NSC LM5045

National Semiconductor
Application Note 2111
Ajay Hari
February 23, 2011
Introduction
Theory of Operation
The LM5045 evaluation board is designed to provide the design engineer with a fully functional power converter based
on the full-bridge topology to evaluate the LM5045 PWM controller. The evaluation board is provided in an industry standard 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.
Power converters based on the full-bridge topology offer highefficiency 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 alternatively 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 utilization 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 primary current enables higher efficiency as compared to a halfbridge at high load currents.
LM5045 Evaluation Board
LM5045 Evaluation Board
30146201
Simplified Full-Bridge Converter
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© 2011 National Semiconductor Corporation
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The secondary side employs synchronous rectification
scheme, which is controlled by the LM5045. In addition to the
basic soft-start already described, the LM5045 contains a
second soft-start function that gradually turns on the synchronous rectifiers to their steady-state duty cycle. This function keeps the synchronous rectifiers off until the error
amplifier on the secondary side soft-starts, allowing a linear
start-up of the output voltage even into pre-biased loads.
Then the SR output duty cycle is gradually increased to prevent output voltage disturbances due to the difference in the
voltage drop between the body diode and the channel resistance of the synchronous MOSFETs. Once the soft-start is
finished, the synchronous rectifiers are engaged with a nonoverlap time programmed by the RD1 and RD2 resistors.
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 opto-coupler. The
LM5045 evaluation board employs peak current mode control
and a standard “type II” network is used for the compensator.
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 (72V) the input
current will be approximately 1.5A. Therefore, to fully test the
LM5045 evaluation board a DC power supply capable of at
least 85V and 4A is required. The power supply must have
adjustments for both voltage and current.
The power supply and cabling must present low impedance
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 under voltage lockout, the cabling
impedance and the inrush current.
Powering and Loading
Considerations
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 that there is sufficient cooling provided for the
load.
When applying power to the LM5045 evaluation board certain
precautions need to be followed. A misconnection 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 evaluation board. Monitor the voltage directly at the output terminals of the evaluation board. The voltage drop across the load connecting wires
Air Flow
Full power loading should never be attempted without providing the specified 200 CFM of air flow over the evaluation
board. A stand-alone fan should be provided.
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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 soon as the appropriate
input voltage is supplied to the board, 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 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 adversely.
Few parameters can be incorrect in a switching power supply
without creating losses and potentially damaging heat.
Over Current Protection
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Conditions: Input Voltage = 48V
Output Current = 25A
Trace 1: Output Voltage Volts/div = 1V
Horizontal Resolution = 2.0 ms/div
The evaluation board is configured with hiccup over-current
protection. In the event of an output overload (approximately
38A) the unit will discharge the SS capacitor, which disables
the power stage. After a delay, programmed by the RES capacitor, the SS capacitor is released. If the overload condition
persists, this process is repeated. Thus, the converter will be
in a loop of shot bursts followed by a sleep time in continuous
overload conditions. The sleep time reduces the average input current drawn by the power converter in such a condition
and allows the power converter to cool down.
FIGURE 2. Soft-Start
Performance Characteristics
Once the circuit is powered up and running normally, the output voltage is regulated to 3.3V with the accuracy determined
by the feedback resistors and the voltage reference. The frequency of operation is selected to be 420 kHz, which is a good
comprise between board size and efficiency. Please refer to
the figure 1. for efficiency curves.
100
36V
EFFICIENCY (%)
90
48V
80
70
30146205
Conditions: Input Voltage = 48V
Output Current = 15A to 22.5A to 15A
Upper Trace: Output Voltage Volts/div = 100mV
Lower Trace: Output Current = 5A/div
Horizontal Resolution = 200 µs/div
72V
VOUT = 3.3V
FIGURE 3. Transient Response
60
50
5 7 9 11 13 15 17 19 21 23 25 27 29
LOAD CURRENT (A)
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FIGURE 1. Application Board Efficiency
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When applying power to the LM5045 evaluation board a certain sequence of events occurs. Soft-start capacitor values
and other components allow for a minimal output voltage for
a short time until the feedback loop can stabilize without overshoot. Figure 2 shows the output voltage during a typical startup with a 48V input and a load of 25A. There is no overshoot
during start-up.
Powering Up
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Figure 4 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 voltages.
30146208
Conditions: Input Voltage = 72V
Output Current = 30A
Trace 1: SW1 Node (Q2 Drain) Voltage Volts/div = 20V
Horizontal Resolution = 1 µs/div
30146206
Conditions: Input Voltage = 48V, Output Current = 30A
Trace 1: Output Voltage Volts/div = 50mV
Bandwidth Limit = 20MHz
Horizontal Resolution = 2 µs/div
FIGURE 6. Switch Node Waveforms
Figure 7 shows a typical startup of the LM5045 into a 2V prebiased load. Trace 2 represents the output current that is
monitored between the output caps of the power converter
and the 2V pre-bias voltage supply. It can be inferred from the
Trace 2 that the SR MOSFET's do not sink any current during
the power-up into pre-biased load.
