RESONANT CONVERTERS

RESONANT CONVERTERS

Resonant converters connect a DC system to an AC system or another DC system and control both the power transfer between them and the output voltage or current. They are used in such applications as: induction heating, very high frequency DC-DC power supplies, sonar transmitters, ballasts for fluorescent lamps, power supplies for laser cutting machines, ultrasonic generators, etc.

There are some common features characterizing the behavior of most, or at least some, of these elements. DC-DC and DC-AC converters have two basic shortcomings when their switches are operating in the switch mode. During the turn-on and turn-off time, high current and voltage appear simultaneously in and across the switches producing high power losses in them, that is, high switching stresses.

The power loss increases linearly with the switching frequency. To ensure reasonable efficiency of the power conversion, the switching frequency has to be kept under a certain maximum value. The second shortcoming in a switching mode operation is the electromagnetic interference (EMI) generated by the large dv/dt and di/dt values of the switching variables.

The drawbacks have been accentuated by the trend which is pushing the switching frequency to higher and higher range in order to reduce the converter size and weight. The resonant converters can minimize these shortcomings. The switches in resonant converters create a square-wave-like voltage or current pulse train with or without a DC component.

A resonant L-C circuit is always incorporated. Its resonant frequency could be close to the switching frequency or could deviate substantially. If the resonant L-C circuit is tuned to approximately the switching frequency, the unwanted harmonics are removed by the circuit. In both cases the variation of the switching frequency is one of the means for controlling the output power and voltage.

The advantages of resonant converters are derived from their L-C circuit and they are as follows: sinusoidal-like wave shapes, inherent filter action, reduced dv/dt and di/dt and EMI, facilitation of the turn-off process by providing zero current crossing for the switches and output power and voltage control by changing the switching frequency.

In addition, some resonant converters e.g., quasi-resonant converters, can accomplish zero current and/or zero voltage across the switches at the switching instant and reduce substantially the switching losses. The literature categorizes these converters as hard switched and soft switched converters. Unlike hard switched converters the switches in soft switched converters, quasiresonant and some resonant converters are subjected to much lower switching stresses.

Note that not all resonant converters offer zero current and/or zero voltage switchings, that is, reduced switching power losses. In return for these advantageous features, the switches are subjected to higher forward currents and reverse voltages than they would encounter in a nonresonant configuration of the same power. The variation in the operation frequency can be another drawback. First, a short review of the two basic resonant circuits, series and parallel, are given. Then the following three types of resonant converters are discussed:

• Load resonant converters

 • Resonant switch converters

• Resonant DC-link converter