AN-9067 Datasheet, PDF (9/12 Pages) Fairchild Semiconductor – Analysis of MOSFET Failure Modes in LLC Resonant Converter

AN-9067
Table 1 shows the comparisons of R_DS(on), reverse recovery
time (t_rr), reverse recovery current (I_rr), and reverse recovery
charge (Q_rr). The reverse recovery charge (Q_rr) of the fast-
recovery body diode MOSFET, called FRFET^®, is
dramatically reduced almost by a factor of 14 compared to a
conventional MOSFET. Waveforms of a FRFET^®
(FQPF13N50CF) and a conventional MOSFET
(FQPF13N50C) are compared at startup state in the LLC
resonant half bridge converter. Figure 20 presents key
waveforms comparing reverse recovery characteristics at
startup between the conventional MOSFET and the
FRFET^®. A peak drain-source voltage of the conventional
MOSFET exceeded rated voltage (500V) and a high level of
shoot-through current is induced. On the contrary, no
voltage spike occurred with the FRFET^®.
APPLICATION NOTE
Experimental Results at Short Circuit
Waveforms of a fast-recovery body diode MOSFET and a
conventional MOSFET are compared in the 520W LLC
resonant half-bridge converter under shorted output condition.
Table 2. Comparisons of Critical Specifications
Comparison of DUTs
Devices
RDS(ON) Max. [Ω] trr [ns] Irr [A] Qrr [µC]
FDP20N50
0.23
507 28.40 7.2
FDP20N50F
0.26
154 6.49 0.5
Note:
2. Test Condition: VGS=0V, IS=13A, di/dt=100A/µs,
TC=25°C.
Id_Q1:2A/div
Vds_Q1:100V/div
Id_Q2:2A/div
Vds_Q2:100V/div
The potential failure mode !
Vds_Peak :516.3V
80ns/div
Figure 24. Shoot-Through Current with Conventional
MOSFET Technology
Id_Q1:2A/div
Vds_Q1:100V/div
Id_Q2:2A/div
Vds_Q2:100V/div
Vds_Peak :408.5V
Figure 26. Waveforms of Conventional Power MOSFET
at Short-Circuit Condition
80ns/div
Figure 25. Shoot-Through Current Improvement with
FRFET^® Technology
In addition, a peak current level of the conventional
MOSFET is almost double that of the FRFET^®. These
negative behaviors of the conventional MOSFET may result
in device failure as mentioned; reverse recovery dv/dt, and
breakdown dv/dt. Finally, the FRFET^® can effectively
minimize shoot-through current, peak drain-source voltage,
and reverse recovery dv/dt - potential causes of failure in
startup state.
Figure 27. Waveforms of FRFET^® Power MOSFETs at
Short-Circuit Condition
Table 2 shows the comparisons of R_DS(on), reverse recovery
time (t_rr), reverse recovery current (I_rr), and reverse recovery
charge (Q_rr). The reverse-recovery charge (Q_rr) of the fast-
recovery type MOSFET is significantly smaller. Figure 26
and Figure 27 show the drain-source voltage and drain
current waveforms of the conventional MOSFET and the
FRFET^® under output-short condition. Operation mode is
changed form ZVS to ZCS after output short. Switching
frequency is reduced and high current flows through the
MOSFETs at short circuit. The current spike of a
conventional MOSFET is several tens of ampere current
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 11/5/09
9
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