AN-9067

APPLICATION NOTE

© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.0 • 11/5/09

6

(c)

(d)

(e)

Figure 13. Potential Failure Operation Mode of LLC

Resonant Half-Bridge Converter at Overload

Condition

Short-Circuit Failure Mode

The worst case is a short-circuit condition. During short

circuit, the MOSFET conducts extremely high (theoretically

unlimited) current and frequency is reduced. When short

continuously conducting in CCM mode at short circuit.

Operation mode during short circuit is almost same as

overload condition, but short-circuit condition is worse

because reverse-recovery current, which flows through the

body diode of the switch, is much higher.

Figure 14. Measured Waveforms of Power MOSFETs at

Short-Circuit Condition

Figure 15. Simulated Waveforms of Power MOSFETs at

Short-Circuit Condition

Figure 14 and Figure 15 show the switching waveforms of

the power MOSFETs at short circuit condition. Waveforms

during short circuit are similar with those during overload

condition,

but

the

current

level

during

short-circuit

condition is much higher and can lead to increased junction

temperature of MOSFETs and make it easier to fail.

Power MOSFET Failure Mechanisms

Body Diode Reverse Recovery dv/dt

The switching process of the diode from on state to reverse

blocking state is called reverse recovery. Figure 16 shows

reverse recovery waveforms of MOSFET body diode.

Firstly, the body diode was forward-conducted for a while.

During this period, charges are stored in the P-N junction of

the diode. When reverse voltage is applied across the diode,

stored charge should be removed to go back to blocking

state. The removal of the stored charge occurs via two

phenomena: the flow of a large reverse current and

recombination. A large reverse-recovery current occurs in

the diode during the process. This reverse-recovery current

flows through the body diode of MOSFET because the

channel is already closed. Some of reverse recovery current

flows right underneath N+ source.