National Semiconductor Application Note 2025 Ajay Hari February 17, 2010 Introduction Theory of Operation The LM5039 evaluation board is designed to provide the design engineer with a fully functional power converter based on the half-bridge topology to evaluate the LM5039 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: 89% @ 30A, 92% @ 15A • Frequency of Operation: 400 kHz • Board Size: 2.28 x 1.45x 0.5 inches • Load Regulation: 0.2% • Line Regulation 0.1% • Line UVLO (31V/30V on/off) • Constant 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 half-bridge topology offer high-efficiency and good power handling capability up to 500W. A simplified half bridge circuit is illustrated below. The capacitors C1 and C2, which form one-half of the bridge, are arranged in series such that the mid-point is at half the input voltage. The other half of the bridge is formed by the switches Q1 and Q2. Switches Q1 and Q2 are turned on alternatively with a pulse-width determined by the input and output voltages and the transformer turns ratio. Each switch, when turned on, applies one-half the input voltage to 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 half-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. LM5039 Evaluation Board LM5039 Evaluation Board 30112701 Simplified Half-Bridge Circuit AN-2025 © 2010 National Semiconductor Corporation 301127 www.national.com AN-2025 The secondary side employs synchronous rectification scheme, which is controlled by the LM5039, during the softstart, the sync FET body diodes act as the secondary rectifiers. Once, the soft-start is finished, the synchronous rectifiers are engaged with a non-overlap time programmed by the DLY resistor. 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 LM5039 controller pulse width modulates the error signal with a ramp signal derived from the line voltage (feed-forward) to reduce the response time. A standard “type III” network is used for the compensator. plication 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. 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. Powering and Loading Considerations 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. When applying power to the LM5039 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. POWERING UP Using the ON/OFF 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 ON/OFF 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 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. 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 LM5039 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 ap- 30112702 FIGURE 1. www.national.com 2 Average Current Limit The major drawback of the half-bridge topology is that during current limit condition, the center-point of the capacitor divider tends to runaway either towards the input voltage rail or towards the ground. This phenomenon saturates the transformer and requires the capacitors in the divider to be rated to at least the input voltage. In an overload condition, the PWM cycle is terminated by the current sense comparator instead of the PWM comparator. This is similar to peak current 30112703 Trace 1 (C1) Output Voltage Trace 2(C2): Voltage on the ACL Capacitor Trace 3 (C3): Output current Trace 4 (C4): Voltage at the center-point of the half-bridge capacitor divider FIGURE 2. 