The
TO-257
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-257 is very similar to the TO-254.
The TO-257 is the metal body equivalent of the
popular plastic-molded TO-220.
Unlike the TO-220, however, the TO-257 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 feed-throughs that insulate them from the metal body.
The TO-257 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-257 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-257 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-257 package has 3
leads, although TO-257's with other lead counts may be encountered. The typical lead pitch (distance between leads) of a
3-lead TO-257 package is 2.54 mm. Several TO-257 variants with different
lead formations (i.e., bending of the leads into specific shapes) are
also offered in the market. TO-257 packages without a metal tab at
the back, also known as tabless TO-257, are also available.
