Datasheet

Vout
T1
T1
Vin
HO1
HO2
LO2LO1
SR1
SR2
Turn-off
controlled by
CLK
Turn-off
controlled by
PWM
Passive to Active
Transition at SW1
Active to Passive
Transition at SW2
SW1 SW2
TPA
=
(Lleakage + Lcommutation) x Cparasitic
'
2
LM5046
SNVS703G –FEBRUARY 2011–REVISED MARCH 2013 www.ti.com
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 voltage 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 inductor 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 to active transition time can be approximated by using the
following formula:
(5)
When tuned appropriately either by deliberately increasing the leakage inductance or by adding an extra
commutating inductor, the sinusoidal resonant waveform peaks such that it is clamped by the body-diode of the
LO1 switch. At this instant, ZVS can be realized by turning on the LO1 switch.
The switching sequence in this CLK cycle is as follows: activation of the switch LO1 turns the diagonal LO1 and
HO2 on, resulting in power transfer. The power transfer cycle ends when PWM turns off HO2, which is followed
by an active to passive transition where LO2 is turned on. In the freewheel mode, LO1 and LO2 are both
activated. From this sequence, it can be inferred that the FETs on the right side of the bridge (HO2 and LO2) are
always terminated by the PWM ending a power transfer cycle and the SW2 node always sees an active to
passive transition. Further, the FETs on the left side of the bridge (HO1 and LO1) are always turned-off by the
CLK ending a freewheel cycle and the SW1 node always sees a passive to active transition.
Figure 21. Simplified PSFB Topology Showing the Turn-Off Mechanism
CONTROL METHOD SELECTION
The LM5046 is a versatile PWM control IC that can be configured for either current mode control or voltage
mode control. The choice of the control method usually depends upon the designer preference. The following
must be taken into consideration while selecting the control method. Current mode control can inherently balance
flux in both phases of the PSFB topology. The PSFB topology, like other double ended topologies, is susceptible
to the transformer core saturation. Any asymmetry in the volt-second product applied between the two alternating
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