To reduce energy costs, equipment designers are constantly seeking innovative ways to improve power density. One common approach is to increase the switching frequency, which helps lower power consumption and shrink the system size. LLC resonant converters are gaining popularity due to their advantages, such as a wide output regulation range, narrow switching frequency variation, and zero voltage switching even under no-load conditions. However, MOSFET failures remain a critical issue in these designs. This article will explore effective strategies to prevent MOSFET failure in LLC resonant converters.
Poor body diode performance in the primary MOSFET can lead to unexpected system failures, including severe shoot-through currents, high body diode dv/dt, breakdown dv/dt, and gate oxide breakdown under abnormal conditions like startup, load transients, or output shorts. These issues can significantly impact reliability and performance.
Figure 1: LLC Resonant Converter Operating Region and Mode
The DC gain characteristics of an LLC resonant converter under different load conditions are illustrated in Figure 2. Depending on the operating frequency and load, the converter can be divided into three distinct regions. The area to the right of the resonance frequency (fr1) represents the zero voltage switching region, while the area to the left of the minimum secondary resonance frequency (fr2) under no-load conditions corresponds to the zero current switching region. The area between fr1 and fr2 can switch between zero voltage and zero current switching based on the load. The purple region indicates inductive loading, and the pink region shows capacitive loading.
Figure 2: DC Gain Characteristics of an LLC Resonant Converter
In the LLC resonant converter, current flows through the body diodes of other MOSFETs before the switch turns on. When the MOSFET is activated, the reverse recovery stress on the body diodes becomes significant. High reverse recovery current spikes occur because the resonant circuit cannot absorb them, leading to high body diode dv/dt. These voltage and current spikes can cause device failure during the reverse recovery process. Therefore, it's crucial to avoid operating in the capacitive load region.
When the switching frequency (fs) is higher than fr1, the input impedance of the resonant tank behaves like an inductive load. As shown in Figure 3(b), the MOSFET turns on at zero voltage (ZVS). This minimizes turn-on losses due to the Miller effect, and the MOSFET’s input capacitance does not increase during this phase. Additionally, the body diode reverse recovery current is a small portion of the sine wave and becomes part of the switching current when it is positive. Hence, zero-voltage switching typically takes precedence over zero-current switching, as it reduces switching losses and stress from reverse recovery current and junction capacitance discharge.
Figure 3: Operating Mode in the LLC Resonant Converter
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