National Semiconductor Application Note 2111 Ajay Hari February 23, 2011 Introduction Theory of Operation The LM5045 evaluation board is designed to provide the design engineer with a fully functional power converter based on the full-bridge topology to evaluate the LM5045 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. Power converters based on the full-bridge topology offer highefficiency and good power handling capability up to 500W. Figure 1 illustrates the circuit arrangement for the full-bridge topology. The switches, in the diagonal, Q1,Q3 and Q2,Q4 are turned alternatively with a pulse width determined by the input and output voltages and the transformer turns ratio. Each diagonal (Q1 and Q3 or Q2 and Q4), when turned ON, applies input voltage across the primary of the transformer. The resulting secondary voltage is then rectified and filtered with an LC filter to provide a smoothened output voltage. In a full-bridge 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 single-ended topologies such as a forward converter. Further, in a half-bridge topology, during power transfer when one of the primary switches is active, the voltage across the primary of the power transformer is 1/2 the input voltage (VIN) compared to a full VIN in a full-bridge topology. Therefore, for a given power, the primary current will be half as much for the full-bridge as compared to the half-bridge. The reduced primary current enables higher efficiency as compared to a halfbridge at high load currents. LM5045 Evaluation Board LM5045 Evaluation Board 30146201 Simplified Full-Bridge Converter AN-2111 © 2011 National Semiconductor Corporation 301462 www.national.com AN-2111 The secondary side employs synchronous rectification scheme, which is controlled by the LM5045. In addition to the basic soft-start already described, the LM5045 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. Once the soft-start is finished, the synchronous rectifiers are engaged with a nonoverlap time programmed by the RD1 and RD2 resistors. 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 LM5045 evaluation board employs peak current mode control and a standard “type II” network is used for the compensator. will give inaccurate measurements. This is especially true for accurate efficiency measurements. 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 (72V) the input current will be approximately 1.5A. Therefore, to fully test the LM5045 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. Powering and Loading Considerations 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 that there is sufficient cooling provided for the load. When applying power to the LM5045 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 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. 30146202 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. Over Current Protection 30146204 Conditions: Input Voltage = 48V Output Current = 25A Trace 1: Output Voltage Volts/div = 1V Horizontal Resolution = 2.0 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. 