To reduce energy costs, equipment designers are constantly exploring new ways to improve power density. A common approach among power supply engineers is to increase the switching frequency, which helps lower power consumption and shrink system size. LLC resonant converters have gained popularity due to their advantages, such as a wide output regulation range, narrow switching frequency variation, and the ability to achieve zero-voltage switching even under no-load conditions. However, one major challenge in these converters is the failure of power MOSFETs. This article will explain how to prevent such failures and ensure reliable operation.
One key issue is the poor performance of the body diode in the primary MOSFET. Under abnormal conditions—such as startup, load transients, or output shorts—this can lead to unexpected system failures, including severe shoot-through current, high body diode dv/dt, breakdown dv/dt, and gate oxide breakdown. These issues can significantly impact the reliability and lifespan of the converter.
Figure 1 shows the operating regions and modes of an LLC resonant converter. The DC gain characteristics under different load conditions are illustrated in Figure 2. Based on the switching frequency and load, the converter can operate in three distinct regions: the zero voltage switching (ZVS) region on the right side of the resonance frequency (fr1), the zero current switching (ZCS) region on the left side of the minimum secondary resonance frequency (fr2) under no-load conditions, and an intermediate area that may switch between ZVS and ZCS depending on the load. The purple and pink areas represent inductive and capacitive load regions, respectively.
When the switching frequency (fs) is below fr1, the input impedance of the resonant tank becomes inductive. As shown in Figure 3(b), the MOSFET turns on at zero voltage, minimizing switching losses due to the Miller effect. The MOSFET’s input capacitance doesn’t increase significantly, reducing stress during turn-on. Additionally, the body diode’s reverse recovery current is a small portion of the sine wave and becomes part of the positive switching current, resulting in minimal stress. Therefore, zero-voltage switching is generally preferred over zero-current switching because it avoids large switching losses and the associated stresses from reverse recovery and junction capacitance discharge.
By understanding the operating modes and avoiding the capacitive load region, designers can enhance the reliability of LLC resonant converters and prevent MOSFET failures.
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