Optimal Selection of Inductor Arrays and Coupled Inductors: Bourns SRF1280A Series and TT Electronics HM78D Series

When selecting a coupled inductor, inductor array, or transformer for an application, an engineer should consider both the technical and procurement requirements of the project. Technically, the smallest and most efficient component is best, while from a procurement standpoint the cheapest and most highly available component is preferable. In this article, a technique for automating much of the component selection process is introduced, which helps engineers find the optimum balance between technical and procurement demands. Two coupled inductors will be used as an example: the SRF1280A4R7Y from Bourns and the HM78D1284R7MLFTR from TT Electronics.

Meeting Technical Requirements with Proper Coupled Inductors Selection

Coupled inductors have two (or more) windings on a common core. They are typically used in DC-DC converters to transfer energy from one winding to the other. Coupled inductors have four (or more) ports, unlike normal inductors which only have two. This makes the measurement and characterization of coupled inductors different from normal inductors.

The coupled inductor is functionally equivalent to a transformer. The windings of the coupled inductor can be wired in different ways to use as isolation transformers or as common mode chokes. Mostly they are used in power conversion circuits including flyback, SEPIC, Ćuk, and other topologies. There are also resonant converter topologies that utilize coupled inductors/transformers.

When an engineer needs to select a coupled inductor for their application, they are looking at several factors. First, their calculations will output a minimum inductance required for the ripple current they are trying to achieve. Along with this, they will know how much power they are trying to convert, which tells them how much DC current each winding of the coupled inductor needs to handle. They will also know the frequency of their application and will look for an inductor with an SRF safely above it.

In an application with strict temperature requirements, the engineer will also look at DC resistance (or the graph of temperature vs. DC current, if available) and the core losses of the coupled inductor to determine the total thermal impact of the product on the system. These calculations are typically reinforced with the thermal testing of a prototype.

To meet space requirements, they will search for a part that meets their length, width, and height requirements. When no such requirements exist, they choose based on the lowest cost, smallest dimensions, and smallest DC resistance (or temperature rise).

The engineers at SourcingBot have collected both datasheet information and lab-measured data to help in the selection of coupled inductors and other passive components. To start, their powerful parametric search helps find all products which meet the technical requirements of the project. For example, an engineer can quickly find all 20 inductor arrays with 4-6uH primary inductance and 7-10A of rated current across an array of manufacturers. From the results, the individual part page can be visited. One inductor which meets these criteria is the TT Electronics HM78D1284R7MLFTR. Its part page can be found at the link.

On the part page, both the datasheet information and lab-tested data can be viewed. For this part, charts for inductance vs. current, inductance vs. frequency, and temperature vs. current are available. With datasheet information and lab-tested data in one place, it is much easier to choose a coupled inductor that meets the technical requirements of the project.

Increase Supply Chain and Procurement Stability with the Right Coupled Inductor 

SourcingBot also helps decrease supply risk and cost by offering information on price and availability at top distributors, as well as cross-references in case the original part goes out of stock. Under the datasheet information in the part, the page is a table of similar parts. In this table, the top specifications have been automatically compared and the top results are shown with a similarity score out of 100 percentage points. Here, we see replacements from Bourns, Pulse Electronics, and Eaton. The top result, the SRF1280A4R7Y (and the non-AEC-Q200 rated equivalent, the SRF12804R7Y) are both pin-to-pin footprint replacements for the original part. And with the ‘Click to compare’ feature, the lab-tested data for both parts can be shown on one easy-to-compare chart. Depending on the requirements of the project, replacements can be found with different objectives in mind, for example, footprint replacements, replacements with smaller height, and replacements with smaller DC resistance.

Using the tools available at SourcingBot, it is easy for engineers to meet both the technical specifications and procurement demands of a project. Research into technical specifications and characteristic charts can be done in one place and a list of suggested replacements is calculated for every part. In addition to coupled inductors, SourcingBot offers part information and replacement parts for over 5 million components, including passives, semiconductors, and connectors.


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