LM5027A

LM5027A Evaluation Board LM5027A Evaluation Board National Semiconductor Application Note 2067 Terry Allinder June 15, 2010 Introduction The LM5027A 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 LM5027A 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: 36 to 78V (100V peak) Output Voltage: 3.3V Output Current: 0 to 30A Measured Efficiency: 90.5% @ 30A, 92.5% @ 15A Frequency of Operation: 250 kHz Board Size: 2.3 X 1.45 x 0.5 inches Load Regulation: 1% Line Regulation: 0.1% Line UVLO, Hiccup Current Limit A 70% Maximum Duty Cycle The printed circuit board consists of 6 layers of 2 ounce copper on FR4 material with a total thickness of 0.050 inches. The unit is designed for continuous operation at rated load at 1.2V), refer to Figure 7. On start-up the primary side soft-start begins and the output voltage rises from the pre-bias voltage level to 3.3 V, refer to Figure 6. At the end of the primary side soft-start period the controller will be at maximum duty cycle and the output voltage will overshoot until the feedback error amplifier has a chance to respond and reduce the output voltage to the regulation set point. Secondary Side Soft-Start Reset When input power is supplied to the LM5027A Evaluation Board the LM5027A’s internal VCC Regulator turns-on providing power to the VCC pin, the primary side soft-start voltage increases, and the output drives are enabled. When the drive outputs are enables the voltage on the transformer secondary increases, the Secondary Bias rises supplying voltage to the reference and error amplifier, refer to Figure 8. During this time FET Q1 is turned-on holding the reference voltage at the positive input to the error amplifier low (zero volts). When the voltage on the secondary bias capacitor (CBIAS) rises above the Zener diode> 3.6 V, the Secondary Bias Power Good (the collector of Q2) goes high. This turns-off FET Q1 allowing the secondary soft-start capacitor to charge up. This solution of reseting the soft-start capacitor to zero (0 V) on start-up works for pre-bias loads as well as loads that do not need to start into a pre-biased condition. This allows for a monotonic start-up under both operating modes. 30126308 FIGURE 6. Pre-bias Secondary Side Soft-Start www.national.com 8 AN-2067 30126309 FIGURE 7. VREF with Pre-Bias Load 9 www.national.com AN-2067 30126310 FIGURE 8. Pre-Bias Schematic Pre-Bias Load-Synchronous Forward MOSFET Enabled The self driven synchronous rectification topology has an issues starting into a pre-bias load. When a pre-bias load is connected across the power supply output, refer to Figure 9, the pre-bias source will conduct current through the output inductor and the self driven gate drive resistors R1 and R2. If the pre-bias voltage is greater than the Vgs of the synchronous MOSFET (M1), the MOSFET will be turned-on sinking current into the power supply. www.national.com 10 AN-2067 30126311 FIGURE 9. Self Driven with Pre-Bias Load Synchronous Forward MOSFET Enabled For the LM5027A Evaluation board we used the Secondary Bias Power Good signal as a flag to indicate that the primary sides MOSFETs are switching providing power to the secondary of the transformer T1. When the flag goes high this indicates that it is time to turn-on the forward conducting MOSFET M1. The Secondary Bias Power Good signal drives the base of an NPN transistor (Q3), refer to Figure 10. The NPN transistor is configured as a Cascod amplifier; when it is turned-on, the voltage on the secondary of the transformer T1 drives the gate of the synchronous