The above advertisement is from the magazine Electronics (May 30, 1974). Unitrode advertises a fast power switch for use in switching regulators. There are a total of eight variants, four for positive currents and four for negative currents. The freewheeling diode of the switching regulator is already integrated in this component.
The Unitrode Semiconductor Data Book from 1980 describes even more variants. Whereas in 1974 only reverse voltages of 60V and 80V were available, in 1980 there were already variants with a reverse voltage of 100V. The permissible currents could be selected between 5A, 15A and 20A. The 5A and 15A variants are in TO-66 packages with four connection pins. The packages of these types are insulated. The 20A variant, on the other hand, uses a TO-3 package with three pins. There, the package is connected to the reference potential. A closer look at the specifications shows that the 15A variants may be permanently loaded with 15A, while the 20A variants are the peak current. No more than 15A may flow continuously here either.
The components contain an NPN power transistor and a driver transistor. The variants for negative load currents use NPN driver transistors and are therefore Darlington transistors. The variants for positive load currents use PNP driver transistors and are therefore Sziklai transistors. All variants also contain a freewheeling diode. The variants shown here use pn diodes. There are also variants with Schottky diodes (PIC730, PIC740). By matching the transistor/diode combination, the current commutation between the two elements is cleaner and cause less noise.
The component shown here is a PIC647. The marking is barely recognizable. The date code refers to the year 1984.
The PIC600 variants are hybrid components. The TO-3 package contains a large heatspreader on which a disk of beryllium oxide is placed. All active elements are located on the ceramic disk.
In order to be able to transmit the high currents, several bondwires lead from the connection pins to the ceramic carrier. The traces on the back of the pins show that the wires have been electrically welded there. Bonding processes are otherwise usually carried out by friction welding.
The PIC647 is one of the Sziklai transistors. The NPN power transistor is located in the upper area of the ceramic carrier. The significantly smaller PNP driver transistor is located below the power transistor. The base-emitter resistors were applied to the ceramic carrier as thick-film resistors. The freewheeling diode is located on the left-hand side. A metal block on the heatspreader enables a bond connection to the package.
The input pin is connected to the collector potential of the power transistor via two bondwires. Four bondwires had to be attached to the output pin, as two end directly on the emitter of the power transistor and the freewheeling diode also had to be contacted.
The edge length of the power transistor is 2,7 mm. The structures of the base and emitter are not unusual. However, it is noticeable that the transistor structures do not extend to the lower edge. There is obviously a second, smaller transistor in the lower area, which is inactive here. Apparently, the design offers the possibility of building a Darlington transistor. If a differently structured metal layer is applied, the small transistor can be used as a driver for the large transistor.
Outside the active area, the base area ends surprisingly quickly.
Where the base surface is contacted, the metal surface has become discolored. There was probably a strong build-up of heat there. A discoloration of the silicon can be clearly seen in the right-hand area. Perhaps a breakthrough occurred at this point.
The edge length of the driver transistor is 0,77mm. The picture above clearly shows that it is a transistor with a so-called perforated emitter. The base area under the emitter surface is contacted via small, square windows in the emitter surface. It is not a large power transistor, but the base current can still be up to 400mA.
The edge length of the freewheeling diode is 2,8 mm. The upper right area appears to be damaged.
The resistors are ordinary thick-film resistors.
https://www.richis-lab.de/BipolarA62.htm 
Home
Help
Search
Login
Register
