lm5046sqx National Semiconductor Corporation, lm5046sqx Datasheet - Page 23

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

to active transition time can be approximated by using the
following formula:
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 in-
stant, ZVS can be realized by turning on the LO1 switch.
The switching sequence in this CLK cycle is as follows: acti-
vation of the switch LO1 turns the diagonal LO1 and HO2 on,
CONTROL METHOD SELECTION
The LM5046 is a versatile PWM control IC that can be con-
figured for either current mode control or voltage mode con-
trol. 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 phases results in flux imbalance that caus-
es a dc buildup in the transformer. This continual dc buildup
may eventually push the transformer into saturation. The volt-
second asymmetry can be corrected by employing current
FIGURE 10. Simplified PSFB Topology Showing the Turn-Off Mechanism
23
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 end-
ing a freewheel cycle and the SW1 node always sees a
passive to active transition.
mode control. In current mode control, a signal representative
of the primary current is compared against an error signal to
control the duty cycle. In steady-state, this results in each
phase being terminated at the same peak current by adjusting
the pulse-width and thus applying equal volt-seconds to both
the phases.
Current mode control can be susceptible to noise and sub-
harmonic oscillation, while voltage mode control employs a
larger ramp for PWM and is usually less susceptible. Voltage-
mode control with input line feed-forward also has excellent
line transient response. When configuring for voltage mode
control, a dc blocking capacitor can be placed in series with
the primary winding of the power transformer to avoid any flux
imbalance that may cause transformer core saturation.
30147863
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