In electronics, TO-3 is a designation for a standardized metal semiconductor package used for transistors and some integrated circuits. The TO element stands for "transistor outline" and refers to a series of technical drawings produced by JEDEC.
The TO-3 metal can package is commonly used for power transistors, silicon-controlled rectifiers, and, occasionally, integrated circuits. The semiconductor die component is mounted on a metal plate with a metal can crimped on top of it, providing good heat conduction and durability. TO-3 packages usually have two leads, the case being the third connection, though devices with four leads exist and more leads are possible. The leads pass through the metal base plate and are sealed with glass. The metal case is connected to the internal device. The TO-3 case has two mounting holes. The design originated at Motorola around 1955. The lead spacing was originally intended to allow plugging the device into a then-common tube socket.
The metal package can be attached to a heat sink, making it suitable for devices dissipating several watts of heat. Thermal compound is used to improve heat transfer between the device case and the heat sink. Since the device case is one of the electrical connections, an insulator may be required to electrically isolate the component from the heatsink. Insulating washers may be made of mica or other materials with good thermal conductivity.
- Can be used in high-power and high-current applications where equivalent components of other cases may be susceptible to damage.
- Almost all surfaces are metal for better heat conduction and durability.
- Metal-glass seal is hermetic to protect the semiconductor from liquids and gases
- More costly than plastic encapsulated components with similar ratings.
- Component size and lead placement makes soldering to a circuit board difficult.
- Off-board space must be dedicated to the components and/or their heatsinks, thereby increasing production costs.
- Large lead spacing puts a limit on operating frequency
Common components that use the TO-3 package
Common voltage regulators:
- LM317, voltage regulator
- LM340, voltage regulator
- 2N3055, NPN power transistor
- 2N2955, PNP power transistor
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