100 36V EFFICIENCY (%) 90 48V 80 70 30146205 Conditions: Input Voltage = 48V Output Current = 15A to 22.5A to 15A Upper Trace: Output Voltage Volts/div = 100mV Lower Trace: Output Current = 5A/div Horizontal Resolution = 200 µs/div 72V VOUT = 3.3V FIGURE 3. Transient Response 60 50 5 7 9 11 13 15 17 19 21 23 25 27 29 LOAD CURRENT (A) 30146203 FIGURE 1. Application Board Efficiency 3 www.national.com AN-2111 When applying power to the LM5045 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-2111 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. 30146208 Conditions: Input Voltage = 72V Output Current = 30A Trace 1: SW1 Node (Q2 Drain) Voltage Volts/div = 20V Horizontal Resolution = 1 µs/div 30146206 Conditions: Input Voltage = 48V, Output Current = 30A Trace 1: Output Voltage Volts/div = 50mV Bandwidth Limit = 20MHz Horizontal Resolution = 2 µs/div FIGURE 6. Switch Node Waveforms Figure 7 shows a typical startup of the LM5045 into a 2V prebiased 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 25A load. Figure 5 shows an input voltage represents an input voltage of 48V and Figure 6 represents an input voltage of 72V. 30146207 Conditions: Input Voltage = 48V Output Current = 30A Trace 1: SW1 Node (Q2 Drain) Voltage Volts/div = 20V Horizontal Resolution = 1µs/div 30146209 Conditions: Input Voltage = 48V, Output Pre-Bias = 2V Trace 1 (Channel 4): Output Voltage Volts/div = 1V Trace 2 (Channel 2): Output Current Amps/div = 200mA Trace 3 (Channel 3): SR Gate Voltage Volts/div = 5V FIGURE 5. Switch Node Waveforms FIGURE 7. Soft-Start into 2V Pre-Biased Load www.national.com 4 Application Circuit: Input 36V to 75V, Output 3.3V at 30A 30146210 AN-2111 5 www.national.com AN-2111 Bill of Materials Item Designator Description Part Number 1 AA 2 C1, C2, C3, C4 Printed Circuit Board Ceramic 2.2uF, X7R, 100V, 10%, 1210 GRM32ER72A225KA35L 3 C5, C35 Ceramic 2.2uF, X7R, 16V, 10%, 0805 GRM21BR71C225KA12L 4 C7, C8 Ceramic, 2.2uF, X5R, 25V, 10%, 0805 GRM21BR71E225KA73L 5 C9 Ceramic, 1uF, X7R, 50V, 10%, 0805 GRM21BR71H105KA12L 6 C6 Ceramic 2.2uF 10V X7R 0603 GRM188R71A225KE15D 7 C10, C11 Ceramic, 1uF, X7R, 16V, 10%, 0603 C1608X7R1C105K 8 C12, C15, C21, C32 Ceramic,0.1uF, X7R, 25V, 10%, 0603 06033C104KAT2A 9 C13 Ceramic, X7R,2000V, 2700pF,10% C1808C272KGRACTU 10 C14 Ceramic 0.1uF, 100V, +/-10%, X7R, 0603 GRM188R72A104KA35D 11 C16, C23 Ceramic, C0G/NP0 470pF, 100V, 10%, 1206 12 C17, C39 Cap 330uF, 4V, AL, 4V, 20%, 0.012 Ohm EEF-UE0G331R ESR 13 C18, C19, C20 14 C22 15 C34, C36 Ceramic 1000pF, 25V, +/-5%, C0G/NP0, C1005C0G1E102J 0402, 16 C26, C27 Ceramic1uF, 16V, +/-20%, X7R, 0805 GRM21BR71C105MA01L 17 C28, R20, D4, L3 NU NU 18 C29 Ceramic 47pF, 50V, +/-5%, C0G/NP0, 0402 GRM1555C1H470JZ01 19 C30, C40 Ceramic 100pF, C0G/NP0, 50V, 5%, 0603 C1608C0G1H101J 20 C24 CAP, CERM, 0.056uF, 6.3V, +/-10%, X7R, 0402 C0402C563K9RACTU 21 C25, C31, C37, C33 22 C38 CAP, CERM, 0.47uF, 6.3V, +/-20%, X5R, C1005X5R0J474K 0402 23 D2 Vr = 30V, Io = 1A, Vf = 0.38V 24 D3, D7, D10, D14 25 D5 SMT 5.1V Zener Diode MMSZ5231B 26 D6 SMT 8.2V Zener Diode CMHZ4694 27 D8, D12 Vr = 100V, Io = 1A, Vf = 0.77V, Schottky DFLS1100-7 diode 28 D9, D13 Vr = 40V, Io = 0.2A, Vf = 0.65V, Common CMPSH-3AE Anode 29 D11 SMT 11V Zener Diode CMHZ4698 30 D16 Vr = 30V, Io = 0.2A, Vf = 0.7V, Schottky BAT54WS-7-F 31 D17 Diode, Zener, 4.7V, 250mW, SOD-323 CMDZ4L7 32 L1 Shielded