LM5025A Evaluation Board
Introduction
The LM5025A evaluation board is designed to provide the
design engineer with a fully functional power converter
based on the Active Clamp Forward topology to evaluate the
LM5025A controller. The evaluation board is provided in an
industry standard half-brick footprint.
The performance of the evaluation board is as follows:
•Input range: 36V to 78V (100V peak)
•Output voltage: 3.3V
•Output current: 0 to 30A
•Measured efficiency: 90.5% at 30A, 92.5% at 15A
•Frequency of operation: 230kHz
•Board size: 2.3 x 2.4 x 0.5 inches
•Load Regulation: 1%
•Line Regulation: 0.1%
•Line UVLO, Hiccup Current Limit
The printed circuit board consists of 4 layers of 3 ounce
copper on FR4 material with a total thickness of 0.050
inches. Soldermask has been omitted from some areas to
facilitate cooling. The unit is designed for continuous opera-
tion at rated load at <40˚C and a minimum airflow of 200
CFM.
Theory of Operation
Power converters based on the Forward topology offer high
efficiency and good power handling capability in applications
up to several hundred Watts. The operation of the trans-
former in a forward topology does not inherently self-reset
each power switching cycle, a mechanism to reset the trans-
former is required. The active clamp reset mechanism is
presently finding extensive use in medium level power con-
verters in the 50 to 200W range.
The Forward converter is derived from the Buck topology
family, employing a single modulating power switch. The
main difference between the topologies are, the Forward
topology employs a transformer to provide input / output
ground isolation and a step down or step up function.
Each cycle, the main primary switch turns on and applies the
input voltage across the primary winding, which has 12
turns. The transformer secondary has 2 turns, leading to a
6:1 step-down of the input voltage. For an output voltage of
3.3V the required duty cycle (D) of the main switch must vary
from approximately 60% (low line) to 25% (high line). The
clamp capacitor along with the reset switch reverse biases
the transformer primary each cycle when the main switch
turns off. This reverse voltage resets the transformer. The
clamp capacitor voltage is Vin / (1-D).
The secondary rectification employs self-driven synchronous
rectification to maintain high efficiency and ease of drive.
Feedback from the output is processed by an amplifier and
reference, generating an error voltage, which is coupled
back to the primary side control through an optocoupler. The
LM5025A voltage mode controller pulse width modulates the
error signal with a ramp signal derived from the input volt-
age. Deriving the ramp signal slope from the input voltage
provides line feed-forward, which improves line transient
rejection. The LM5025A also provides a controlled delay
necessary for the reset switch.
The evaluation board can be synchronized to an external
clock with a recommended frequency range of 190 to
300KHz.
National Semiconductor
Application Note 1345
Robert Bell
December 2004
LM5025A Evaluation Board AN-1345
© 2004 National Semiconductor Corporation AN201276 www.national.com
Theory of Operation (Continued)
20127601
Simplified Active Clamp Forward Converter
Powering and Loading
Considerations
When applying power to the LM5025A evaluation board
certain precautions need to be followed. A failure or mis-
connection can present itself in a very alarming manner.
Proper Connections
When operated at low input voltages the UUT can draw up to
3.5A of current at full load. The maximum rated output cur-
rent for the evaluation board is 30A. Be sure to choose the
correct connector and wire size when attaching the source
supply and the load. Monitor the current into and out of the
UUT (evaluation board or unit under test). Monitor the volt-
age directly at the output terminals of the UUT. The voltage
drop across the load connecting wires will give inaccurate
measurements, this is especially true for accurate efficiency
measurements.
Source Power
The evaluation board can be viewed as a constant power
load. At low input line voltage (35V) the input current can
reach 3.5A, while at high input line voltage (78V) the input
current will be approximately 1.5A. Therefore to fully test the
LM5025A evaluation board a DC power supply capable of at
least 80V and 4A is required. The power supply must have
adjustments for both voltage and current. An accurate read-
out of output current is desirable since the current is not
subject to loss in the cables as voltage is.
The power supply and cabling must present a low imped-
ance to the UUT. Insufficient cabling or a high impedance
power supply will droop during power supply application with
the UUT inrush current. If large enough, this droop will cause
a chattering condition upon power up. This chattering condi-
tion is an interaction with the UUT undervoltage lockout, the
cabling impedance and the inrush current
Loading
An appropriate electronic load with specified operation down
to 3.0V minimum is desirable. The resistance of a maximum
load is 0.11Ω. You need thick cables! Consult a wire chart if
needed. If resistor banks are used there are certain precau-
tions to be taken. The wattage and current ratings must be
adequate for a 30A, 100W supply. Monitor both current and
voltage at all times.
Be careful!! The high temperatures reached
by even the most adequately rated resistors
may burn you or melt your benchtop.
