)

SW1

LO2

HO2

HO1

LO1

SW1

VIN

LO2

HO2

HO1

LO1

SW2

SR1

SR2

VOUT

VIN

SW2

SW1

VIN

LO2

HO2

HO1

LO1

SW2

SR1

SR2

VOUT

GND

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

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)

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