TO-257 Package

TO-254 Package

The TO-254 is a type of "transistor outline" package that features a metallic body with a flat metal tab at its back (see Figure 1) to enable it to dissipate relatively large amounts of heat. The TO-254 is very similar to the TO-257, but it is bigger than the latter.

The TO-254 is hermetically sealed to protect the device from environmental factors such as contaminants and moisture. The leads protrude from the bottom of the package through glass and ceramic eyelets that insulate them from the metal body.

The TO-254 is commonly used for housing transistors, thyristors, and integrated circuits with low lead counts such as voltage regulators that are intended for high-reliability and mission-critical circuits. The metal body of the TO-254 and its hermetic sealing make it more expensive than an ordinary TO-220, but also make it more reliable.

The metal tab at the back of the TO-254 serves as a heat sink itself, but it has a hole so that it can be screwed onto a larger heat sink if higher power handling capability is required.

A typical TO-254 package has 3 leads, although TO-254's with other lead counts may be encountered. The typical lead pitch (distance between leads) of a 3-lead TO-254 package is 3.81 mm. Several TO-254 variants with different lead formations (i.e., bending of the leads into specific shapes) are also offered in the market. TO-254 packages without a metal tab at the back, also known as tabless TO-254, are also available.

Table 1. Properties of a Typical TO-254

Body Size (HxW)

excl. Metal Tab

Body Size (HxW)incl. Metal Tab

Body Thickness

(inc. tab)

Metal Tab Thickness

Lead Length

20.2 mm x 13.7 mm

13.7 mm x 13.7 mm

6.46 mm

1.15 mm

13.7 mm

Figure 1. Graphical Representation of a TO-254 Package

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