MOSFET, M1. The MOSFET gate drive voltage is: V-GATE_DRIVE_M1 = V_Secondary_Bias_Power_GoodVBE_Q3 An NPN transistor needs to be selected so that the transistors collector to emitter voltage under the worst case operating condition does not exceed it’s VCE ratings, and that the collector current (Icc) can handle the maximum peak current to drive the gate of MOSFET M1. For the LM5027A Evaluation board the transistor is a 30 V, 1.5 ampere transistor. The maximum VCE is: Where: Vin = 100 V under transient conditions n is the transformer turns ratio = 6 A diode D1 is connected from the collector to the emitter of Q3 to handle any voltage spikes as a result of circuit inductance. Without this diode inductive voltage spike may damage the Cascod amplifier Q3. An NPN transistor was use instead of an N-Channel MOSFET because the Vgs drop, typically 4 to 5 volts; this would reduce the gates drive voltage to M1. Under minimum input line conditions M1 may not be fully turned-on and there would be an increase in the I2 x RDS(ON) losses. Figure 11 shows the start-up waveforms for the Evaluation board. After the input power is supplied to the Evaluation board the secondary bias voltage rises, when the secondary bias is greater than 3.6 V, the Secondary Power Good output goes high. This turns-on M1 and enables the secondary side soft-start circuit allowing the output voltage to increase after Vout > Vpre-bias. 11 www.national.com AN-2067 30126313 FIGURE 10. Isolated Synchronous MOSFET www.national.com 12 AN-2067 30126314 FIGURE 11. Pre-Bias Load Waveforms An alternative to using the circuit in Figure 10 is shown in Figure 12; an additional winding can be added to the power transformer which can be used to drive the Forward Synchronous Rectifier MOSFET (M1). This is a simple solution and should not add a lot of complexity to the transformer design. 13 www.national.com AN-2067 30126315 FIGURE 12. Isolated Synchronous MOSFET Drive Using a Transformer Pre-Bias Load Test Set-Up For the Pre-bias start-up test, the circuit in Figure 13 was used. An external bias supply, through a 1.0 ohm resistor, was connected across the output terminals of the Evaluation Board. Pre-Bias Load Start-Up Requirements The Evaluation board Pre-Bias start-up requirements are: During converter start-up the output shall rise monotonically and not sink current (into the converter) of more than 50 mA . www.national.com 14 AN-2067 30126316 FIGURE 13. Isolated Synchronous MOSFET Drive Using a Transformer Evaluation Board Results Figure 14 shows the output of the Evaluation Board starting with a pre-bias voltage of 2.7 V. Under these conditions the output voltage starts at 2.7 V and then increases monotonically to 3.3 V. The current into the Evaluation board (sinking) is less than 50 mA. When the output voltage rise above the pre-bias voltage there is approximately 400 mA of current out of (sourced) the Evaluation Board to charge the external 220 µF capacitor. After the external capacitor is charge to 3.3 V the current out of the power supply drop to approximately 50 mA. 30126317 FIGURE 14. Pre-Bias StartUp 15 www.national.com AN-2067 www.national.com 30126318 16 Application Schematic: Input 36-76, Voutput 6.3A, 30A AN-2067 Performance Characteristics TURN-ON WAVEFORMS When applying power to the LM5027A 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 15 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 16 shows the transient response for a load of change from 2A to 25A. The lower trace shows minimal output voltage droop and overshoot during the sudden change in output current shown by the upper trace. 