Anecdote of inductor development [No.3] The LQS series realized super tight tolerance by trimming
As introduced in "Anecdote of inductor development [No.1] Birth of the LQH and its peculiar shape" (March 28. 2012), Murata has expanded its inductor lineup from the creatively shaped LQH series to include inductors of various sizes and performances. Of these, the second-generation 3225(in mm)/1210(in inch) size (3.2 mm × 2.5 mm) square-shaped wire-wound chip inductor was used in 1992 as the base to commercialize the LQS series of inductors that have a magnetically shielded structure and tight inductance tolerance (±2%).
At the time, the state of technology for winding copper wire around a ferrite core was such that inductance tolerance could only be reduced to ±5%. Development to realize a tolerance of ±2% commenced in 1989 by combining this technology with ferrite cores for closed magnetic circuit configurations and adjusting the inductance by trimming.
The inductance changes according to the gap and the trimming amount.
The optimal gap is determined by the design of the ferrite core for magnetically shielded types, and the inductance is adjusted downward by trimming.
First, we designed and manufactured prototype ferrite cores for closed magnetic circuit configurations that could be combined with 3225(in mm)/1210(in inch) size square-shaped wire-wound chip inductors, and repeated trial and error to see if we could stabilize the trimming amount and the amount of change in the inductance. Of course, we had no trimming equipment, so we worked every day by hand, rubbing dozens of product samples against polishing paper with our fingertips and then measuring the change in inductance to obtain the relationship between the trimming amount and the amount of change in the inductance. We also performed simulations using the finite element method (FEM) to optimize the dimensions of the ferrite cores for closed magnetic circuit configurations. By the time we had repeated enough trials to achieve a stable relationship between the trimming amount and the amount of change in the inductance, we felt that there were good prospects for realizing an inductance tolerance of ±2%, albeit with handmade products. On the other hand, in order to realize mass production we needed to figure out how to assemble 3225(in mm)/1210(in inch) size square-shaped wire-wound chip inductors together with ferrite cores for closed magnetic circuit configurations, decide what trimming equipment to use, and create a production line. In particular with regards to the trimming equipment, we repeatedly visited a plant producing ferrite cores to investigate mass production using various types of polishing equipment. There were numerous issues, including how to fix the products to the equipment, how to increase the trimming accuracy, and how to remove the products from the equipment and wash and dry them. However, we were able to resolve these numerous issues with cooperation from various divisions, and succeeded in commercialization.
In closing, the LQS series' role as a product has already reached its end, but the technology for combining chip inductors with ferrite cores for closed magnetic circuit configurations lives on in the form of magnetically shielded structure wire-wound products (LQH66S, DLW5BS/T).
Written by: T.H., Miyazaki Plant, Fukui Murata Manufacturing Co., Ltd.
The information presented in this article was current as of the date of publication. Please note that it may differ from the latest information.