Drum Core, 2.2uH 4.15A, 0.0165 Ohm DR73-2R2-R 33 L2 Shielded Drum Core, 0.08A, 11 Ohm LPS5030-225MLB 34 L3 NU NU 35 L4 Inductor, Shielded E Core, Ferrite, 800nH, 45A, 0.0009 ohm, SMD SER2010-801MLB www.national.com 12061A471KAT2A Ceramic 47uF, X7R, 6.3V, 10% GCM32ER70J476KE19L Ceramic 0.022uF, 16V, +/-10%, X7R, 0402 C1005X7R1C223K CAP, CERM, 0.01uF, 16V, +/-10%, X7R, C1005X7R1C1103K 0402 B130LAW-7-F Vr = 40V, Io = 0.2A, Vf = 0.65V, Common CMPSH-3CE Cathode 6 Designator Description Part Number 36 P1, P3, P5, P6 37 P2 PCB Pin 3104-2-00-34-00-00-08-0 Test Point, SMT, Miniature 38 5015 P4, P7 PCB Pin 3231-2-00-34-00-00-08-0 39 Q1, Q3 NPN, 2A, 45V FCX690BTA 40 Q2 PNP, 0.2A, 40V CMPT3906 41 Q4, Q5, Q10, Q11 42 Q6, Q7, Q8, Q9 43 Q12, Q13 44 R1 RES 10 Ohm 1%, 0.125W, 0805 CRCW080510R0FKEA 45 R2, R28, R33, R34, R35, R36 RES 10K Ohm 1%, 0.063W, 0402 CRCW040210k0FKED 46 R3, R4 RES 5.1K Ohm 5%, 0.125W, 0805 ERJ-6GEYJ512V 47 R5 RES 1K Ohm, 5.1, 0.125W, 0805 CRCW08051K00FKEA 48 R6 RES 100K Ohm,1%, 0.125W, 0805 CRCW0805100KFKEA 49 R7 RES, 2.61k ohm, 1%, 0.063W, 0402 CRCW04022K61FKED 50 R8 RES 20 OHM 1/8W 5% 0805 SMD ERJ-6GEYJ200V 51 R9 RES, 1.58k ohm, 1%, 0.063W, 0402 CRCW04021K58FKED 52 R10, R12 RES, 0 ohm, 5%, 0.063W, 0402 RC0402JR-070RL 53 R11, R17 RES 4.99 Ohm,1%, 0.25W, 1206 CRCW12064R99FNEA 54 R13 RES, 1.69k ohm, 1%, 0.063W, 0402 CRCW04021K69FKED 55 R14 RES 24K, 5%, 0.063W, 0402 CRCW040224k0JNED 56 R15 RES, 30.1k ohm, 1%, 0.063W, 0402 CRCW040230K1FKED 57 R16 RES 20k Ohm,1%, 0.063W, 0402 CRCW040220k0FKED 58 R18 RES, 15.0 ohm, 1%, 0.063W, 0402 CRCW040215R0FKED 59 R19, R31 RES 10.0 ohm, 1%, 0.063W, 0402 CRCW040210R0FKED 60 R21 RES 1.0K OHM 1/16W 5% 0402 SMD CRCW04021K00JNED 61 R22 RES 25.5k ohm,1%, 0.063W, 0402 CRCW040225k5FKED 62 R23 RES 499 ohm, 1%, 0.063W, 0402 CRCW0402499RFKED 63 R24 RES 5.11k ohm, 1%, 0.063W, 0402 CRCW04025k11FKED 64 R25, R26 NU NU 65 R27 RES 47 OHM .25W 5% 0603 SMD CRCW060347R0JNEAHP 66 R32 RES 100 ohm, 1%, 0.063W, 0402 CRCW0402100RFKED 67 R29 RES 15k ohm,1%, 0.063W, 0402 CRCW040215k0FKED 68 R30 RES 1.82k ohm,1%, 0.063W, 0402 CRCW04021k82FKED 69 R37 RES 0.0 ohm, 5%, 0.063W, 0402 CRCW04020000Z0ED 70 D1 RES 0.0 ohm, 5%, 0.063W, 1206 CRCW12060000Z0EA 71 T1 High Frequency Planar Transformer PA0876.003NL 72 T2 SMT Current Sense Transformer PA1005.100NL 73 U1 Full-Bridge PWM Controller LM5045MH 74 U2 Dual 5A Compound Gate Driver with Negative Output Voltage Capability LM5110-1SD 75 U3 Low Input Current, High CTR Photocoupler PS2811-1-M-A 76 U4 RRIO, High Output Current & Unlimited Cap Load Op Amp in SOT23-5 LM8261M5 77 U5 Precision Micro-power Shunt Voltage Reference LM4041BIM3-1.2 78 U6 ISO-Pro Low-Power Dual-Channel Digital Si8420BB-D-IS Isolator 4.5A, 36nC, rDS(on) @ 4.5V =0.004 ohm SI7336ADP-GE3 MOSFET, N-CH, 100V, 9.3A, PQFN 8L 5x6 A IRFH5053TRPBF 0.31A, 0.7nC, rDS(on) @ 4.5V =2.5 Ohm NTZD5110NT1G 7 www.national.com AN-2111 Item AN-2111 PCB Layouts 30146211 Top Side Assembly 30146212 Bottom Side Assembly www.national.com 8 AN-2111 30146213 Layer 1 (Top Side) 30146214 Layer 2 9 www.national.com AN-2111 30146215 Layer 3 30146216 Layer 4 www.national.com 10 AN-2111 30146217 Layer 5 30146218 Layer 6 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