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Custom Power Transformers Sizes & Dimensions


Custom transformers are usually built with some standardized ferrite core sizes with or without bobbins. If a custom transformer is for RF (radio frequency) application, it is usually designed and built with toroidal core for high frequency operations. CWS stocks thousands of such toroidal cores for these RF applications, and we will design and do winding for our customers with the fastest possible delivery. For more information or to purchase over the internet the various types of toroidal core, please visit:

However, if a custom transformer is for switch mode power application, or application where the switching frequency is below 500 Khz, paired core is usually used. The paired core use a bobbin to put the windings on. These bobbins have pins for either through hole mount or surface mount. Paired cores are available in hundreds of different shapes and sizes, ranging from EFD, to PQ to TF. The ferrite industry has adopted some standardized core shapes and sizes. For example, the EFD20's core is shaped like a "EFD" with a size of 20mm in usually the largest dimension. Every ferrite manufacturer's EFD20 will have the same dimension, and common third party bobbins can therefore be used. The advantage of using paired core is lower cost and ease in manufacturing, especially for high volume. Because of lower cost, if the volume is very large, paired core custom transformer is the way to go. For small size types, the packaging can also be in tape and reel.

The list below shows 41 different types of standardized sizes in PDF format that are maintained at CWS' facilities in California, Malaysia, Taiwan or China. The ferrite core material used in these transformers are industry standard MnZn with permeabilities of 2000 to 2500. The cores used are from manufacturers such as Magnetics Inc, Epcos, TDK, Tokin, Kawatetsu, Siemens, Philips ( Ferroxcube), etc. Depending on the application and switching frequencies, most of these manufacturers' product has similar characteristics. Some may have lower losses when frequency increases, while others have lower losses when operating at higher flux densities and ambient temperatures, etc.

When we receive a request for custom built transformers using paired cores, we will try to use one of the core types listed below. In this way, we can deliver final products to our customer with the shortest lead time. The critical dimensions are shown in the drawings. These include the maximum height and width, and pin size and spacing. These data are important for design engineers to plan the space on the PCB's.

The number of pins on the bobbins are shown inside the parenthesis beside the core type. i.e. EFD30(12P) means the core is EFD, size is 30mm width and bobbin has 12 pins. The number of pins are just a guide. Depending on the requirement, we can choose other types of bobbins with different number of pins. In many cases, surface mount bobbins are also available, even though they are not listed here.


EFD20 (8P) EFD25 (10P) EFD30 (12P)
EI10 (8P) EI13 (10P) EI14 (6P)
EI16 (10P) EI19 (6P) EI22 (10P)
EI24 (6P) EI25 (8P) EI30 (10P)
EI33 (12P) EI33 (14P) EI40 (12P)
EI40 (16P) EL19 (10P) EL19 (7P)
EL25 (10P) EL25 (7P) EP10 (8P)
EP13 (10P) EPC19 (11P) ER11.5 (10P)
ER14.5 (10P) ER28 (10P) ER28 (12P)
ER28 (13P) ER35 (12P) ER35 (14P)
ER35 (16P) ER9.5 (8P) PQ2020 (16P)
PQ2620 (12P) PQ32 (12P) RM08 (12P)
RM10 (12P) TF16 (10P) TF16 (5P)
UI-15 UU10.5 (4P)    

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