Texas Instruments AN-1387 LM5026 User manual

User's Guide
SNVA117A–September 2005–Revised May 2013
AN-1387 LM5026 Evaluation Board
1 Introduction
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 operation at rated load at < 40°C and a minimum airflow of 200 CFM.
2 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 transformer in a forward topology does
not inherently self-reset each power switching cycle, a mechanism to reset the transformer is required.
The active clamp reset mechanism is presently finding extensive use in medium level power converters 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
current 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 compensation network. The
LM5026 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
230 to 300KHz.
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T1
CS
VIN
36 - 78V VOUT
3.3V
SYNC I/O
LM5026
UVLO
PGND AGND
COMP
OUT_A
OUT_B
VCC
SS
RT
SYNC
REF
TIME
RES
CS
VIN
DCL
ERROR
AMP and
ISOLATION
Powering and Loading Considerations www.ti.com
Figure 1. Schematic
3 Powering and Loading Considerations
When applying power to the LM5026 evaluation board certain precautions need to be followed. A mis-
connection can damage the assembly.
4 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 will give inaccurate measurements, this is especially true for accurate efficiency
measurements.
5 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 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 undervoltage lockout, the cabling impedance and the
inrush current.
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Scope Volt-meter Volt-meter Current-meter
80 Volt, 5 Amp
Power Supply
with Current
Meter
200 Watt, 60 Amp
Electronic Load
-
+
+
-
+
Evaluation Board
ON/OFF
(SHUTDOWN)
IN OUT
Jumper
www.ti.com Loading
6 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.
7 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.
8 Powering Up
Using the provided shutdown pin 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 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.
9 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 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.
Figure 2. Typical Evaluation Setup
10 Performance Characteristics
10.1 Turn-On Waveforms
When applying power to the LM5026 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 3 shows the output voltage during a typical start-up
with a 48V input and a load of 5A. There is no overshoot during startup.
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Performance Characteristics www.ti.com
10.2 Output Ripple Waveforms
Figure 4 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 5 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.
Figure 6 and Figure 7 show the drain voltage of Q1 with a 25A load. Figure 6 represents an input voltage
of 38V and Figure 7 represents an input voltage of 78V.
Figure 8 shows the gate voltages of the synchronous rectifiers. 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.
Conditions: Input Voltage = 48VDC Output Current = 5A to
Conditions: Input Voltage = 48VDC Output Current = 5A 25A Trace 1: Output Voltage Volts/div = 0.5V Trace 2:
Trace 1: Output Voltage Volts/div = 1V Horizontal Resolution Output Current, Amps/div = 5A Horizontal Resolution =
= 1msec/div 1msec/div
Figure 3. Output Voltage Figure 4. Transient Response
Conditions: Input Voltage = 48VDC Output Current = 30A Conditions: Input Voltage = 38VDC Output Current = 25A
Bandwidth Limit = 25MHz Trace 1: Output Ripple Voltage Trace 1: Q1 drain voltage Volts/div = 20V Horizontal
Volts/div = 50mV Horizontal Resolution = 2µs/div Resolution = 1µs/div
Figure 5. Typical Output Ripple Figure 6. Drain Voltage of Q1
