FAN7621 Datasheet, PDF (11/17 Pages) Fairchild Semiconductor – PFM Controller for Half-Bridge Resonant Converters

Latch-Mode Protection: Once this protection is
triggered, switching is terminated and the gate output
signals remain off. The latch is reset only when LVCC is
discharged below 5V.
LV _CC
7
11 / 14 V
+
LV _CC good _VREF
-
Internal
Bias
OCP
OLP
OVP
LV _CC good
CON
20k
Auto-restart
protection
SQ
R -Q
F/F
Latch
protection
QS
-Q R
F/F
Shutdown
AOCP
TSD
LV _CC < 5V
Figure 24. Protection Blocks
Current Sensing Using Resistor: FAN7621 senses
drain current as a negative voltage, as shown in Figure
25 and Figure 26. Half-wave sensing allows low power
dissipation in the sensing resistor, while full-wave
sensing has less switching noise in the sensing signal.
C _DL
LV_CC
HV _CC
RT
HO
CON
CTR
V _CS
LO
CS
SG PG
R_sense
I _ds
I _ds
V _CS
Figure 25. Half-Wave Sensing
I _ds
C _DL
L V_CC
HV _CC
RT
HO
CON
CTR
V _CS
LO
CS
SG PG
V _CS
I _ds
R_sense
Figure 26. Full-Wave Sensing
Current Sensing Using Resonant Capacitor Voltage:
For high-power applications, current sensing using a
resistor may not be available due to the severe power
dissipation in the resistor. In that case, indirect current
sensing using the resonant capacitor voltage can be a
good alternative because the amplitude of the resonant
capacitor voltage (Vcrp-p) is proportional to the resonant
current in the primary side (Ipp-p) as:
V p−p
Cr
=
I p−p
p
2π f_sC_r
(6)
To minimize power dissipation, a capacitive voltage
divider is generally used for capacitor voltage sensing,
as shown in Figure 27.
C_DL
LVCC
HV _CC
RT
HO
I_p
CON
CTR
LO
CS
SG PG
C_sense
C_B
V_sense
100
C_r
I_p
V_Cr
V_Crp-p
V_sense
V pk
sense
V p− p
Cr
=
C_B
C_sense+ C _B
V pk
sense
2
= V_CON
V_CON
V_sensepk
V_sensepk
t_Delay =R_dC_d
Figure 27. Current Sensing Using Resonant
Capacitor Voltage
5.1 Over-Current Protection (OCP): When the sensing
pin voltage drops below -0.6V, OCP is triggered and the
MOSFETs remain off. This protection has a shutdown
time delay of 1.5µs to prevent premature shutdown
during startup.
5.2 Abnormal Over-Current Protection: (AOCP): If the
secondary rectifier diodes are shorted, large current with
extremely high di/dt can flow through the MOSFET
before OCP or OLP is triggered. AOCP is triggered
without shutdown delay when the sensing pin voltage
drops below -0.9V. This protection is latch mode and
reset when LVCC is pulled down below 5V.
© 2009 Fairchild Semiconductor Corporation
FAN7621 • Rev. 1.0.1
11
www.fairchildsemi.com