not available through normal parametric search websites, and offer a significant advantage to any design engineer who needs to make a protection device selection.
Use of the iDesign tool is very straightforward. The first step is to create a login, which allows saving of results and sharing parts with co-workers. Once logged in, the tool currently offers a Fuse Selection Tool and an ESD Selection tool.
Note that the flexibility of the tool allows new features and product lines to be added at any time, so it is worthwhile to check in with the tool when researching any circuit protection project.
Starting with the Fuse Selection Tool, the first step is to choose whether a standard fuse is applicable or if a specialty fuse is needed. If specialty fuses are needed for applications such as Military, UMF, Hazardous Locations, High Surge withstand, or Telecom applications, a link is supplied at the bottom of the page that takes the user directly to the applicable specialty fuse. To continue using the tool for standard fuses, the maximum AC or DC operating voltage, Nominal operating current, Maximum available fault current, and the maximum ambient operating temperature can be entered in available fields. Each of these has a help button which links to definitions and other guidance. On the next page, Agency certifications can be chosen, such as UL Listed or UL Recognized, as well as other European or Asian compliance marks.
Check boxes are supplied for these choices so that multiple agency certifications can be chosen. Once these parameters are entered, the tool then checks for the form factor desired by the user. SMD, Axial/Cartridge, Axial/PICO, and Radial lead fuses can be selected.
As with other pages, if the user needs some more information, additional details on each type including the Littelfuse series is available by clicking on the pictures of the fuse types. The page also displays a drawing of the type with common dimensioning indicators. Hitting next brings up a series selection page that shows all of the available fuse series that meet the parameters input earlier in the process, along with dimensions and a brief description and link to the datasheet. Once the series is selected, the tool then prompts the user for information on the application’s inrush spikes, including the number of pulses the design is expected to see over its lifetime and the wave shape and amplitude of the pulse. A number of standard waveform shapes are available, and the tool also allows the user to draw their own custom waveform or directly enter a specific i²t value, if it is known.
The tool then provides a list of specific fuses along with their amperage and i²t ratings, interrupt rating, and any agency certifications they have. The user can then select one of the options, and the tool will advance to a calculated time/current graph that calculates if the fuse will open on time to provide appropriate protection for the circuit.
Results are shown in simple graphical form, along with a plain-text PASS/FAIL rating to aid in analysis of the prediction. If the fuse value is too high, the tool provides guidance on how to go back and select a smaller value.
ESD PROTECTION TOOL
The ESD Protection tool is similarly easy to use. It starts by collecting information on the levels of ESD robustness desired, the HBM ESD rating and equivalent internal dynamic resistance of the protected ASIC, any series resistance between the TVS and ASIC, and the DC operating voltage of the circuit.
Using models which have captured Transmission Line Pulse (TLP) response, an engineer may input the descriptive of their interface design to drive to the best choices to protect the interface. The tool prompts for specific information such as the number of lines to be protected, the nominal application ( queries a list which cover most modern and legacy interfaces) and the desired level of robustness in kilovolts. Important parametrics such as maximum line capacitance and leakage current may also be entered. Optional parameters include choice for the maximum dynamic resistance and surge rating. All available parts meeting these criteria are then displayed, along with their critical parameters, pinout, and schematic. Up to three parts can be selected, and the tool will graphically display a plot showing the expected and theoretical maximum protection the devices can provide with proper layout and circuit design
Given the advantages present in Littelfuse’s iDesign tool, it is a critical stop when selecting circuit protection devices. It quickly allows engineers at all levels of experience to narrow down a fuse or TVS device based on the end application, provides a customized simulation to validate the selection, and gives guidance along the way to help users of the tool understand the logic behind the selection process. While in-system testing and validation of the component choice is recommended, the use of IDesign can greatly increase the probability of “First-Time-Right” and permit the circuit designers to move on to work on more challenging portions of their design.