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www.ti.com Performance Characteristics
Conditions: Input Voltage = 78VDC Output Current = 25A Conditions: Input Voltage = 48VDC Output Current = 5A
Trace 1: Q1 drain voltage Volts/div = 20V Horizontal Synchronous rectifier, Q3 gate Volts/div = 5V Trace 1:
Resolution = 1µs/div Synchronous rectifier, Q3 gate Volts/div = 5V Trace 2:
Synchronous rectifier, Q5 gate Volts/div = 5V Horizontal
Resolution = 1µs/div
Figure 7. Drain Voltage of Q1 Figure 8. Gate Voltages
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Application Circuit www.ti.com
11 Application Circuit
Figure 9. Application Circuit: Input 36 to 78V, Output 3.3V, 30A
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www.ti.com Layout and Bill of Materials (BOM)
12 Layout and Bill of Materials (BOM)
The Bill of Materials is shown in Table 1 and includes the manufacturer and part number. The layers of the
printed circuit board (PCB) are shown in top down order. The view is from the top down except for the
bottom silkscreen, which is shown viewed from the bottom. The 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 (BOM)
QTY DESIGNATOR PART NUMBER DESCRIPTION VALUE
4 C1-C4 C4532X7R2A225M CAPACITOR, CER, TDK 2.2µ, 100V
1 C5 C4532X7R3A103K CAPACITOR, CER, TDK 0.01µ, 1000V
2 C6,C15 C3216X7R2E104K CAPACITOR, CER, TDK 0.1µ, 250V
1 C7 C4532X7R1E156M CAPACITOR, CER, TDK 15µ, 25V
1 C8 C2012X7R2A103K CAPACITOR, CER, TDK 0.01µ, 100V
3 C9,C30,C33 C2012X7R2A102K CAPACITOR, CER, TDK 1000p, 100V
4 C10,C14,C28, C31 C2012X7R1H104K CAPACITOR, CER, TDK 0.1µ, 50V
2 C11, C12 C2012X7R1H473K CAPACITOR, CER, TDK 0.047µ, 50V
2 C13,C18 C1206C104K5RAC CAPACITOR, CER, KEMET 0.1µ, 50V
3 C16, C17, C29 C0805C471J5GAC CAPACITOR, CER, KEMET 470p, 50V
2 C19,C20 T520D337M006AS4350 CAPACITOR,TANT,KEMET 330µ, 6.3V
3 C21,C22,C23 C4532X7S0G686M CAPACITOR, CER, TDK 68µ, 4V
C24, C25 OPEN NOT USED
1 C26 C0805C101J5GAC CAPACITOR, CER, KEMET 100p, 50V
1 C27 C1206C333K5RAC CAPACITOR, CER, KEMET 0.033µ, 50V
1 C32 C0805C330J5GAC CAPACITOR, CER, KEMET 33p, 50V
7 D1- D7 CMPD2838 DIODE, SIGNAL, CENTRAL
1 D8 CMPD7000 DIODE, SIGNAL, CENTRAL
1 D9 CMR1U-02 DIODE, 200V, CENTRAL
1 L1 SLF10145T-5R6M3R2 INPUT CHOKE, TDK 5.6µH, 3.5A
1 L2 B0358-C CHOKE with AUX, 2µH, 33A
COILCRAFT
1 Q1 SI7846DP N-FET, SILICONIX 150V, 50m
1 Q2 ZVP2120GTA P-FET, ZETEX 200V, 20
4 Q3 - Q6 SI7866DP FET, SILICONIX 20V, 3m
4 R1, R22, R24, R28 CRCW120610R0F RESISTOR 10Ω
R2, R13, R25 OPEN NOT USED
2 R3, R4 CRCW120615R0F RESISTOR 15Ω
1 R5 CRCW12062000F RESISTOR 200Ω
1 R6 CRCW120649R9F RESISTOR 49.9Ω
1 R7 CRCW12061003F RESISTOR 100kΩ
1 R8 CRCW12063831F RESISTOR 3.83kΩ
2 R9, R15 CRCW12061001F RESISTOR 1kΩ
1 R10 CRCW12062212F RESISTOR 22.1kΩ
1 R11 CRCW12063921F RESISTOR 3.92kΩ
1 R12 CRCW12061652F RESISTOR 16.5kΩ
5 R14,R18,R19,R29,R3 CRCW12061002F RESISTOR 10kΩ
3,R35
2 R16, R17 CRCW12065R60F RESISTOR 5.6Ω
2 R20, R21 CRCW2512100J RESISTOR 10Ω, 1W
1 R23 CRCW12061000F RESISTOR 100Ω
1 R26 CRCW12062492F RESISTOR 24.9kΩ
1 R27 CRCW12061502F RESISTOR 15kΩ
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PCB Layouts www.ti.com
Table 1. Bill of Materials (BOM) (continued)
QTY DESIGNATOR PART NUMBER DESCRIPTION VALUE
3 R30, R31, R34 CRCW12064991F RESISTOR 4.99kΩ
1 R32 CRCW12062002F RESISTOR 20kΩ
1 T1 P8208T CURRENT XFR, PULSE ENG 100:01
1 T2 B0357-B POWER XFR, COILCRAFT 12:02
1 U1 LM5026 CONTROLLER, Texas
Instruments
1 U2 MOCD207M OPTO-COUPLER, QT OPTO
1 U3 LM6132 OPAMP, Texas Instruments
1 U4 LM4041 REFERENCE, Texas
Instruments
13 PCB Layouts
Figure 10.
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www.ti.com PCB Layouts
Figure 11.
Figure 12.
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PCB Layouts www.ti.com
Figure 13.
Figure 14.
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