Air Flow
Full rated power should never be attempted without provid-
ing the specified 200 CFM of air flow over the evaluation
board. This can be provided by a stand-alone fan.
Powering Up
Using the shutdown pin provided will allow powering up the
source supply with the current level set low. It is suggested
that the load be kept low during the first power up. Set the
current limit of the source supply to provide about 1.5 times
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Powering Up (Continued)
the wattage of the load. As you remove the connection from
the shutdown pin to ground, immediately check for 3.3 volts
at the output.
A most common occurrence, that will prove unnerving, is
when the current limit set on the source supply is insufficient
for the load. The result is similar to having the high source
impedance referred to earlier. The interaction of the source
supply folding back and the UUT going into undervoltage
shutdown will start an oscillation, or chatter, that may have
highly undesirable consequences. A quick efficiency check is
the best way to confirm that everything is operating properly.
If something is amiss you can be reasonably sure that it will
affect the efficiency adversely. Few parameters can be in-
correct in a switching power supply without creating losses
and potentially damaging heat.
Over Current Protection
The evaluation board is configured with delayed hiccup over-
current protection. In the event of an output overload (ap-
proximately 33A) the unit will discharge the softstart capaci-
tor, which disables the power stage. After a delay the
softstart is released. The shutdown, delay and slow recharge
time of the softstart capacitor protects the unit, especially
during short circuit event where the stress is highest.
20127602
Typical Evaluation Setup
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Performance Characteristics
TURN-ON WAVEFORMS
When applying power to the LM5025A evaluation board a
certain sequence of events must occur. Soft-start capacitor
values and other components allow the feedback loop to
stabilize without overshoot. Figure 1 shows the output volt-
age during a typical start-up with a 48V input and a load of
5A. There is no overshoot during startup.
OUTPUT RIPPLE WAVEFORMS
Figure 2 shows the transient response for a load of change
from 5A to 25A. The upper trace shows output voltage droop
and overshoot during the sudden change in output current
shown by the lower trace.
Figure 3 shows typical output ripple seen directly across the
output capacitor, for an input voltage of 48V and a load of
30A. This waveform is typical of most loads and input volt-
ages.
Figure 4 and Figure 5 show the drain voltage of Q1 with a
25A load. Figure 4 represents an input voltage of 38V and
Figure 5 represents an input voltage of 78V.
Figure 6 shows the gate voltages of the synchronous recti-
fiers. The drive from the main power transformer is delayed
slightly at turn-on by a resistor interacting with the gate
capacitance. This provides improved switching transitions
for optimum efficiency. The difference in drive voltage is
inherent in the topology and varies with line voltage.
20127607
Conditions: Input Voltage = 48VDC Output Current = 5A
Trace 1: Output Voltage Volts/div = 0.5V
Horizontal Resolution = 1msec/div
FIGURE 1.
20127608
Conditions: Input Voltage = 48VDC Output Current = 5A to 25A
Trace 1: Output Voltage Volts/div = 0.5V
Trace 2: Output Current, Amps/div = 10.0A
Horizontal Resolution = 1µs/div
FIGURE 2.
20127609
Conditions: Input Voltage = 48VDC
Output Current = 30A
Bandwidth Limit = 25MHz
Trace 1: Output Ripple Voltage Volts/div = 50mV
Horizontal Resolution = 2µs/div
FIGURE 3.
20127604
Conditions: Input Voltage = 38VDC Output Current = 25A
Trace 1: Q1 drain voltage Volts/div = 20V Horizontal Resolution = 1µs/div
FIGURE 4.
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Performance Characteristics
(Continued)
20127605
Conditions: Input Voltage = 78VDC Output Current = 25A
Trace 1: Q1 drain voltage Volts/div = 20V Horizontal Resolution = 1µs/div
FIGURE 5.
20127606
Conditions: Input Voltage = 48VDC Output Current = 5A
Synchronous rectifier, Q3 gate Volts/div = 5V
Trace 1: Synchronous rectifier, Q3 gate Volts/div = 5V
Trace 2: Synchronous rectifier, Q5 gate Volts/div = 5V
Horizontal Resolution = 1µs/div
FIGURE 6.
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Application Circuit
20127603
Application Circuit: Input 36 to 78V, Output 3.3V, 30A
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Layout and Bill of Materials
The Bill of Materials is shown below and includes the manu-
facturer and part number. The layers of the printed circuit
board are shown in top down order. View is from the top
down except for the bottom silkscreen which is shown
viewed from the bottom. Scale is approximately X1.5. The
printed circuit board consists of 4 layers of 3 ounce copper
on FR4 material with a total thickness of 0.050 inches.
