LM5026 Evaluation Board
Introduction
The LM5026 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
LM5026 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
•Output voltage: 3.3V
•Output current: 0 to 30A
•Measured efficiency: 90% 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 65% (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
COMP input to the LM5026 greatly increases the achievable
loop bandwidth. The capacitance effect (and associated
pole) of the optocoupler is greatly reduced by holding the
voltage across the optocoupler constant. The LM5026 cur-
rent mode controller pulse width modulates the error signal
with a ramp signal derived from the transformer primary. A
standard “type II” (pole-zero-pole) is used as a compensa-
tion network. The LM5026 provides a controlled delay nec-
essary for the reset switch.
The evaluation board can be synchronized to an external
clock with a recommended frequency range of 230 to
300KHz.
National Semiconductor
Application Note 1387
Robert Bell
September 2005
LM5026 Evaluation Board AN-1387
© 2005 National Semiconductor Corporation AN201546 www.national.com
Theory of Operation (Continued)
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Schematic
Powering and Loading
Considerations
When applying power to the LM5026 evaluation board cer-
tain precautions need to be followed. A mis-connection can
damage the assembly.
Proper Connections
When operated at low input voltages the evaluation board
can draw up to 3.5A of current at full load. The maximum
rated output current 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 evalu-
ation board. Monitor the voltage directly at the output termi-
nals of the evaluation board. 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 (36V) 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
LM5026 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.
The power supply and cabling must present a low imped-
ance to the evaluation board. Insufficient cabling or a high
impedance power supply will droop during power supply
application with the evaluation board inrush current. If large
enough, this droop will cause a chattering condition upon
power up. This chattering condition is an interaction with the
evaluation board undervoltage lockout, the cabling imped-
ance 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. The high output current requires
thick cables! If resistor banks are used there are certain
precautions to be taken. The wattage and current ratings
must be adequate for a 30A, 100W supply. Monitor both
current and voltage at all times. Ensure there is sufficient
cooling provided for the load.
Air Flow
Full power loading should never be attempted without pro-
viding the specified 200 CFM of air flow over the evaluation
board. A stand-alone fan should be provided.
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
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
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Powering Up (Continued)
supply folding back and the evaluation board going into
undervoltage shutdown will start an oscillation, or chatter,
that may have 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 ad-
versely. Few parameters can be incorrect in a switching
power supply without creating losses and potentially damag-
ing heat.
Over Current Protection
The evaluation board is configured with hiccup over-current
protection. In the event of an output overload (approximately
33A) the unit will discharge the softstart capacitor, which
disables the power stage. After a delay the softstart is re-
leased. The shutdown, delay and slow recharge time of the
softstart capacitor protects the unit, especially during short
circuit event where the stress is highest.
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Typical Evaluation Setup
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Performance Characteristics
TURN-ON WAVEFORMS
When applying power to the LM5026 evaluation board a
certain sequence of events occurs. Soft-start capacitor val-
ues and other components allow for a minimal output voltage
for a short time until the feedback loop can stabilize without
overshoot. Figure 1 shows the output voltage 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 minimal 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.
20154604
Conditions: Input Voltage = 48VDC Output Current = 5A
Trace 1: Output Voltage Volts/div = 1V
Horizontal Resolution = 1msec/div
FIGURE 1.
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Conditions: Input Voltage = 48VDC Output Current = 5A to 25A
Trace 1: Output Voltage Volts/div = 0.5V
Trace 2: Output Current, Amps/div = 5A
Horizontal Resolution = 1msec/div
FIGURE 2.
20154605
Conditions: Input Voltage = 48VDC
Output Current = 30A
Bandwidth Limit = 25MHz
Trace 1: Output Ripple Voltage Volts/div = 50mV
Horizontal Resolution = 2µs/div
FIGURE 3.
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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)
20154607
Conditions: Input Voltage = 78VDC Output Current = 25A
Trace 1: Q1 drain voltage Volts/div = 20V Horizontal Resolution = 1µs/div
FIGURE 5.
