LM5072EVAL National Semiconductor, LM5072EVAL Datasheet - Page 6

• Manufacturer Part Number: LM5072EVAL
• Description: BOARD EVALUATION LM5072
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM5072EVAL

• Main Purpose: Special Purpose DC/DC, Power Over Ethernet
• Outputs And Type: 1, Isolated
• Power - Output: 9.9W
• Voltage - Output: 3.3V
• Current - Output: 3A
• Voltage - Input: 38 ~ 60V
• Regulator Topology: Flyback
• Frequency - Switching: 250kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM5072
• Lead Free Status / RoHS Status: Not applicable / Not applicable

www.national.com
Auxiliary Input “OR-ing” Diode
Selection
diodes and prevent false states at the ICL_FAUX and RAUX
pins. Please see the LM5072 datasheet for more details
about the selection of these two resistors.
Flyback Converter Topology
The dc-dc converter stage of the LM5072 evaluation board
features the flyback topology, which employs the minimum
number of power components to implement an isolated
power supply at the lowest possible cost.
A unique characteristic of the flyback topology is its power
transformer. Unlike an ordinary power transformer that si-
multaneously transfers the power from the primary to the
secondary, the flyback transformer first stores the energy in
the transformer core while the main switch is turned on, then
releases the stored energy to the load during the rest of the
cycle. When the stored energy is not completely released
before the main switch is turned on again, it is said that the
flyback converter operates in continuous conduction mode
(CCM). Otherwise, it is in discontinuous conduction mode
(DCM).
Major advantages of CCM over DCM include (i) lower ripple
current and ripple voltage, requiring smaller input and output
filter capacitors; and (ii) lower RMS current, thus reducing
the conduction losses. To keep the flyback converter in CCM
at light load, the transformer’s primary inductance should be
designed as large as is practical.
Major drawbacks of CCM, as compared to DCM, are (i) the
presence of the right-half-plane zero, which may limit the
achievable bandwidth of the feedback loop, and (ii) the need
for slope compensation to stabilize the feedback loop at duty
cycles greater than 50%.
The flyback topology can have multiple secondary windings
for several isolated outputs. One or more of these secondary
channels are normally utilized internally by the converter
itself to provide necessary bias voltages for the controller.
The evaluation board uses a small power transformer having
a primary inductance of 32 µH. This is a compromise made
to allow the small transformer to operate over a wide input
voltage range from 13V to 60V. However, with this trans-
former, the flyback converter runs in CCM at full load for
input voltages lower than 42V, and in DCM for higher input
voltages or light loads. The LM5072’s built-in slope compen-
sation helps stabilize the feedback loop when the duty cycle
exceeds 50% during low input voltage operation.
A transformer winding is used to provide the bias voltage
(V
includes an internal startup regulator which can support the
bias requirement indefinitely, the transformer winding pro-
duces an output about 2V higher than the startup regulator
output, thus shutting off the startup regulator and reducing
the power dissipation inside the IC. Given the low current
limit value (15 mA nominal) of the high voltage startup regu-
lator, the V
external loads.
Factors Limiting the Minimum
Operating Input Voltage
The LM5072 supports operation with low voltage auxiliary
power sources. The minimum FAUX voltage is 24V for the
maximum output current. It can be reduced to 18V if the
output current is reduced to 2A. The voltage drop caused by
CC
) to the LM5072 IC. Although the LM5072 controller
CC
line is not meant to be a linear regulator for
(Continued)
6
the FAUX input OR-ing diode D3 reduces the VIN pin poten-
tial to 17V. This is because at lower FAUX input voltages the
maximum power that can be delivered to the load will be
greatly reduced by the hot swap MOSFET’s current limit
function. This is one of the drawbacks of FAUX operation,
though DC current limit can be adjusted by adding / chang-
ing the value of the DCCL resistor.
The minimum RAUX voltage of the evaluation board is 16V
(voltage drops caused by the inrush limit resistors R1 and R2
and the RAUX input OR-ing diode D1 reduce the VIN pin
potential to about 15V), although the LM5072 can function
with a minimum of 9V seen at VIN. The 13V RAUX operating
voltage limit is mainly determined by three factors; the value
of the RAUX inrush current limit resistor R1 and R2, the
flyback power transformer design, the values of the current
sense resistors R14 and R15, and mainly the dropout of the
startup regulator.
The installed R1 and R2 are 2Ω resistors. Under full load
conditions, these two resistors significantly reduce the effec-
tive RAUX voltage seen by the DC-DC converter stage. In
order to operate the evaluation board at a lower input volt-
age, it is required to reduce R1 and R2 to 1Ω or lower values.
The installed EP13 type power transformer (DA2257-AL or
DCT13EP-U12S005) is a low cost, area efficient solution to
operate with a wide auxiliary input voltage range. However,
the small cross-sectional area of the EP13 magnetic core
also limits the maximum flux it can support. To use such a
small transformer from 16V to 60V input under the full load
condition, a compromise between the minimum operating
input voltage and maximum inductance of the transformer
must be made such that the peak current at 16V input will not
cause the peak flux density to exceed 3000 Gauss (satura-
tion). A drawback of this low cost solution is that the RMS
current flowing through the dc-dc converter stage is in-
creased and the efficiency of the dc-dc converter suffers by
about 3%.
Replacing the installed transformer with the optional power
transformer DA2383-AL from Coilcraft improves the effi-
ciency, but the minimum operating input voltage will be
limited to 24V. To use this optional transformer for lower input
voltage, the load level should be scaled down accordingly,
as shown in Figure 3.
FIGURE 3. Maximum Load Current vs. Minimum Input
Voltage as Limited by Different EP13 Type Power
Transformers
20187203