Why do some fast recovery diodes have to be made into three legs?

Why do some fast recovery diodes have to be made into three legs?

Fast recovery diodes are divided into chip diodes and axial diodes. So, why do some fast recovery diodes have three legs?

Fast diodes are also called fast recovery diodes. They are generally used as rectifying diodes in high-frequency circuits such as switching power supplies. Ordinary rectifying transistors have slow response speeds and are generally low-frequency transistors, which are easily burned out when used in high-frequency circuits. The fast diode is made into three legs, which uses a transistor packaging frame. Some have one leg suspended, some have two legs shorted, and some have two diodes packaged together, also called a half-bridge.

For example, the TO-220 package commonly used for fast recovery diodes is divided into two pins TO-220AC and three pins TO-220AB. The TO-220AC package generally has only one chip, so it only needs two pins. One pin is positive and the other pin is negative; The TO-220AB package is different from the TO-220AC package. It has two chips inside, so in addition to one pin being the positive pole and the other pin being the negative pole, one pin is also needed to connect the two chips together. It is the pin in the middle of the template in the example picture, which is displayed as the positive pole.

The connection method of the three pins of the fast recovery diode is as follows:

The diode with three legs is a combination of two diodes. It is a full-wave rectifier bridge of a dual-winding (with a center tap) transformer. The middle electrode is the positive pole of the DC output, and both sides are connected to the transformer respectively.