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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
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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.
TABLE 1. Bill of Materials
DESIGNATOR QTY PART NUMBER DESCRIPTION VALUE
C1-C4 4 C4532X7R2A225M CAPACITOR, CER, TDK 2.2µ, 100V
C5 1 C4532X7R3A103K CAPACITOR, CER, TDK 0.01µ, 1000V
C6,C15 2 C3216X7R2E104K CAPACITOR, CER, TDK 0.1µ, 250V
C7 1 C4532X7R1E156M CAPACITOR, CER, TDK 15µ, 25V
C8 1 C2012X7R2A103K CAPACITOR, CER, TDK 0.01µ, 100V
C9,C30,C33 3 C2012X7R2A102K CAPACITOR, CER, TDK 1000p, 100V
C10,C14,C28, C31 4 C2012X7R1H104K CAPACITOR, CER, TDK 0.1µ, 50V
C11, C12 2 C2012X7R1H473K CAPACITOR, CER, TDK 0.047µ, 50V
C13,C18 2 C1206C104K5RAC CAPACITOR, CER, KEMET 0.1µ, 50V
C16, C17, C29 3 C0805C471J5GAC CAPACITOR, CER, KEMET 470p, 50V
C19,C20 2 T520D337M006AS4350 CAPACITOR,TANT,KEMET 330µ, 6.3V
C21,C22,C23 3 C4532X7S0G686M CAPACITOR, CER, TDK 68µ, 4V
C24, C25 OPEN NOT USED
C26 1 C0805C101J5GAC CAPACITOR, CER, KEMET 100p, 50V
C27 1 C1206C333K5RAC CAPACITOR, CER, KEMET 0.033µ, 50V
C32 1 C0805C330J5GAC CAPACITOR, CER, KEMET 33p, 50V
D1- D7 7 CMPD2838 DIODE, SIGNAL, CENTRAL
D8 1 CMPD7000 DIODE, SIGNAL, CENTRAL
D9 1 CMR1U-02 DIODE, 200V, 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 ZVP2120GTA P-FET, ZETEX 200V, 20
Q3 - Q6 4 SI7866DP FET, SILICONIX 20V, 3m
R1, R22, R24, R28 4 CRCW120610R0F RESISTOR 10Ω
R2, R13, R25 OPEN NOT USED
R3, R4 2 CRCW120615R0F RESISTOR 15Ω
R5 1 CRCW12062000F RESISTOR 200Ω
R6 1 CRCW120649R9F RESISTOR 49.9Ω
R7 1 CRCW12061003F RESISTOR 100kΩ
R8 1 CRCW12063831F RESISTOR 3.83kΩ
R9, R15 2 CRCW12061001F RESISTOR 1kΩ
R10 1 CRCW12062212F RESISTOR 22.1kΩ
R11 1 CRCW12063921F RESISTOR 3.92kΩ
R12 1 CRCW12061652F RESISTOR 16.5kΩ
R14,R18,R19,R29,R33,R35 5 CRCW12061002F RESISTOR 10kΩ
R16, R17 2 CRCW12065R60F RESISTOR 5.6Ω
R20, R21 2 CRCW2512100J RESISTOR 10Ω,1W
R23 1 CRCW12061000F RESISTOR 100Ω
R26 1 CRCW12062492F RESISTOR 24.9kΩ
R27 1 CRCW12061502F RESISTOR 15kΩ
R30, R31, R34 3 CRCW12064991F RESISTOR 4.99kΩ
R32 1 CRCW12062002F RESISTOR 20kΩ
T1 1 P8208T CURRENT XFR, PULSE ENG 100:01
T2 1 B0357-B POWER XFR, COILCRAFT 12:02
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Layout and Bill of Materials (Continued)
TABLE 1. Bill of Materials (Continued)
DESIGNATOR QTY PART NUMBER DESCRIPTION VALUE
U1 1 LM5026MM 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
20154609
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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)
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PCB Layouts (Continued)
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National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
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-1387 LM5026 Evaluation Board
