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

2024.06.12

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

LLC converters are normally in ZVS operation during turn-on, so that switching losses can be reduced. However, if off-resonance occurs, a shoot-through current flows due to the recovery current, so that switching losses increase, and in the worst case MOSFET destruction may result. In order to reduce the risk of such destruction, it is effective to select MOSFETs with excellent body diode recovery characteristics for use as primary-side switching elements, so as to reduce shoot-through currents.

Below are summarized the key points explained in the articles in this series, together with links to the articles.

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

Key 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.

Basic Configuration of an LLC Converter

Key Points of This Article

・In an LLC converter, a series circuit of two inductors (L) and a capacitor (C) is formed.

・Because of the ZCS operation of the diodes used on the secondary side, recovery losses do not occur.

・As a result, when diodes with a low forward voltage VF are selected, losses can be reduced.

Features of LLC Converter Operation

Key Points of This Article

・Reduced switching losses are one advantage of LLC converters, but off-resonance may cause increased switching losses, possibly leading to MOSFET destruction.

・LLC converters use a PFM method to control the output voltage. Because the gain-frequency characteristic of an LLC has two resonance frequencies, operation is classified by the fsw into three operating regions.

Basic Operation of LLC Converters

Key Points of This Article

・LLC converters generally operate in region (2) where the gain is greater than 1 and MOSFET turn-on ZVS operation occurs.

・Circuit operation of an LLC converter in region (2) is divided into ten modes.

Importance of MOSFET Recovery Characteristics for Off-Resonance of LLC Converters

Key Points of This Article

・In an LLC converter, when there is deviation from the anticipated resonance condition, a shoot-through current occurs due to the recovery current in the MOSFET body diode, thus increasing switching losses and, in the worst case, culminating in MOSFET destruction.

・In order to reduce the risk of such destruction, a MOSFET having a body diode with excellent recovery characteristics should be selected, to reduce the shoot-through current value.

References

  • ・Koichi Morita, “LLC 共振コンバータの設計,” 電源回路設計 2009, CQ 出版.
  • ・W.S Choi, S.M Young, D.W Kim, “Analysis of MOSFET Failure Modes in LLC Resonant Converter,” INTELEC 2009, October 2009.
  • ・W.S Choi, S.M Young, “Improving System Reliability Using FRFETR in LLC Resonant Converters,” PESC 2008, June 2008.

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