National Semiconductor Application Note 2115 Ajay Hari February 23, 2011 Introduction Theory of Operation The LM5046 evaluation board is designed to provide the design engineer with a fully functional power converter based on the phase-shifted full-bridge topology to evaluate the LM5046 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. The Phase-Shifted Full-Bridge (PSFB) topology is a derivative of the classic full-bridge topology. When tuned appropriately the PSFB topology achieves zero voltage switching (ZVS) of the primary FETs while maintaining constant switching frequency. The ZVS feature is highly desirable as it reduces both the switching losses and EMI emissions. Figure 1 illustrates the circuit arrangement for the PSFB topology. The power transfer mode of the PSFB topology is similar to the hard switching full-bridge i.e., when the FETs in the diagonal of the bridge are turned-on (Q1 & Q3 or Q2 & Q4), it initiates a power transfer cycle. At the end of the power transfer cycle, PWM turns off the switch Q3 or Q4 depending on the phase with a pulse width determined by the input and output voltages and the transformer turns ratio. In the freewheel mode, unlike the classic full-bridge where all the four primary FETs are off, in the PSFB topology the primary of the power transformer is shorted by activating either both the top FETs (Q1 and Q4) or both the bottom FETs (Q2 and Q3) alternatively. In a PSFB 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 singleended topologies. Further, the ZVS of the primary FETs results in low EMI compared to the conventional hard-switching full-bridge topology. LM5046 Evaluation Board LM5046 Evaluation Board 30149701 Simplified Full-Bridge Converter AN-2115 © 2011 National Semiconductor Corporation 301497 www.national.com AN-2115 The secondary side employs synchronous rectification scheme, which is controlled by the LM5046. In addition to the basic soft-start already described, the LM5046 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. 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 LM5046 evaluation board employs peak current mode control and a standard “type II” network is used for the compensator. Source Power Powering and Loading Considerations Loading 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 LM5046 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. 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 LM5046 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 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. 30149702 www.national.com 2 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. 30149704 Over Current Protection Conditions: Input Voltage = 48V Output Current = 25A Trace 1: Output Voltage Volts/div = 1V Horizontal Resolution = 5.0 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. 30149705 100 Conditions: Input Voltage = 48V Output Current = 15A to 22.5A to 15A Upper Trace: Output Voltage Volts/div = 100mV Lower Trace: Output Current = 10A/div Horizontal Resolution = 200 µs/div 36V EFFICIENCY (%) 90 48V 80 70 72V FIGURE 3. Transient Response VOUT = 3.3V 60 50 5 7 9 11 13 15 17 19 21 23 25 27 29 LOAD CURRENT (A) 30149703 FIGURE 1. Application Board Efficiency 3 www.national.com AN-2115 When applying power to the LM5046 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 AN-2115 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. 