lm5045sqx National Semiconductor Corporation, lm5045sqx Datasheet - Page 16

• Manufacturer Part Number: lm5045sqx
• Description: Full-bridge Pwm Controller With Integrated Mosfet Drivers
• Manufacturer: National Semiconductor Corporation

www.national.com
Synchronous Rectifier Control
Outputs (SR1 & SR2)
Synchronous rectification (SR) of the transformer secondary
provides higher efficiency, especially for low output voltage
converters, compared to the diode rectification. The reduction
of rectifier forward voltage drop (0.5V - 1.5V) to 10mV -
200mV VDS voltage for a MOSFET significantly reduces
rectification losses. In a typical application, the transformer
secondary winding is center tapped, with the output power
inductor in series with the center tap. The SR MOSFETs pro-
vide the ground path for the energized secondary winding and
the inductor current. From Figure 5 it can be seen that when
the HO1/LO2 diagonal is turned ON, power transfer is en-
abled from the primary. During this period, the SR1 MOSFET
is enabled and the SR2 MOSFET is turned-off. The sec-
ondary winding connected to the SR2 MOSFET drain is twice
the voltage of the center tap at this time. At the conclusion of
the HO1/LO2 pulse, the inductor current continues to flow
through the SR2 MOSFET body diode. Since the body diode
causes more loss than the SR MOSFET, efficiency can be
improved by minimizing the T2 period while maintaining suf-
ficient timing margin over all conditions (component toler-
ances, etc.) to prevent the shoot-through current. When HO2/
LO1 enables power transfer from the primary, the SR2 MOS-
FET is enabled and the SR1 MOSFET is off.
During the freewheeling period, the inductor current is almost
equally shared between both the SR1 and SR2 MOSFETs
which effectively shorts the transformer secondary. The SR2
FIGURE 5. Synchronous Rectifier Timing Diagram
16
MOSFET is disabled before HO1/LO2 is turned-on. The SR2
MOSFET body diode continues to carry about the half induc-
tor current until the primary power raises the SR2 MOSFET
drain voltage and reverse biases the body diode. Ideally,
dead-time T1 would be set to the minimum time that allows
the SR MOSFET to turn off before the SR MOSFET body
diode starts conducting.
The SR drivers are powered by the REF regulator and each
SR output is capable of sourcing 0.1A and sinking 0.4A peak.
The amplitude of the SR drivers is limited to 5V. The 5V SR
signals enable the LM5045 to transfer SR control across the
isolation barrier either through a solid-state isolator or a pulse
transformer. The actual gate sourcing and sinking currents for
the synchronous MOSFETs are provided by the secondary-
side bias and gate drivers.
T1 and T2 can be programmed by connecting a resistor be-
tween RD1 and RD2 pins and AGND. It should be noted that
while RD1 effects the maximum duty cycle, RD2 does not.
The RD1 and RD2 resistors should be located very close to
the device. The formula for RD1 and RD2 resistors are given
below:
If the desired dead-time for T1 is 60ns, then the RD1 will be
20 kΩ.
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