Transistors｜Evaluation

Features of LLC Converter Operation

2024.02.08

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.

This article is on the second topic, “Features of LLC Converter Operation”.

Features of LLC Converter Operation

The following table summarizes the advantages and disadvantages of LLC converters when focusing on the primary-side MOSFETs in the basic circuit presented in the previous article. The LLC converter achieves ZVS operation using a partial resonance method for charge/discharge of the output capacitance Coss of the MOSFETs using a magnetizing current. Switching losses can thus be reduced, and the sizes of MOSFET packages and heat sinks for heat dissipation can be shrunk.

Advantages Disadvantages
  • • Because this is a bridge circuit, the MOSFET drain-source voltage VDS is clamped at the power supply voltage.
  • • In essence, ZVS operation occurs while turned on, so switching losses can be reduced.
  • • Switching losses can be reduced, so that package and heat sink sizes can be made smaller.
  • • Off-resonance results in shoot-through currents in MOSFETs, with increased switching losses. In the worst case, MOSFET destruction is possible.

In an LLC converter, frequency modulation (Pulse Frequency Modulation, hereafter “PFM”) which changes the switching frequency fsw of the MOSFETs is used to control the output voltage. This circuit design has two resonance frequencies, represented by equations (1) and (2) using the values of Lr, Lm, and Cr; the circuit operation changes depending on the setting of the switching frequency.

f_0=1/(2π√(L_r C_r ))　・・・・・・・・（1）f_m=1/(2π√((L_r+L_m ) C_r ))　・・・・・・・・（2）

Fig. 2 is a graph of the gain-frequency characteristics. Operation is classified into three regions according to the switching frequency and the DC gain.


Fig. 2. Graph of LLC converter gain-frequency characteristics

The operating regions are explained below.

Region（1） Inductive load region. Through partial resonance using a magnetizing current, ZVS operation is achieved.
Region（2） Similarly to region (1), ZVS operation occurs. In this region, G>1, so step-up operation is performed. Normally, LLC converters use this region.
Region（3） Capacitive load region. Here ZCS rather than ZVS operation occurs. Because current flows in the negative direction when the MOSFETs are turned off, currents flow in the MOSFET body diodes, and a shoot-through current due to the reverse recovery characteristic of the body diode flows in the opposite arm, so that turn-on losses increase.

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