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AN1300

APPLICATION NOTE

October 2000

Operation of the Resonant Converters

Full-resonant converters are based upon the half-bridge and full-bridge PWM topologies. They are driven with

a symmetrical waveform whose frequency is changed to control the output voltage. There is a deadtime provid-

ed between the conduction of the upper and lower power switches to turn-on with zero voltage witching. Here

the drain voltage of the MOSFET turning off immediately swings to the opposite voltage and causes the oppos-

ing antiparallel diode to conduct The opposing MOSFET can then turn-on with a very low drain-to-source volt-

age.

There are two major types of resonant converters: the parallel- resonant converter shown in figure 1a, and the

series-resonant converter shown in figure 1b.

Figure 1. The Two Types of Resonant Converters

(a) Parallel

(b) Series

by Eric Danstrom

L6598 BASED 12V/3A RESONANT APPLICATION

Off-line switching power supplies have two major problems: high switching losses, and an operating en-

vironment, which is very sensitive to the radiation of Electromagnetic Interference (EMI) and Radio Fre-

quency Interference (RFI). The switching losses are a largest creator of EMI (conducted) and RFI

(radiated), so their control and reduction is a major benefit to the power supply designer. Resonant tech-

niques offer a hope for greatly reducing the switching losses and hence the factors contributing to the

EMI and RFI. This paper will present and organized approach to the design of a 36-watt, full-resonant,

off-line converter that can be used for portable PC applications.