National Semiconductor Application Note 1345 Robert Bell December 2004 Introduction presently finding extensive use in medium level power converters in the 50 to 200W range. 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 operation at rated load at < 40C 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 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 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 voltage. 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. LM5025A Evaluation Board LM5025A Evaluation Board AN-1345 (c) 2004 National Semiconductor Corporation AN201276 www.national.com AN-1345 Theory of Operation (Continued) 20127601 Simplified Active Clamp Forward Converter 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 condition is an interaction with the UUT undervoltage lockout, the cabling impedance and the inrush current Powering and Loading Considerations When applying power to the LM5025A evaluation board certain precautions need to be followed. A failure or misconnection can present itself in a very alarming manner. Loading Proper Connections An appropriate electronic load with specified operation down to 3.0V minimum is desirable. The resistance of a maximum load is 0.11. You need thick cables! Consult a wire chart if needed. 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. Be careful!! The high temperatures reached by even the most adequately rated resistors may burn you or melt your benchtop. When operated at low input voltages the UUT can draw up to 3.5A of current at full load. The maximum rated output current 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 voltage 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. Air Flow Source Power Full rated power should never be attempted without providing the specified 200 CFM of air flow over the evaluation board. This can be provided by a stand-alone fan. 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 readout 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 impedance to the UUT. Insufficient cabling or a high impedance www.national.com 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 2 affect the efficiency adversely. Few parameters can be incorrect in a switching power supply without creating losses and potentially damaging heat. (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 Over Current Protection The evaluation board is configured with delayed hiccup overcurrent 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. 20127602 Typical Evaluation Setup 3 www.national.com AN-1345 Powering Up AN-1345 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 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 output voltage droop and overshoot during the sudden change in output current shown by the lower trace. 20127609 Conditions: Input Voltage = 48VDC Output Current = 30A Bandwidth Limit = 25MHz Trace 1: Output Ripple Voltage Volts/div = 50mV Horizontal Resolution = 2s/div FIGURE 3. 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 voltages. 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 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. 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 20127604 Trace 2: Output Current, Amps/div = 10.0A Conditions: Input Voltage = 38VDC Output Current = 25A Horizontal Resolution = 1s/div Trace 1: Q1 drain voltage Volts/div = 20V Horizontal Resolution = 1s/div FIGURE 2. www.national.com FIGURE 4. 4 AN-1345 Performance Characteristics (Continued) 20127606 Conditions: Input Voltage = 48VDC Output Current = 5A Synchronous rectifier, Q3 gate Volts/div = 5V Trace 1: Synchronous rectifier, Q3 gate Volts/div = 5V 20127605 Conditions: Input Voltage = 78VDC Output Current = 25A Trace 2: Synchronous rectifier, Q5 gate Volts/div = 5V Trace 1: Q1 drain voltage Volts/div = 20V Horizontal Resolution = 1s/div Horizontal Resolution = 1s/div FIGURE 5. FIGURE 6. 5 www.national.com www.national.com 6 Application Circuit Application Circuit: Input 36 to 78V, Output 3.3V, 30A 20127603 AN-1345 The Bill of Materials is shown below and includes the manufacturer and part number. The layers of the printed circuit board are shown in top down order. View is from the top Bill of Materials PART NUMBER DESCRIPTION C1-C4 DESIGNATOR QTY 4 C4532X7R2A225M CAPACITOR, CER, TDK 2.2u, 100V C5 1 C4532X7R3A103K CAPACITOR, CER, TDK 0.01, 1000V C6 1 C0805C221J5GAC CAPACITOR, CER, KEMET 220p, 50V C7 1 C2012X7R1E224K CAPACITOR, CER, TDK 0.22, 25V C8,C16 2 C3216X7R2E104K CAPACITOR, CER, TDK 0.1, 250V C9 1 C4532X7R1E156M CAPACITOR, CER, TDK 15, 25V C10,C17,C18, C31 4 C0805C471J5GAC CAPACITOR, CER, KEMET 470p, 50V C11 1 C2012X7R2A103K CAPACITOR, CER, TDK 0.01, 100V C12,C15,C30, C33 4 C2012X7R1H104K CAPACITOR, CER, TDK 0.1, 50V C13 1 C2012X7R2A223K CAPACITOR, CER, TDK 0.022, 100V C14 1 C3216X7R1H334K CAPACITOR, CER, TDK 0.33, 50V C19,C20 2 C1206C104K5RAC CAPACITOR, CER, KEMET 0.1, 50V C21,C22 2 T520D337M006AS4350 CAPACITOR,TANT, KEMET 330, 6.3V C23,C24,C25 3 C4532X7S0G686M CAPACITOR, CER, TDK 68, 4V 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 L2 1 B0358-C CHOKE with AUX, COILCRAFT 2H, 33A Q1 1 SI7846DP N-FET, SILICONIX 150V, 50m Q2 1 IRF6217 P-FET, IR 150V, 2.4 20V, 3m C26 VALUE 5.6H, 3.5A Q3 - Q6 4 SI7866DP FET, SILICONIX 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 7 www.national.com AN-1345 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. Layout and Bill of Materials AN-1345 Layout and Bill of Materials Bill of Materials DESIGNATOR (Continued) (Continued) PART NUMBER DESCRIPTION R4 QTY 1 CRCW120615R0F RESISTOR VALUE 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 8.06K 15 R11 1 CRCW12068061F RESISTOR 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 www.national.com 8 AN-1345 PCB Layouts 20127610 9 www.national.com AN-1345 PCB Layouts (Continued) 20127611 www.national.com 10 AN-1345 PCB Layouts (Continued) 20127612 11 www.national.com AN-1345 PCB Layouts (Continued) 20127613 www.national.com 12 AN-1345 PCB Layouts (Continued) 20127614 13 www.national.com LM5025A Evaluation Board PCB Layouts (Continued) 20127615 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. 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