To cut energy costs, equipment designers are constantly searching for new ways to optimize power density. Typically power supply designers have reduced power losses and system size by moving to higher switching frequencies. LLC resonant converters (Figure 1) are becoming more and more popular because of its many benefits such as wide output regulation with a narrow switching frequency range and guaranteed ZVS even at no load. However, power MOSFET failures have also been an issue in LLC resonant converters. Poor body diode performance of primary MOSFETs can lead to some unexpected system or device failures associated with severe shoot-through current, body diode dv/dt, breakdown dv/dt, and gate oxide breakdown in various abnormal conditions such as start-up, load transient, and output short circuit. In this blog, we will explain how to avoid these MOSFET failures.

 

Fig1 LLC

Figure 1. LLC resonant converter

Operation region & mode in a LLC resonant converter
DC gain characteristics of an LLC resonant converter at different loads are shown in Figure 2. They are classified into three regions according to different operating frequencies and load conditions. The right side (blue box) of the resonant frequency, fr1, is the ZVS region and the left side (red box) of the minimum second resonant frequency, fr2 at no load, is the ZCS region. The region between fr1 and fr2 can be either the ZVS or ZCS region, depending on the load condition. The purple region represents the inductive load region and the pink region represents the capacitive load region. For switching frequency, fs<fr2, the input impedance of the resonant tank represents a capacitive load, and the current through the resonant circuit leads the fundamental component of the voltage applied to the MOSFET. The MOSFETs are turned off at zero current (ZCS), as shown in Figure 3 (a).

Fig2 DC gain

Figure 2. DC Gain Characteristics of LLC Resonant Converters

Prior to the MOSFET turn-on, the current flows through the body diode of the other MOSFET. When the MOSFET switch turns on, reverse recovery stress of the other MOSFET’s body diode is very severe. This high reverse recovery current spike flows through the other MOSFET switch because it cannot flow through the resonant circuit. It creates a high body diode dv/dt and its current and voltage spike can cause device failure during reverse recovery of the body diode. Therefore, the converter should avoid operating in the capacitive region. For fs>fr1, input impedance of the resonant tank is an inductive load. As shown in Figure 3 (b), the MOSFETs are turned on at zero voltage (ZVS). The turn-on switching loss is minimized because Miller’s effect is absent and the MOSFET input capacitance is not increased by Miller’s effect. In addition, the body diode reverse recovery current is a fraction of a sine wave and becomes a part of the switch current when the switch current is positive. As a result, ZVS is usually preferred to ZCS because it can eliminate the major switching losses and stress due to the reverse recovery current and the discharging of its junction capacitance.

Fig 3 Operation modeFigure 3. Operation mode in the LLC resonant converter

Failure mode in the LLC resonant converter
1) Start-up

During start-up, the ZVS operation can be lost and MOSFETs can fail due to reverse recovery dv/dt.

Resonant capacitance and output capacitance are completely discharged before start-up. These empty capacitances cause further conduction of the Q2 body diode and it is not recovered completely before Q1 turns on. This reverse recovery current is very high and is enough to make shoot-through problems during start-up, as shown in Figure 4.

Fig 4 Wavforms

Figure 4. Waveforms during start-up in the LLC resonant converter

Recommended solution for failure mode in start-up:

  • Using a Fast recovery MOSFET
  • Reducing the resonant capacitor
  • Control the drive signal of high-side and low-side MOSFETs to make a complete body diode recovery

2) Output short

During the output short circuit, the MOSFET conducts an extremely high current. When the output short circuit occurs, Lm is shunted in resonance. The LLC resonant converter can be simplified as a series resonant tank by Cr and Lr because Cr resonates with only Lr. This condition usually results in the ZCS operation (capacitive mode). The most severe drawback of the ZCS operation is hard commutation at turn-on, which can lead to the diode reverse recovery stress (dv/dt) and huge current and voltage stress as shown in Figure 5. Also, the device can be damaged by gate over voltage stress because of the high di/dt and dv/dt during its body diode reverse recovery.

Fig5 Waveforms

Figure 5. Waveforms during output short in the LLC resonant converter

Recommended solution for failure mode in start-up:

  • Using a Fast recovery MOSFET
  • Increasing the turn-on resistor to reduce reverse recovery di/dt & dv/dt, body diodereverse current (Irm) and peak Vgs as shown in Figure 6
  • Increasing the minimum switching frequency to prevent capacitive mode
  • Reducing the Vgs turn-off delay as fast as possible after output short
  • Reducing the over current protection level

Fig6 Turn on gate

Figure 6. Turn-on gate resistor effects during reverse recovery

Fig7 Comparisons reverse

Figure 7. Comparisons of reverse recovery characteristics between the FRFET (FCH072N60F) and a normal MOSFET (FCH072N60)

Replacing a normal MOSFET with a Fast recovery MOSFET (FRFET® MOSFET) is very simple and effective to implement because additional circuits or devices are not necessary. Figure 7 shows an improvement of reverse recovery characteristics of a FRFET MOSFET compared to a normal MOSFET. The reverse recovery charge of the FRFET MOSFET (FCH072N60F) is reduced by 90% compared to a normal MOSFET (FCH072N60). The body diode ruggedness of the FRFET MOSFET is much better than a normal MOSFET. In addition, the peak gate-source voltage of low-side MOSFET can decrease from 54V to 26V when the high-side MOSFET is changed to the FRFET MOSFET from the normal MOSFET during reverse recovery. With all of these improved characteristics, the FRFET MOSFET provides enhanced reliability in the LLC resonant half-bridge converter.

For more information on how SuperFET® II MOSFETs provide improved reliability and efficiency, please visit the following application note: 650V Fast Recovery SuperFET II MOSFET for High System Efficiency and Reliability in Resonant Topologies.

Visit the Fairchild page to learn more about SuperFET II MOSFET products.

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