30126321 Conditions: Input Voltage = 48VDC Output Current = 30A Bandwidth Limit = 25 MHz Trace 1: Output Voltage Volts/div = 50 mV Horizontal Resolution = 2 µs/div FIGURE 17. Figure 17 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 18 and Figure 19 show the drain voltage of Q1 with a 25A load. Figure 18 represents an input voltage of 38V and Figure 19 represents an input voltage of 78V. Figure 20 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 30126319 Conditions: Input Voltage = 48VDC Output Current = 5A Trace 1: Output Voltage Volts/div = 1.0V Horizontal Resolution =1 ms/div FIGURE 15. 30126322 30126320 Conditions: Input Voltage = 48VDC Output Current = 2A to 25A Trace1: Output Voltage Volts/div = 0.2V Trace 2: Output Current Amps/Div = 5.0 A Horizontal Resolution = 1 ms/div Conditions: Input Voltage = 38VDC Output Current = 25A Trace 1: Q1 Drain Voltage volts/Div = 20V Horizontal Resolution = 1 µs/div FIGURE 18. FIGURE 16. 17 www.national.com AN-2067 30126323 30126324 Conditions: Input Voltage = 78VDC Trace 1: Q1 Drain Voltage Volts/Div = 20V Horizontal Resolution = 1 µs/div FIGURE 19. Conditions: Input Voltage = 48VDC Output Current = 5A Trace 3: (gate) Synchronous Rectifier, Q3/Q4 Volts/Div = 2V Trace 2: (gate) Synchronous Rectifier, Q5/Q6 Volts/Div = 2V Horizontal Resolution = 1 µs/div FIGURE 20. 30126336 FIGURE 21. Efficiency www.national.com 18 AN-2067 Bill of Materials ITEM C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C D D D D D D D D D J 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 71 35 36 1 2 3 4 5 6 7 8 9 1 PART NUMBER C4532X7R2A225M C4532X7R2A225M C4532X7R2A225M C4532X7R2A225M APXE4R0ARA681MH80G C1210C476M8PACTU C1210C476M8PACTU C0603C471J5GAC C0603C103K3RAC C0603C223K3RAC C0603C473K3RAC C1608X7R1H104K C0603C101J5GAC C0603C104K3RAC C3216X7R2E104K C1608X7R1H104K C1210C476M8PACTU C1210C476M8PACTU C0603C221J3GAC OPEN C3216X7R2E104K C1608X7R1H104K C0603C103K3RAC C0603C473K3RAC C0603C473K3RAC C4532X7R3D222K GRM188R61E105KA12D C0603C224K3RAC C0603C102K3RAC C0603C102K3RAC C0805C471J5GAC C0805C471F5GAC C2012X7R2A332K OPEN C4532X7R1E156M C0603C102K3RAC GRM188R61E105KA12D ZHCS350 ZHCS350 ZHCS350 ZHCS350 ZHCS350 CMD2836 ZHCS350 ZHCS350 ZHCS350 3104-2-00-01-00-00-08-0 CAPACITOR, CER, TDK CAPACITOR, CER, KEMET CAPACITOR, CER, MURATA DIODE, SIGNAL, ZETEX DIODE, SIGNAL, ZETEX DIODE, SIGNAL, ZETEX DIODE, SIGNAL, ZETEX DIODE, SIGNAL, ZETEX DIODE, DUAL SIGNAL, CENTRAL DIODE, SIGNAL, ZETEX DIODE, SIGNAL, ZETEX DIODE, SIGNAL, ZETEX PIN, BRICK, 0.040D, MILL-MAX 15µ, 25V 1000p, 25V 1.0u, 25V 40V, 500mA 40V, 500mA 40V, 500mA 40V, 500mA 40V, 500mA 120V, 200mA 40V, 500mA 40V, 500mA 40V, 500mA MOUNT ON SOLDER SIDE OF PCB CAPACITOR, CER, TDK CAPACITOR, CER, KEMET CAPACITOR, CER, KEMET CAPACITOR, CER, KEMET CAPACITOR, CER, KEMET CAPACITOR, CER, TDK CAPACITOR, CER, MURATA CAPACITOR, CER, TDK CAPACITOR, CER, KEMET CAPACITOR, CER, KEMET CAPACITOR, CER, KEMET CAPACITOR, CER, KEMET CAPACITOR, CER, TDK 0.1µ, 250V 0.1µ, 25V 0.01µ, 25V 0.047µ, 25V 0.047µ, 25V 2200p, 2000V 1.0µ, 25V 0.22µ, 25V 1000p, 25V 1000p, 25V 470p, 50V 470p, 50V 3300p, 100V DESCRIPTION CAPACITOR, CER, TDK CAPACITOR, CER, TDK CAPACITOR, CER, TDK CAPACITOR, CER, TDK CAPACITOR, CER, United Chemi-Con CAPACITOR,CER,KEMET CAPACITOR,CER,KEMET CAPACITOR, CER, KEMET CAPACITOR, CER, KEMET CAPACITOR, CER, KEMET CAPACITOR, CER, KEMET CAPACITOR, CER, TDK CAPACITOR, CER, KEMET CAPACITOR, CER, KEMET CAPACITOR, CER, TDK CAPACITOR, CER, TDK CAPACITOR, CER, TDK CAPACITOR, CER, TDK