National Semiconductor Application Note 1387 Robert Bell September 2005 Introduction 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. 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 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 presently finding extensive use in medium level power converters in the 50 to 200W range. 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. LM5026 Evaluation Board LM5026 Evaluation Board AN-1387 (c) 2005 National Semiconductor Corporation AN201546 www.national.com AN-1387 Theory of Operation (Continued) 20154601 Schematic Powering and Loading Considerations evaluation board undervoltage lockout, the cabling impedance and the inrush current. When applying power to the LM5026 evaluation board certain precautions need to be followed. A mis-connection can damage the assembly. Loading An appropriate electronic load, with specified operation down to 3.0V minimum, is desirable. The resistance of a maximum load is 0.11. 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. 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. 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. Source Power Powering Up 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 www.national.com 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 2 Over Current Protection (Continued) 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. 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. 20154602 Typical Evaluation Setup 3 www.national.com AN-1387 Powering Up AN-1387 Performance Characteristics 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 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. 20154605 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 andFigure 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. 20154604 Conditions: Input Voltage = 48VDC Output Current = 5A Trace 1: Output Voltage Volts/div = 1V Horizontal Resolution = 1msec/div FIGURE 1. 20154606 Conditions: Input Voltage = 38VDC Output Current = 25A Trace 1: Q1 drain voltage Volts/div = 20V Horizontal Resolution = 1s/div 20154615 Conditions: Input Voltage = 48VDC Output Current = 5A to 25A FIGURE 4. Trace 1: Output Voltage Volts/div = 0.5V Trace 2: Output Current, Amps/div = 5A Horizontal Resolution = 1msec/div FIGURE 2. www.national.com 4 AN-1387 Performance Characteristics (Continued) 20154608 Conditions: Input Voltage = 48VDC Output Current = 5A Synchronous rectifier, Q3 gate Volts/div = 5V Trace 1: Synchronous rectifier, Q3 gate Volts/div = 5V 20154607 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 20154603 AN-1387 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 TABLE 1. Bill of Materials DESIGNATOR QTY PART NUMBER DESCRIPTION VALUE C1-C4 4 C4532X7R2A225M CAPACITOR, CER, TDK 2.2, 100V C5 1 C4532X7R3A103K CAPACITOR, CER, TDK 0.01, 1000V C6,C15 2 C3216X7R2E104K CAPACITOR, CER, TDK 0.1, 250V C7 1 C4532X7R1E156M CAPACITOR, CER, TDK 15, 25V C8 1 C2012X7R2A103K CAPACITOR, CER, TDK 0.01, 100V 1000p, 100V C9,C30,C33 3 C2012X7R2A102K CAPACITOR, CER, TDK C10,C14,C28, C31 4 C2012X7R1H104K CAPACITOR, CER, TDK 0.1, 50V C11, C12 2 C2012X7R1H473K CAPACITOR, CER, TDK 0.047, 50V C13,C18 2 C1206C104K5RAC CAPACITOR, CER, KEMET 0.1, 50V C16, C17, C29 3 C0805C471J5GAC CAPACITOR, CER, KEMET 470p, 50V C19,C20 2 T520D337M006AS4350 CAPACITOR,TANT,KEMET 330, 6.3V C21,C22,C23 3 C4532X7S0G686M CAPACITOR, CER, TDK 68, 4V OPEN NOT USED C26 1 C0805C101J5GAC CAPACITOR, CER, KEMET 100p, 50V C27 1 C1206C333K5RAC CAPACITOR, CER, KEMET 0.033, 50V 33p, 50V C24, C25 C32 1 C0805C330J5GAC CAPACITOR, CER, KEMET 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 L2 1 B0358-C CHOKE with AUX, COILCRAFT 2H, 33A Q1 1 SI7846DP N-FET, SILICONIX 150V, 50m 5.6H, 3.5A Q2 1 ZVP2120GTA P-FET, ZETEX 200V, 20 Q3 - Q6 4 SI7866DP FET, SILICONIX 20V, 3m R1, R22, R24, R28 4 CRCW120610R0F RESISTOR 10 OPEN NOT USED R3, R4 2 CRCW120615R0F RESISTOR 15 R5 1 CRCW12062000F RESISTOR 200 R6 1 CRCW120649R9F RESISTOR 49.9 R7 1 CRCW12061003F RESISTOR 100k R8 1 CRCW12063831F RESISTOR 3.83k R9, R15 2 CRCW12061001F RESISTOR 1k R2, R13, R25 R10 1 CRCW12062212F RESISTOR 22.1k R11 1 CRCW12063921F RESISTOR 3.92k R12 1 CRCW12061652F RESISTOR 16.5k R14,R18,R19,R29,R33,R35 5 CRCW12061002F RESISTOR 10k R16, R17 2 CRCW12065R60F RESISTOR 5.6 R20, R21 2 CRCW2512100J RESISTOR 10, 1W R23 1 CRCW12061000F RESISTOR 100 R26 1 CRCW12062492F RESISTOR 24.9k R27 1 CRCW12061502F RESISTOR 15k R30, R31, R34 3 CRCW12064991F RESISTOR 4.99k R32 1 CRCW12062002F RESISTOR 20k T1 1 P8208T CURRENT XFR, PULSE ENG 100:01 T2 1 B0357-B POWER XFR, COILCRAFT 12:02 7 www.national.com AN-1387 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-1387 Layout and Bill of Materials (Continued) TABLE 1. Bill of Materials (Continued) DESIGNATOR QTY PART NUMBER DESCRIPTION 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 www.national.com 8 VALUE AN-1387 PCB Layouts 20154609 9 www.national.com AN-1387 PCB Layouts (Continued) 20154610 www.national.com 10 AN-1387 PCB Layouts (Continued) 20154611 11 www.national.com AN-1387 PCB Layouts (Continued) 20154612 www.national.com 12 AN-1387 PCB Layouts (Continued) 20154613 13 www.national.com LM5026 Evaluation Board PCB Layouts (Continued) 20154614 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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