FIGURE 4. Output Ripple
Figures 5 and 6 show the typical SW node voltage waveforms
with a 25A load. Figure 5 shows an input voltage represents
an input voltage of 48V and Figure 6 represents an input voltage of 72V.
30146207
Conditions: Input Voltage = 48V
Output Current = 30A
Trace 1: SW1 Node (Q2 Drain) Voltage Volts/div = 20V
Horizontal Resolution = 1µs/div
30146209
Conditions: Input Voltage = 48V, Output Pre-Bias = 2V
Trace 1 (Channel 4): Output Voltage Volts/div = 1V
Trace 2 (Channel 2): Output Current Amps/div = 200mA
Trace 3 (Channel 3): SR Gate Voltage Volts/div = 5V
FIGURE 5. Switch Node Waveforms
FIGURE 7. Soft-Start into 2V Pre-Biased Load
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Application Circuit: Input 36V to 75V, Output 3.3V at 30A
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Bill of Materials
Item
Designator
Description
Part Number
1
AA
2
C1, C2, C3, C4
Printed Circuit Board
Ceramic 2.2uF, X7R, 100V, 10%, 1210
GRM32ER72A225KA35L
3
C5, C35
Ceramic 2.2uF, X7R, 16V, 10%, 0805
GRM21BR71C225KA12L
4
C7, C8
Ceramic, 2.2uF, X5R, 25V, 10%, 0805
GRM21BR71E225KA73L
5
C9
Ceramic, 1uF, X7R, 50V, 10%, 0805
GRM21BR71H105KA12L
6
C6
Ceramic 2.2uF 10V X7R 0603
GRM188R71A225KE15D
7
C10, C11
Ceramic, 1uF, X7R, 16V, 10%, 0603
C1608X7R1C105K
8
C12, C15, C21, C32
Ceramic,0.1uF, X7R, 25V, 10%, 0603
06033C104KAT2A
9
C13
Ceramic, X7R,2000V, 2700pF,10%
C1808C272KGRACTU
10
C14
Ceramic 0.1uF, 100V, +/-10%, X7R, 0603 GRM188R72A104KA35D
11
C16, C23
Ceramic, C0G/NP0 470pF, 100V, 10%,
1206
12
C17, C39
Cap 330uF, 4V, AL, 4V, 20%, 0.012 Ohm EEF-UE0G331R
ESR
13
C18, C19, C20
14
C22
15
C34, C36
Ceramic 1000pF, 25V, +/-5%, C0G/NP0, C1005C0G1E102J
0402,
16
C26, C27
Ceramic1uF, 16V, +/-20%, X7R, 0805
GRM21BR71C105MA01L
17
C28, R20, D4, L3
NU
NU
18
C29
Ceramic 47pF, 50V, +/-5%, C0G/NP0,
0402
GRM1555C1H470JZ01
19
C30, C40
Ceramic 100pF, C0G/NP0, 50V, 5%,
0603
C1608C0G1H101J
20
C24
CAP, CERM, 0.056uF, 6.3V, +/-10%,
X7R, 0402
C0402C563K9RACTU
21
C25, C31, C37, C33
22
C38
CAP, CERM, 0.47uF, 6.3V, +/-20%, X5R, C1005X5R0J474K
0402
23
D2
Vr = 30V, Io = 1A, Vf = 0.38V
24
D3, D7, D10, D14
25
D5
SMT 5.1V Zener Diode
MMSZ5231B
26
D6
SMT 8.2V Zener Diode
CMHZ4694
27
D8, D12
Vr = 100V, Io = 1A, Vf = 0.77V, Schottky DFLS1100-7
diode
28
D9, D13
Vr = 40V, Io = 0.2A, Vf = 0.65V, Common CMPSH-3AE
Anode
29
D11
SMT 11V Zener Diode
CMHZ4698
30
D16
Vr = 30V, Io = 0.2A, Vf = 0.7V, Schottky
BAT54WS-7-F
31
D17
Diode, Zener, 4.7V, 250mW, SOD-323
CMDZ4L7
32
L1
Shielded Drum Core, 2.2uH 4.15A,
0.0165 Ohm
DR73-2R2-R
33
L2
Shielded Drum Core, 0.08A, 11 Ohm
LPS5030-225MLB
34
L3
NU
NU
35
L4
Inductor, Shielded E Core, Ferrite,
800nH, 45A, 0.0009 ohm, SMD
SER2010-801MLB
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12061A471KAT2A
Ceramic 47uF, X7R, 6.3V, 10%
GCM32ER70J476KE19L
Ceramic 0.022uF, 16V, +/-10%, X7R,
0402
C1005X7R1C223K
CAP, CERM, 0.01uF, 16V, +/-10%, X7R, C1005X7R1C1103K
0402
B130LAW-7-F
Vr = 40V, Io = 0.2A, Vf = 0.65V, Common CMPSH-3CE
Cathode
6
Designator
