lm5046sqx National Semiconductor Corporation, lm5046sqx Datasheet - Page 22

• Manufacturer Part Number: lm5046sqx
• Description: Phase-shifted Full-bridge Pwm Controller With Integrated Mosfet Drivers
• Manufacturer: National Semiconductor Corporation

www.national.com
Applications Information
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 fre-
quency. 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 fol-
lows:
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 diag-
onal of the bridge are turned-on (HO1 & LO2 or HO2 & LO1),
a power transfer cycle from the primary to the secondary is
initiated.
es 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, whichev-
er 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
+I
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:
Lpeak
act as a current source charging the parasitic capacitor
Figure 9
depicts the case where the diagonal switch-
FIGURE 9. Operating States of the PSFB Topology
mag
22
Where, I
transformer’s turns ratio, I
current and C
SW2.
Operating State 3 (Freewheel/Passive Mode)
In the freewheel mode, unlike the conventional full-bridge
topology where all the four primary FETs are off, in the PSFB
topology the primary of the power transformer is shorted by
activating either both the top FETs (HO1 and HO2) or both of
the bottom FETs (LO1 and LO2) alternatively. In the current
CLK cycle, the top FETs HO1 and HO2 are kept on together.
Further in this mode, on the secondary side, similar to the
classic full-bridge topology the synchronous FETs are both
activated. During this state there is no energy transfer from
the primary and the filter inductor current in the secondary
freewheels through both the synchronous FETs.
Operating State 4 (Passive to Active Transition)
At the end of the switching cycle i.e. after the oscillator times
out the current CLK cycle, the primary switch HO1 and the
secondary FET SR1 are turned-off simultaneously. The volt-
age at the node SW1 begins to fall towards the GND. This is
due to the resonance between leakage inductance of the
power transformer plus any additional commutation inductor
and the parasitic capacitances at SW1. The magnetizing in-
ductor is shorted in the freewheel mode and therefore it does
not play any role in this transition. The LC resonance results
in a half-wave sinusoid whose period is determined by the
leakage inductor and parasitic capacitor. The peak of the half-
wave sinusoid is a function of the load current. The passive
m
is the magnetizing current, N
parasitic
is the parasitic capacitance at the node
Lpeak
is the peak output filter inductor
TR
is the power
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