30149708 Conditions: Input Voltage = 72V Output Current = 30A Trace 1: SW1 Node (Q2 Drain) Voltage Volts/div = 50V Trace 1: SW2 Node (Q3 Drain) Voltage Volts/div = 50V Horizontal Resolution = 1 µs/div 30149706 Conditions: Input Voltage = 48V, Output Current = 30A Trace 1: Output Voltage Volts/div = 20mV Bandwidth Limit = 20MHz Horizontal Resolution = 2µs/div FIGURE 6. 72V Switch Node Waveforms Figure 7 shows a typical startup of the LM5046 evaluation board into a 2V pre-biased 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 30A load. Figure 5 shows an input voltage represents an input voltage of 48V and Figure 6 represents an input voltage of 72V. When one SW node is at the input voltage and the other SW node at the GND, it implies power transfer cycle, i.e., FETs in the diagonal, Q1 and Q3, or Q2 and Q4, are activated. Further, when both the SW nodes are the same potential, i.e., either at the input voltage or at the GND, it implies freewheeling mode. 30149709 Conditions: Input Voltage = 48V, Output Pre-Bias = 2V Trace 1 (Channel 1): Output Voltage Volts/div = 1V Trace 2 (Channel 2): Output Current Amps/div = 200mA Trace 3 (Channel 3): SR Gate Voltage Volts/div = 5V 30149707 Conditions: Input Voltage = 48V Output Current = 30A Trace 1: SW1 Node (Q2 Drain) Voltage Volts/div = 20V Trace 2: SW2 Node (Q3 Drain) Voltage Volts/div = 20V Horizontal Resolution = 1µs/div FIGURE 7. Soft-Start into 2V Pre-Biased Load FIGURE 5. 48V Switch Node Waveforms www.national.com 4 Application Circuit: Input 36V to 75V, Output 3.3V at 30A 30149710 AN-2115 5 www.national.com AN-2115 Bill of Materials Item Designator 1 AA 2 C1, C2, C3, C4 3 Description Manufacturer Printed Circuit Board TBD Part Number Ceramic 2.2uF X7R 100V 10% MuRata 1210 GRM32ER72A225KA35L C35 Ceramic 4.7uF X7R 16V 10% 0805 MuRata GRM21BR71C475KA73L 4 C5 Ceramic 2.2uF X7R 16V 10% 0805 MuRata GRM21BR71C225KA12L 5 C7, C8 Ceramic 2.2uF X5R 25V 10% 0805 TDK GRM21BR71E225KA73L 6 C9 7 C10, C11 8 CAP CERM 1uF X7R 50V 10% MuRata 0805 GRM21BR71H105KA12L Ceramic 1uF X7R 16V 10% 0603 TDK C1608X7R1C105K AVX 06033C104KAT2A C12, C15, C21, C32 Ceramic 0.1uF X7R 25V 10% 0603 9 C13 CAP CERM X7R 2000V 2700pF 10% Kemet C1808C272KGRACTU 10 C14 CAP CERM 0.1uF 100V +/-10% X7R 0603 MuRata GRM188R72A104KA35D 11 C16, C23 Ceramic C0G/NP0 470pF 100V AVX 10% 1206 12061A471KAT2A 12 C17, C39 CAP 330uF 4V AL 4V 20% 0.012 Ohm ESR Panasonic EEF-UE0G331R 13 C18, C19, C20 CAP CERM 47uF X7R 6.3V 10% MuRata GCM32ER70J476KE19L 14 C22 Ceramic 0.022uF 16V +/-10% X7R 0402 TDK C1005X7R1C223K 15 C34, C36 Ceramic 1000pF 25V +/-5% C0G/NP0 0402 TDK C1005C0G1E102J 16 C26, C27 Ceramic 1uF 16V +/-20% X7R MuRata 0805 GRM21BR71C105MA01L 17 C28, R20, D4, L3 NU NU 18 C29 19 C30, C40 20 C24 21 NU Ceramic 47pF 50V +/-5% C0G/ MuRata NP0 0402 GRM1555C1H470JZ01 Ceramic 100pF C0G/NP0 50V TDK 5% 0603 C1608C0G1H101J CAP CERM 0.056uF 6.3V +/-10% X7R 0402 Kemet C0402C563K9RACTU TDK C1005X7R1C1103K C25, C31, C37, C33 CAP CERM 0.01uF 16V +/-10% X7R 0402 22 C38 CAP CERM 0.47uF 6.3V +/-20% X5R 0402 TDK C1005X5R0J474K 23 D1 Vr=100V Ir=150mA Vf=0.7V Schottky Vishay BAT46JFILM 24 D2 Vr=30V Io=1A Vf=0.38V Diodes Inc B130LAW-7-F 25 D3, D7, D10 Vr=40V Io=0.2A Vf=0.65V Common Cathode Central Semiconductor CMPSH-3CE 26 D5 SMT 5.1V Zener Diode Diodes Inc MMSZ5231B 27 D6 SMT 8.2V Zener Diode Central Semiconductor CMHZ4694 28 D8, D12 Vr=100V Io=1A Vf=0.77V Schottky diode Diodes Inc DFLS1100-7 www.national.com 6 Designator 29 D9, D13 30 31 AN-2115 Item Description Manufacturer Part Number Vr=40V Io=0.2A Vf=0.65V Common Anode Central Semiconductor CMPSH-3AE D11 SMT 11V Zener Diode Central Semiconductor CMHZ4698 D16 Vr=30V Io=0.2A Vf=0.7V Schottky Diodes Inc BAT54WS-7-F 32 D17 Zener Diode 4.7V 250mW SOD-323 Central