3 www.national.com AN-2025 mode control, which inherently results in an on-time between both the phases of the half-bridge topology. Any such imbalance, for an extended period, will cause the voltage at the center-point of the capacitor divider to drift either towards the input voltage or the ground. However, in an average current limit scheme, the PWM cycle is terminated through the PWM comparator, by pulling down the PWM control input. Because of its averaging nature, the PWM control input voltage is slow moving and is essentially held at a constant dc voltage. This results in the on-time between the both the phases to be equal and thus balances the center-point of the capacitor divider. Figure 2 shows the current limit waveforms in a soft-short condition and Figure 3 shows the current limit waveforms in a hard-short condition. It can be observed from the Figures 2 and 3 that the centerpoint of the half-bridge capacitor divider is balanced in both soft-short and hard-short conditions. The response of average current limit circuit is same whether the short is soft or hard. During an overload event, the average current limit scheme converts the power supply from a constant voltage source to a constant current source. This scheme is often known as “brickwall current limiting.” A VOUT vs IOUT curve, shown in Figure 4, illustrates the brickwall current limiting. OVER CURRENT PROTECTION The evaluation board is not configured with over current protection and will be in continuous current limit condition. Therefore, 200 CFM of airflow is a must during the over current condition. If the customer desires to configure the evaluation board with the hiccup mode enabled, a 4700pF capacitor needs to be connected from RES pin to AGND. In the event of an output overload (approximately 35A) the unit will discharge the soft start capacitor, which disables the power stage. After a delay the softstart is released. The shutdown, delay and slow recharge time of the soft start capacitor reduces the average power consumption of the unit in an overload condition. AN-2025 30112704 Trace 1 (C1) Output Voltage Trace 2(C2): Voltage on the ACL Capacitor Trace 3 (C4): Voltage at the center-point of the half-bridge capacitor divider FIGURE 3. The LM5039 evaluation board is configured to be in constant current limiting. To configure the board for hiccup mode restart, remove the zero ohm resistor from the RES pin to the AGND and install a 4700pF capacitor from the RES pin to the AGND. The RES capacitor should be selected such that the time taken for the RES capacitor to reach 2.5V is greater than the time taken for the average current mode control circuit to be in control. This will ensure that center-point of the halfbridge capacitor is balanced. Figure 5 illustrates a balanced half-bridge capacitor divider at the inception of a hiccup mode restart. While Figure 6 shows the same over multiple hiccup mode restarts. The RES capacitor should be selected such that the time taken for the RES capacitor to reach 2.5V and hence start the hiccup mode is greater than the time taken for the ACL pin to get into control. 30112705 FIGURE 4. www.national.com 4 AN-2025 30112706 Trace 1 (C1): Output voltage Trace 2(C2): Voltage on the ACL capacitor Trace 3(C3): Voltage on the RES capacitor Trace 4(C4): Voltage at the center-point of the half-bridge capacitor divider FIGURE 5. 30112707 Trace 1 (C1): Output voltage Trace 2(C2): Voltage on the ACL capacitor Trace 3(C3): Voltage on the RES capacitor Trace 4(C4): Voltage at the center-point of the half-bridge capacitor divider FIGURE 6. 5 www.national.com AN-2025 Other Performance Characteristics When applying power to the LM5039 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 7 shows the output voltage during a typical startup with a 48V input and a load of 30A. There is no overshoot during start-up. Figure 8 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. 