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Selecting MOSFETs for half-bridge resonant converters

Posted: 12 Jun 2009 ?? ?Print Version ?Bookmark and Share

Keywords:MOSFET selection? resonant converters? LLC half-bridge?

When designed to run at high operating frequencies with soft switching for high efficiency, the half-bridge resonant converter can be built using small heat sinks and low-profile, horizontal-style transformers. Shrinking these normally bulky components leads to high power density and helps to reduce BOM costs. These characteristics have made the half-bridge resonant converter a popular topology in switching power supplies designed for consumer applications such as low-profile LCD TVs.

In this type of power converter, the power stage consists of two MOSFETsone high side and one low sidewhich are alternately switched on and off using soft-switching techniques. To ensure proper operation of the converter, special attention must be given to the selection of these MOSFETs. Key device parameters such as Coss, Trr and Qrr of internal body diodes influence the power supply's dynamic performance at turn-on and turn-off and when short-circuit protection is applied.

When specifying MOSFETs for a half-bridge resonant converter, it's also important to consider the impact of other component choices on the MOSFETs. For example, improper selection of component values for the LLC resonant tank circuit can cause the converter to change from an inductive mode with soft switching into a capacitive mode with hard switching. The latter may result in damage to the MOSFETs.

To illustrate the impact of these MOSFET-related issues on converter design, this article studies the performance of a half-bridge LLC resonant DC/DC converter developed for use in an open-frame power supply. The term "LLC" distinguishes this style of converter from the LC series resonant converter. In addition to the resonant inductance in the circuit (Lr), the LLC resonant converter makes use of the magnetizing inductance (Lm) associated with the converter's transformer during dead time to achieve soft switching. The converter described here generates a 12Vdc output at loads up to 11.25A, while operating from an input of 400Vdc.

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