LM5045 Datasheet, PDF (1/12 Pages) National Semiconductor (TI) – Input operating range: 36V to 75V Board size: 2.28 x 1.45 x 0.5 inches

LM5045 Evaluation Board
National Semiconductor
Application Note 2111
Ajay Hari
February 23, 2011
Introduction
The LM5045 evaluation board is designed to provide the de-
sign engineer with a fully functional power converter based
on the full-bridge topology to evaluate the LM5045 PWM con-
troller. The evaluation board is provided in an industry stan-
dard quarter brick footprint.
The performance of the evaluation board is as follows:
• Input operating range: 36V to 75V
• Output voltage: 3.3V
• Measured efficiency at 48V: 92% @ 30A
• Frequency of operation: 420kHz
• Board size: 2.28 x 1.45 x 0.5 inches
• Load Regulation: 0.2%
• Line Regulation: 0.1%
• Line UVLO (34V/32V on/off)
• Hiccup Mode Current Limit
The printed circuit board consists of 6 layers; 2 ounce copper
outer layers and 3 ounce copper inner layers on FR4 material
with a total thickness of 0.062 inches. The unit is designed for
continuous operation at rated load at <40°C and a minimum
airflow of 200 CFM.
Theory of Operation
Power converters based on the full-bridge topology offer high-
efficiency and good power handling capability up to 500W.
Figure 1 illustrates the circuit arrangement for the full-bridge
topology. The switches, in the diagonal, Q1,Q3 and Q2,Q4
are turned alternatively with a pulse width determined by the
input and output voltages and the transformer turns ratio.
Each diagonal (Q1 and Q3 or Q2 and Q4), when turned ON,
applies input voltage across the primary of the transformer.
The resulting secondary voltage is then rectified and filtered
with an LC filter to provide a smoothened output voltage. In a
full-bridge topology, the primary switches are turned on alter-
natively energizing the windings in such a way that the flux
swings back and forth in the first and the third quadrants of
the B-H curve. The use of two quadrants allows better utiliza-
tion of the core resulting in a smaller core volume compared
to the single-ended topologies such as a forward converter.
Further, in a half-bridge topology, during power transfer when
one of the primary switches is active, the voltage across the
primary of the power transformer is 1/2 the input voltage (VIN)
compared to a full VIN in a full-bridge topology. Therefore, for
a given power, the primary current will be half as much for the
full-bridge as compared to the half-bridge. The reduced pri-
mary current enables higher efficiency as compared to a half-
bridge at high load currents.
Simplified Full-Bridge Converter
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