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The Importance of the Recovery Characteristics of Primary-side Switching Elements in LLC Converters －Introduction－

2024.01.22

Points of this article

・LLC converters can be used to construct soft switching circuits using comparatively few components, and switching losses and voltage ringing can be reduced compared with hard switching methods.

・However, depending on the circuit input/output conditions, there is the concern that MOSFET switching elements may ultimately be destroyed.

・In this section, LLC converter circuit configurations and basic operation are first explained, and the importance of the recovery characteristics of primary-side MOSFETs in relation to off-resonance, which is a problem specific to LLC circuits, is described.

The Importance of the Recovery Characteristics of Primary-side Switching Elements in LLC Converters 　－Introduction－

An “LLC” converter is so called because of the use it makes of the resonance due to two inductors (L) and one capacitor (C). By utilizing the resonance of these components, zero-voltage switching (hereafter ZVS) operation when the primary-side switching element of the transformer is turned on, as well as zero-current switching (ZCS) of a diode on the secondary side, are possible.

Switching methods employing such resonance operations are called “soft switching methods”; compared with general hard switching methods, they make possible reductions in switching losses and voltage ringing, and so are used in the power supplies of thin form-factor TVs, office equipment and the like for which small size, high efficiency, and low noise are required, as well as in the DC-DC sections of onboard chargers (OBCs) and similar applications.

However, depending on the circuit input/output conditions, hard switching operation may occur, and if the recovery characteristics of the body diode of the MOSFET switching element are poor, in a worst-case scenario the MOSFET may be destroyed.
In this section on “The Importance of the Recovery Characteristics of Primary-side Switching Elements in LLC Converters”, we will first explain the circuit configuration and basic operation of LLC converters. Then, we will explain the importance of the recovery characteristics of primary-side MOSFETs for off-resonance, which is a problem specific to LLC converters.

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