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

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

Fast diodes, also known as  fast recovery diodes, are generally used as rectifier diodes in high-frequency circuits such as switching power supplies. Ordinary rectifier tubes have a slow response speed and are generally low-frequency diode, which can easily burn out when used in high-frequency circuits. The fast diode is made into three legs by using the packaging framework of the transistor. Some legs are suspended, some legs are short circuited, or two diodes are packaged together, also known as half bridge.

The commonly used TO-220 package for  fast recovery diodes is divided into two pins TO-220AC and three pins TO-220AB. The TO-220AC package usually only has one chip, so it only needs two pins, one for the positive pole and the other for the negative pole; The TO-220AB package is different from the TO-220AC in that it contains two chips. Therefore, in addition to one pin being the positive pole and one pin being the negative pole, another pin is needed to connect the two chips together, which is shown as the positive pole in the middle of the sample in the example image.

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

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