Semiconductor CMDZ4L7 33 L1 Shielded Drum Core 2.2uH 4.15A 0.0165 Ohm Coiltronics DR73-2R2-R 34 L2 Shielded Drum Core 0.08A 11 Ohm Coilcraft Inc LPS5030-225MLB 35 L4 Inductor, Shielded E Core, Ferrite, 800nH 45A 0.0009 Ohm SMD Coilcraft SER2010-801MLB 36 P1, P3, P5, P6 PCB Pin Mill-Max 3104-2-00-34-00-00-08-0 37 P2 Test Point, SMT, Miniature Keystone Electronics 5015 38 P4, P7 PCB Pin Mill-Max 3231-2-00-34-00-00-08-0 39 Q1, Q3 NPN 2A 45V Diodes Inc FCX690BTA 40 Q2 PNP 0.2A 40V Central Semiconductor CMPT3906 41 Q4, Q5, Q10, Q11 4.5A 36nC rDS(on) @ 4.5V =0.004 Ohm Vishay-Siliconix SI7336ADP-GE3 42 Q6, Q7, Q8, Q9 MOSFET N-CH 100V 9.3A PQFN 8L 5x6 A International Rectifier IRFH5053TRPBF 43 44 R1 RES 10 Ohm 1% 0.125W 0805 Vishay-Dale R2, R28, R33, R36 RES 10K Ohm 1% 0.063W 0402 CRCW080510R0FKEA Vishay-Dale CRCW040210K0FKED 45 R3, R4 RES 5.1K Ohm 5% 0.125W 0805 Panasonic ERJ-6GEYJ512V 46 R5 RES 1.0K Ohm 5% 0.125W 0805 Vishay-Dale CRCW08051K00FKEA 47 R6 RES 100K Ohm 1% 0.125W 0805 Vishay-Dale CRCW0805100KFKEA 48 R7 RES 2.61K Ohm1% 0.063W 0402 Vishay-Dale CRCW04022K61KFKED 49 R8 RES 20 Ohm 1/8W 5% 0805 SMD Panasonic ERJ-6GEYJ200V 50 R9 RES 1.58K Ohm, 1% 0.063W 0402 Vishay-Dale CRCW04021K58FKED 51 R10, R12 RES 0 Ohm, 5% 0.063W 0402 Yageo America RC0402JR-070RL 52 R11, R17 RES 4.99 Ohm, 1% 0.25W 1206 Vishay-Dale CRCW12064R99FNEA 53 R13 RES 3.4K Ohm, 1% 0.063W 0402 Vishay-Dale CRCW0402340FKED 54 R14 RES 24K 5% 0.063W 0402 Vishay-Dale CRCW040224K0JNED 55 R15, R16 RES 20K Ohm, 1% 0.063W 0402 Vishay-Dale CRCW040220K0FKED 56 R18 RES 15.0 Ohm 1% 0.063W 0402 Vishay-Dale CRCW040215R0FKED 57 R19, R31 RES 10.0 Ohm, 1% 0.063W 0402 Vishay-Dale CRCW040210R0FKED 58 R21 RES 1.0K Ohm 1/16W 5% 0402 Vishay-Dale SMD CRCW04021K00JNED 7 www.national.com AN-2115 Item Designator Description Manufacturer Part Number 59 R22 RES 25.5K Ohm,1% 0.063W 0402 Vishay-Dale CRCW040225K5FKED 60 R23 RES 499 Ohm, 1% 0.063W 0402 Vishay-Dale CRCW0402499RFKED 61 R24 RES 5.11K Ohm, 1% 0.063W 0402 Vishay-Dale CRCW04025K11FKED 62 R25, R26 NU Vishay-Dale NU 63 R27 RES 47 Ohm .25W 5% 0603 SMD Vishay-Dale CRCW060347R0JNEAHP 64 R32 RES 100 Ohm, 1% 0.063W 0402 Vishay-Dale CRCW0402100RFKED 65 R29 RES 15K Ohm,1% 0.063W 0402 Vishay-Dale CRCW040215K0FKED 66 R30 RES 1.82K Ohm,1% 0.063W 0402 Vishay-Dale CRCW04021K82FKED 67 R37 RES 0.0 Ohm, 5% 0.063W 0402 Vishay-Dale CRCW04020000Z0ED 68 T1 High Frequency Planar Transformer Pulse Engineering PA0876.003NL 69 T2 SMT Current Sense Transformer Pulse Engineering PA1005.100NL 70 U1 Phase Shifted Full-Bridge PWM National Semiconductor Controller LM5046MH 71 U2 Dual 5A Compound Gate Driver National Semiconductor with Negative Output Voltage Capability LM5110-1SD 72 U3 Low Input Current, High CTR Photocoupler PS2811-1-M-A 73 U4 RRIO, High Output Current & National Semiconductor Unlimited Cap Load Op Amp in SOT23-5 LM8261M5 74 U5 Precision Micropower Shunt Voltage Reference National Semiconductor LM4041BIM3-1.2 75 U6 ISOPro Low-Power DualChannel Digital Isolator Silicon Laboratories Inc Si8420BB-D-IS www.national.com 8 NEC AN-2115 PCB Layouts 30149711 Top Side Assembly 30149712 Bottom Side Assembly 9 www.national.com AN-2115 30149713 Layer 1 (Top Side) 30149714 Layer 2 www.national.com 10 AN-2115 30149715 Layer 3 30149716 Layer 4 11 www.national.com AN-2115 30149717 Layer 5 30149718 Layer 6 (Bottom Side) www.national.com 12 AN-2115 Notes 13 www.national.com LM5046 Evaluation Board Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Design Support Amplifiers www.national.com/amplifiers 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