Description
Part Number
36
P1, P3, P5, P6
37
P2
PCB Pin
3104-2-00-34-00-00-08-0
Test Point, SMT, Miniature
38
5015
P4, P7
PCB Pin
3231-2-00-34-00-00-08-0
39
Q1, Q3
NPN, 2A, 45V
FCX690BTA
40
Q2
PNP, 0.2A, 40V
CMPT3906
41
Q4, Q5, Q10, Q11
42
Q6, Q7, Q8, Q9
43
Q12, Q13
44
R1
RES 10 Ohm 1%, 0.125W, 0805
CRCW080510R0FKEA
45
R2, R28, R33, R34, R35,
R36
RES 10K Ohm 1%, 0.063W, 0402
CRCW040210k0FKED
46
R3, R4
RES 5.1K Ohm 5%, 0.125W, 0805
ERJ-6GEYJ512V
47
R5
RES 1K Ohm, 5.1, 0.125W, 0805
CRCW08051K00FKEA
48
R6
RES 100K Ohm,1%, 0.125W, 0805
CRCW0805100KFKEA
49
R7
RES, 2.61k ohm, 1%, 0.063W, 0402
CRCW04022K61FKED
50
R8
RES 20 OHM 1/8W 5% 0805 SMD
ERJ-6GEYJ200V
51
R9
RES, 1.58k ohm, 1%, 0.063W, 0402
CRCW04021K58FKED
52
R10, R12
RES, 0 ohm, 5%, 0.063W, 0402
RC0402JR-070RL
53
R11, R17
RES 4.99 Ohm,1%, 0.25W, 1206
CRCW12064R99FNEA
54
R13
RES, 1.69k ohm, 1%, 0.063W, 0402
CRCW04021K69FKED
55
R14
RES 24K, 5%, 0.063W, 0402
CRCW040224k0JNED
56
R15
RES, 30.1k ohm, 1%, 0.063W, 0402
CRCW040230K1FKED
57
R16
RES 20k Ohm,1%, 0.063W, 0402
CRCW040220k0FKED
58
R18
RES, 15.0 ohm, 1%, 0.063W, 0402
CRCW040215R0FKED
59
R19, R31
RES 10.0 ohm, 1%, 0.063W, 0402
CRCW040210R0FKED
60
R21
RES 1.0K OHM 1/16W 5% 0402 SMD
CRCW04021K00JNED
61
R22
RES 25.5k ohm,1%, 0.063W, 0402
CRCW040225k5FKED
62
R23
RES 499 ohm, 1%, 0.063W, 0402
CRCW0402499RFKED
63
R24
RES 5.11k ohm, 1%, 0.063W, 0402
CRCW04025k11FKED
64
R25, R26
NU
NU
65
R27
RES 47 OHM .25W 5% 0603 SMD
CRCW060347R0JNEAHP
66
R32
RES 100 ohm, 1%, 0.063W, 0402
CRCW0402100RFKED
67
R29
RES 15k ohm,1%, 0.063W, 0402
CRCW040215k0FKED
68
R30
RES 1.82k ohm,1%, 0.063W, 0402
CRCW04021k82FKED
69
R37
RES 0.0 ohm, 5%, 0.063W, 0402
CRCW04020000Z0ED
70
D1
RES 0.0 ohm, 5%, 0.063W, 1206
CRCW12060000Z0EA
71
T1
High Frequency Planar Transformer
PA0876.003NL
72
T2
SMT Current Sense Transformer
PA1005.100NL
73
U1
Full-Bridge PWM Controller
LM5045MH
74
U2
Dual 5A Compound Gate Driver with
Negative Output Voltage Capability
LM5110-1SD
75
U3
Low Input Current, High CTR Photocoupler
PS2811-1-M-A
76
U4
RRIO, High Output Current & Unlimited
Cap Load Op Amp in SOT23-5
LM8261M5
77
U5
Precision Micro-power Shunt Voltage
Reference
LM4041BIM3-1.2
78
U6
ISO-Pro Low-Power Dual-Channel Digital Si8420BB-D-IS
Isolator
4.5A, 36nC, rDS(on) @ 4.5V =0.004 ohm SI7336ADP-GE3
MOSFET, N-CH, 100V, 9.3A, PQFN 8L
5x6 A
IRFH5053TRPBF
0.31A, 0.7nC, rDS(on) @ 4.5V =2.5 Ohm NTZD5110NT1G
7
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Item
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PCB Layouts
30146211
Top Side Assembly
30146212
Bottom Side Assembly
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30146213
Layer 1 (Top Side)
30146214
Layer 2
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30146215
Layer 3
30146216
Layer 4
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30146217
Layer 5
30146218
Layer 6 (Bottom Side)
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LM5045 Evaluation Board
Notes
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