AC-DC power supply mosfet mounting clarification

Hello Balasubramani,

The center leg of high-voltage transistors is usually extended from the other two legs in order meet safety agency creepage and clearance requirements at the pcb level. Commercial PSU designs are subject to the same world-wide requirements and they also extend the center legs in a similar manner.

Whether the transistor is mounted stand-up (as shown) or lay-down (for low board height), the PSU manufacturer is responsible for cost-effective assembly of these parts. They can purchase the transistors in standard form and bend the legs at the factory, or purchase them pre-formed if the volume is high enough. Their manufacturing engineering will design high-volume assembly fixtures to hold these and other large components in place for wave-soldering or reflow soldering, depending on their process. Low volume assembly and soldering is typically done manually.

Since most manufacturers have different and unique assembly processes, the exact fixturing and process flow will be different from one vendor to another. They may be similar in concept but differ in details, so each manufacturer will have a custom solution for preparing and soldering these heat-sinked transistors to their boards.

Regards,
Ulrich