CAPACITOR, CER, KEMET VALUE 2.2µ, 100V 2.2µ, 100V 2.2µ, 100V 2.2v, 100V 680µ, 4V 47µ, 10V 47µ, 10V 470p, 50V 0.01µ, 25V 0.022µ, 25V 0.047µ, 25V 0.1µ, 50V 100p, 50V 0.1µ, 25V 0.1µ, 250V 0.1µ, 50V 47µ, 10V 47µ, 10V 220p, 25V 19 www.national.com AN-2067 ITEM J J J J J J L L L Q Q Q Q Q Q Q Q Q Q R R R R R R R R R R R R R R R R R R R R R R R R R R 2 4 5 6 8 9 1 2 3 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 PART NUMBER 3104-2-00-01-00-00-08-0 3104-2-00-01-00-00-08-0 3231-2-00-01-00-00-08-0 3104-2-00-01-00-00-08-0 3104-2-00-01-00-00-08-0 3231-2-00-01-00-00-08-0 SRU1048-6R8Y 7443556130 SDR0503-332JL SI7846DP SI3475 SI7866DP SI7866DP SI7866DP SI7866DP MMBT2907A QSX6 2N7002VA MMBT2907A CRCW120610R0F CRCW08059093F CRCW06032002F CRCW06034992F CRCW06034991F CRCW08059093F CRCW06031001F CRCW06036191F CRCW06035R60F CRCW060352302F CRCW06032002F CRCW06031001F CRCW06035R60F CRCW120649R9F CRCW06036R34F OPEN CRCW06032200F CRCW06031002F CRCW06034R70F SHORT (0 Ohms) CRCW06031001F CRCW06032000F CRCW06031002F CRCW06031502F CRCW06032492F CRCW060310R0F 20 DESCRIPTION PIN, BRICK, 0.040D, MILL-MAX PIN, BRICK, 0.040D, MILL-MAX PIN, BRICK, 0.080D, MILL-MAX PIN, BRICK, 0.040D, MILL-MAX PIN, BRICK, 0.040D, MILL-MAX PIN, BRICK, 0.080D, MILL-MAX INPUT CHOKE, Bourns CHOKE, WURTH CHOKE, Bourns N-FET, SILICONIX P-FET, IR FET, SILICONIX FET, SILICONIX FET, SILICONIX FET, SILICONIX Bipolar, PNP, 60V, 600mA Bipolar, NPN, 30V, 1.5A FET, N_Channel, Fairchild Bipolar, PNP, 60V, 600mA RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR, 0 OHMS RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR VALUE MOUNT ON SOLDER SIDE OF PCB MOUNT ON SOLDER SIDE OF PCB MOUNT ON SOLDER SIDE OF PCB MOUNT ON SOLDER SIDE OF PCB MOUNT ON SOLDER SIDE OF PCB MOUNT ON SOLDER SIDE OF PCB 6.8uH, 4.8Arms 1.2µH, 37A 3.3mH, 0.045 A 150V, 50m 200V, 1.6 20V, 3m 20V, 3m 20V, 3m 20V, 3m ROHM 60V 280mA 10 90.9k 20k 49.9k 4.99k 90.9K 1K 6.19K 5.6 52.3K 20K 1K 5.6 49.9 6.34 220 10k 4.7 0 ohms 1K 200 10k 15k 24.9k 10 www.national.com AN-2067 ITEM R R R R R R R R R R R R R R R R R T T T U U U U U Z 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 T1 1 2 3 1 2 3 4 5 2 PART NUMBER CRCW060310R0F CRCW06031001F CRCW06032002F CRCW06031002F CRCW06034990F OPEN SHORT (0 Ohms) CRCW1218110ROFKEK CRCW1218110ROFKEK CRCW06031001F CRCW06033011F CRCW06034990F CRCW06034702F CRCW06034702F CRCW06034702F CRCW06031002F NTCG164BH103H HA4000-Al DA2319-ALB P8208T, Pulse LM5027AMH PS2811-1M LM8261M5 LM4040CEM3-4.1 LM4041CEM3-1.2 MM5Z3V6 DESCRIPTION RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR, 0 OHMS RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR RESISTOR NTC, 10k @25°C, 1k@100°C, TDK POWER XFMR W/AUX, COILCRAFT Gate Drive, Coilcraft CURRENT XFR, PULSE ENG CONTROLLER, NATIONAL SEMI OPTO-COUPLER, NEC OPAMP, NATIONAL SEMI REFERENCE, NATIONAL SEMI REFERENCE, NATIONAL SEMI DIODE, ZENER 3.6V VALUE 10 1k 20.0k 10.0k 499 0 ohms 10, 1W 10, 1W 1k 3.01k 499 47k 47k 47k 10k 10k 12:2 100:1 Fairchild 21 www.national.com AN-2067 Printed Circuit Layout 30126326 Tassy 30126327 Bottom Layer www.national.com 22 AN-2067 30126328 Bottom Silk Layer 30126329 Mid 1 Layer 23 www.national.com AN-2067 30126330 Mid 2 Layer 30126331 Mid 3 Layer www.national.com 24 AN-2067 30126332 Mid 4 Layer 30126333 TASSY 25 www.national.com AN-2067 30126334 Top Layer 30126335 Top Silk Layer www.national.com 26 AN-2067 Notes 27 www.national.com LM5027A Evaluation Board Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Amplifiers Audio Clock and Timing Data 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