Bill of Materials
DESIGNATOR QTY PART NUMBER DESCRIPTION VALUE
C1-C4 4 C4532X7R2A225M CAPACITOR, CER,
TDK
2.2u, 100V
C5 1 C4532X7R3A103K CAPACITOR, CER,
TDK
0.01µ, 1000V
C6 1 C0805C221J5GAC CAPACITOR, CER,
KEMET
220p, 50V
C7 1 C2012X7R1E224K CAPACITOR, CER,
TDK
0.22µ, 25V
C8,C16 2 C3216X7R2E104K CAPACITOR, CER,
TDK
0.1µ, 250V
C9 1 C4532X7R1E156M CAPACITOR, CER,
TDK
15µ, 25V
C10,C17,C18, C31 4 C0805C471J5GAC CAPACITOR, CER,
KEMET
470p, 50V
C11 1 C2012X7R2A103K CAPACITOR, CER,
TDK
0.01µ, 100V
C12,C15,C30, C33 4 C2012X7R1H104K CAPACITOR, CER,
TDK
0.1µ, 50V
C13 1 C2012X7R2A223K CAPACITOR, CER,
TDK
0.022µ, 100V
C14 1 C3216X7R1H334K CAPACITOR, CER,
TDK
0.33µ, 50V
C19,C20 2 C1206C104K5RAC CAPACITOR, CER,
KEMET
0.1µ, 50V
C21,C22 2 T520D337M006AS4350 CAPACITOR,TANT,
KEMET
330µ, 6.3V
C23,C24,C25 3 C4532X7S0G686M CAPACITOR, CER,
TDK
68µ, 4V
C26 OPEN NOT USED
C27,C32 2 C2012X7R2A102K CAPACITOR, CER,
TDK
1000p, 100V
C28 1 C0805C101J5GAC CAPACITOR, CER,
KEMET
100p, 50V
C29 1 C2012X7R2A332K CAPACITOR, CER,
TDK
3300p, 100V
D1- D8 8 CMPD2838-NSA DIODE, SIGNAL,
CENTRAL
L1 1 SLF10145T-5R6M3R2 INPUT CHOKE, TDK 5.6µH, 3.5A
L2 1 B0358-C CHOKE with AUX,
COILCRAFT
2µH, 33A
Q1 1 SI7846DP N-FET, SILICONIX 150V, 50m
Q2 1 IRF6217 P-FET, IR 150V, 2.4
Q3 - Q6 4 SI7866DP FET, SILICONIX 20V, 3m
R1,R25,R29 3 CRCW120610R0F RESISTOR 10
R2,R16,R17, R21,R22,
R34
6 CRCW12061002F RESISTOR 10K
R19,R20, R36 3 CRCW12065R60F RESISTOR 5.6
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Layout and Bill of Materials (Continued)
Bill of Materials (Continued)
DESIGNATOR QTY PART NUMBER DESCRIPTION VALUE
R4 1 CRCW120615R0F RESISTOR 15
R5 1 CRCW12062000F RESISTOR 200
R6 1 CRCW12062003F RESISTOR 200K
R8 1 CRCW120649R9F RESISTOR 49.9
R9,R10 2 CRCW12061003F RESISTOR 100K
R3 1 CRCW120618R2F RESISTOR 18.2
R7 1 CRCW12063012F RESISTOR 30.1K
R11 1 CRCW12068061F RESISTOR 8.06K
R12,R15,R18,R26 4 CRCW12061001F RESISTOR 1K
R13 1 CRCW12062672F RESISTOR 26.7K
R14 1 CRCW12061652F RESISTOR 16.5K
R23,R24 2 CRCW2512100J RESISTOR 10, 1W
R27 1 CRCW12062492F RESISTOR 24.9K
R28 1 CRCW12061502F RESISTOR 15K
R30 1 CRCW12063012F RESISTOR 30.1K
R31,R32 2 CRCW12064991F RESISTOR 4.99K
R33 1 CRCW12062002F RESISTOR 20K
R35 1 CRCW12061000F RESISTOR 100
T1 1 P8208T CURRENT XFR,
PULSE ENG
100:1
T2 1 B0357-B POWER XFR,
COILCRAFT
12:02
U1 1 LM5025A CONTROLLER,
NATIONAL SEMI
U2 1 MOCD207M OPTO-COUPLER, QT
OPTO
U3 1 LM6132AIM OPAMP, NATIONAL
SEMI
U4 1 LM4041CEM3-1.2 REFERENCE,
NATIONAL SEMI
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PCB Layouts
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PCB Layouts (Continued)
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PCB Layouts (Continued)
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PCB Layouts (Continued)
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PCB Layouts (Continued)
20127614
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PCB Layouts (Continued)
20127615
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the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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AN-1345 LM5025A Evaluation Board
