Datasheet

T
AP
=
C
parasitic
x V
IN
I
Lpeak
N
TR
)(I
m
+
L
Leak
SW1
L
Mag
LO2
HO2
I
O
+ I
mag
HO1
LO1
SW1
VIN
L
Mag
LO2
HO2
HO1
LO1
SW2
SR1
SR2
VOUT
VIN
SW2
SW1
VIN
L
Mag
LO2
HO2
HO1
LO1
SW2
SR1
SR2
VOUT
L
Leakage
GND
C
Parasitic
L
Mag
HO1
HO2
LO2
LO1
VIN
SW1
SW2
Power Transfer/Active Mode
Active to Passive
Transition
Freewheel/Passive Mode
Passive to Active
Transition
LM5046
www.ti.com SNVS703G –FEBRUARY 2011–REVISED MARCH 2013
APPLICATION INFORMATION
Figure 20. Operating States of the PSFB Topology
PHASE-SHIFTED FULL-BRIDGE OPERATION
The phase shifted full-bridge topology is a derivative of the conventional full-bridge topology. When tuned
appropriately the PSFB topology achieves zero voltage switching (ZVS) of the primary FETs while maintaining
constant switching frequency. The ZVS feature is highly desirable as it reduces both the switching losses and the
EMI emissions. The realization of the PSFB topology using the LM5046 is explained as follows:
Operating State 1 (Power Transfer/Active Mode)
The power transfer mode of the PSFB topology is similar to the hard switching full-bridge i.e. When the FETs in
the diagonal of the bridge are turned-on (HO1 & LO2 or HO2 & LO1), a power transfer cycle from the primary to
the secondary is initiated. Figure 20 depicts the case where the diagonal switches HO1 and LO2 are activated. In
this state, full VIN is applied to the primary of the power transformer, which is typically stepped down on the
secondary winding.
Operating State 2 (Active to Passive Transition)
At the end of the power transfer cycle, PWM turns off switch LO2. In the primary side, the reflected load current
plus the magnetizing current propels the SW2 node towards VIN. The active to passive transition is finished
when either the body diode of HO2 is forward-biased or HO2 is turned-on, whichever happens earlier. A delay
can be introduced by setting RD2 to an appropriate value, such that HO2 is turned-on only after the body-diode
is forward biased. In this mode, the I
mag
+I
Lpeak
act as a current source charging the parasitic capacitor located at
the node SW2. At light load conditions, it takes a longer time to propel SW node towards VIN.
The active to passive transition time can be approximated by using the following formula:
(4)
Where, I
m
is the magnetizing current, N
TR
is the power transformer’s turns ratio, I
Lpeak
is the peak output filter
inductor current and C
parasitic
is the parasitic capacitance at the node SW2.
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