30112710 Conditions: Input Voltage =36V Output Current=5A Trace 1: Q1 Drain Voltage Volts/div=10V Horizontal Resolution= 1 us/div FIGURE 9. 30112708 Conditions: Input Voltage=48V Output Current=5A Trace 1: Output Voltage Volts/div=500mV Horizontal Resolution =2.0 ms/div FIGURE 7. 30112721 Conditions: Input Voltage =72V Output Current=5A Trace 1: Q1 Drain Voltage Volts/div=10V Horizontal Resolution= 1 us/div FIGURE 10. Figure 11 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. 30112709 Conditions: Input Voltage=48V Output Current=15A to 22.5A Upper Trace: Output Voltage Volts/div=50mV Lower Trace: Output Current = 15A to 22.5A to 15A Horizontal Resolution =0.5 ms/div FIGURE 8. Figures 9 and 10 show the drain voltage of Q1 with a 25A load. Figure 9 represents an input voltage represents an input voltage of 36V and Figure 10 represents an input voltage of 72V. 30112724 Conditions: Input Voltage =48V Output Current=5A Trace 1: Output Voltage Volts/div=20mV Horizontal Resolution= 1 us/div FIGURE 11. www.national.com 6 FIGURE 12. 30112711 AN-2025 7 www.national.com AN-2025 Bill of Materials # Designator Qty Part # Description 1 U1 2 U2 1 NSC LM5039MH LM5039 Controller 1 NSC LM5110-1M 3 LM5110-1M Dual Driver U3 1 NSC LM8261M5 4 LM8261M5 Op Amp U5 1 NSC LM4041AIM3-.12 LM4041AIM3-1.2 Ref Amp 5 U4 1 NEC PS2811-1M Opto-Coupler PS2811-1M 6 C21 1 TDK C1608COG1H470J Cer Cap 47pF 50V COG 7 C26 1 TDK C1608COG1H151J Cer Cap 150pF 50V COG 8 C34 1 TDK C1608COG1H471J Cer Cap 470pF 50V COG 9 C19, C37 2 TDK C1608X7R1H102K Cer Cap 1000pF 50V X7R 10 C27 2 TDK C1608COG1H222J Cer Cap 2200pF 50V COG 11 C28 1 TDK C1608COG1H682J Cer Cap 6800pF 50V COG 12 C20, C23, C29, C30 1 TDK C1608X7R1E473K Cer Cap 0.047uF 25V COG 13 C2, C33, C31, C35 3 TDK C1608X7R1H104K Cer Cap 0.1uF 50V X7R 14 C25 2 TDK C1608X7R1C105K Cer Cap 1.0uF 16V X7R 15 C36 3 TDK C1608X7R1C474K Cer Cap 0.47uF 50V X7R 16 C32 1 Vishay CRCW06030000Z0TA Res 0 Ohm 0.1W,5% 17 C39 2 TDK C2012x714224K Cer Cap 0.22uF 25V COG 18 C15, C16 2 KEMT C0805C471M5RAC Cer Cap 470pF 50V COG 19 C17, C24 2 TDK C2012X7R2A104K Cer Cap 0.1uF 100V X7R 20 C7 1 TDK C2012X7R1H334K Cer Cap 0.33uF 50V X7R 21 C1, C22 2 TDK C2012X7R1C225K Cer Cap 2.2uF 16V X7R 22 C18 1 TDK C3216X7R1C475K Cer Cap 4.7uF 16V X7R 23 C11–C14 4 TDK C3216X5R0J226M Cer Cap 22uF 6.3V X5R 24 C38 1 TDK C4532X7R3D222K Cer Cap 2200pF 2000V X7R 25 C3–C6 4 TDK C4532X7R1H685M Cer Cap 6.8uF 50V X7R 26 C8–C10 3 Sanyo 6TPE220MI POSCAP 220uF 6.3V 27 R12 1 Vishay CRCW06035R60FKTA Res 5.6 Ohm 0.1W 1% 28 R17, R35 2 Vishay CRCW060310R0F Res 10 Ohm 0.1W 1% 29 R25, R27, R28 3 Vishay CRCW06031000F Res 100 Ohm 0.1W 1% 30 R21 1 Vishay CRCW06035490F Res 549 Ohm 0.1W 1% 31 R13–14, R18–19 4 Vishay CRCW06031001F Res 1K Ohm 0.1W 1% 32 R24 1 NU NU 33 R31 1 Vishay CRCW06032001F Res 2.0K Ohm 0.1W 1% 34 R20 1 Vishay CRCW06034121F Res 4.12K Ohm 0.1W 1% 35 R32 1 Vishay CRCW06035111F Res 5.11K Ohm 0.1W 1% 36 R22 1 Vishay CRCW06038061F Res 8.06K Ohm 0.1W 1% 37 R7, R30 2 Vishay CRCW06031002F Res 10K Ohm 0.1W 1% 38 R26 1 Vishay CRCW06031022F Res 10.2K Ohm 0.1W 1% 39 R33 1 Vishay CRCW06032492F Res 24.9K Ohm 0.1W 1% 40 R29 1 Vishay CRCW06031502F Res 15K Ohm 0.1W 1% 41 R34 1 Vishay CRCW06032002F Res 20K Ohm 0.1W 1% 42 R23 1 Vishay CRCW06032552F Res 25.5K Ohm 0.1W 1% 43 R3, R4 2 Vishay CRCW06031003F Res 100K Ohm 0.1W 1% 44 R1, R11, R15 3 Vishay CRCW080510R0F Res 10 OHM 1/10W 1% 45 R5 1 Vishay CRCW080549R9F Res 49.9 OHM 1/10W 1% 46 R2 1 Vishay CRCW08052001F Res 2K OHM 1/10W 1% 47 R6 1 Vishay CRCW08051002F Res 10K OHM 1/10W 1% www.national.com 8 R16 1 Vishay CRCW08056492F