Parallel Schottkys as secondary rectifiers

Chargers for smartphones and tablet PCs nowadays are designed for an increased output power, as fast charging cycles make a real difference to users. 10 W to 12.5 W are typical power ratings and mainly Switched Mode Power Supply (SMPS) flyback topology is used. Schottky rectifiers are popular as secondary side rectifiers in these designs, as they are cost-efficient solutions and offer significantly lower forward power losses compared to PN-diodes, increasing power efficiency.

Chargers also need to be compact and circuit elements in small packages are preferred. One issue with slim designs is the high junction temperature of the secondary side rectifiers under full load conditions. A common solution to reduce losses and generation of heat is to use two parallel rectifiers instead of one. But how much will the losses decrease and what is the impact on junction temperatures of the rectifiers? Certainly, the assumption that losses and temperatures are cut in half is too simple. One reason is because nonlinear effects play a certain role on power losses. On the other hand, because of limited heat sink capabilities of Printed-Circuit Boards (PCB), devices will heat up each other.

This document gives an overview of the main effects on the performance of two parallel rectifiers.

View the PDF document for more information.

Originally published by NXP Semiconductors at www.nxp.com as "Parallel Schottkys as secondary rectifiers in flyback adapter: power losses and junction temperatures".