Res 64.9K OHM 1/10W 1% 49 R10 2 Vishay CRCW08051003F Res 100K OHM 1/10W 1% 50 R8, R9 2 Vishay CRCW201010R0F Res 10 OHM 1% 51 D1 1 BAV70-TP Schottky, Diode, 75V 150mA 52 D2, D4 2 Central CMDD4448 Diode, 75V 250mA 53 D3 1 BAT54A Schottky Diode, 30V 200mA 54 BR1 1 BAT54BRW Diodes, Rectifier, Bridge, 30V 55 Z1 1 Central CMPZ4694 Zener 8.2V 5% 56 Z2 1 Central CMPZ4698 Zener 11V 5% 57 Q1, Q2 2 Vishay Si7456DP N-FET 100V 25m ohm 58 Q4–7 4 Vishay Si7336ADP N-FET 30V 3m ohm 59 Q3, Q8 2 ZETEX FCX690B NPN, ZETEX 45V 2A 60 L1 1 TDK RLF7030T-2R2M5R4 Inductor 2.2uH 5.4A 61 L2 1 Coilcraft SER2010-122MX Inductor 1.2uH 37A 62 T1 1 Coilcraft DA2025-AL Transformer 8:5:2:2 63 T2 1 Pulse Engr P8208 Current XFR 100:1, 10A 64 T3, T4 2 Coilcraft DA2319-ALB Gate XFR 1:1 65 J1–3, J5–7 6 Mill-Max 3104-2-00-80-00-00-08-0. Test Pin, Brick 66 J4, J8 2 Mill-Max 3231-2-00-01-00-00-08-0 Test Pin, Brick 9 AN-2025 48 www.national.com AN-2025 PCB Layouts 30112713 Top Silk 30112714 Bottom Silk www.national.com 10 AN-2025 30112715 Top Side 30112716 Layer 2 11 www.national.com AN-2025 30112717 Layer 3 30112718 Layer 4 www.national.com 12 AN-2025 30112719 Layer 5 30112720 Bottom 13 www.national.com LM5039 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 WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage References www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Applications & Markets www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise® Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic PLL/VCO www.national.com/wireless www.national.com/training PowerWise® Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. NO LICENSE, WHETHER EXPRESS, IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS DOCUMENT. TESTING AND OTHER QUALITY CONTROLS ARE USED TO THE EXTENT NATIONAL DEEMS NECESSARY TO SUPPORT NATIONAL’S PRODUCT WARRANTY. EXCEPT WHERE MANDATED BY GOVERNMENT REQUIREMENTS, TESTING OF ALL PARAMETERS OF EACH PRODUCT IS NOT NECESSARILY PERFORMED. NATIONAL ASSUMES NO LIABILITY FOR APPLICATIONS ASSISTANCE OR BUYER PRODUCT DESIGN. BUYERS ARE RESPONSIBLE FOR THEIR PRODUCTS AND APPLICATIONS USING NATIONAL COMPONENTS. PRIOR TO USING OR DISTRIBUTING ANY PRODUCTS THAT INCLUDE NATIONAL COMPONENTS, BUYERS SHOULD PROVIDE ADEQUATE DESIGN, TESTING AND OPERATING SAFEGUARDS. EXCEPT AS PROVIDED IN NATIONAL’S TERMS AND CONDITIONS OF SALE FOR SUCH PRODUCTS, NATIONAL ASSUMES NO LIABILITY WHATSOEVER, AND NATIONAL DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY RELATING TO THE SALE AND/OR USE OF NATIONAL PRODUCTS INCLUDING LIABILITY OR WARRANTIES RELATING TO FITNESS FOR A PARTICULAR PURPOSE, MERCHANTABILITY, OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF THE CHIEF EXECUTIVE OFFICER AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: Life support devices or systems are devices which (a) are intended for surgical implant into the body, or (b) support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness. National Semiconductor and the National Semiconductor logo are registered trademarks of National Semiconductor Corporation. All other brand or product names may be trademarks or registered trademarks of their respective holders. AN-2025 Copyright© 2010 National Semiconductor Corporation For the most current product information visit us at www.national.com National Semiconductor Americas Technical Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Technical Support Center Email: [email protected] National Semiconductor Asia Pacific Technical Support Center Email: [email protected] National Semiconductor Japan Technical Support Center Email: [email protected]