National Semiconductor Application Note 2043 Ajay Hari March 18, 2010 Introduction (LO) modulating power switches with independent pulse width timing. The main difference between the topologies are, the Half Bridge topology employs a transformer to provide input / output ground isolation and a step down or step up function. Each cycle, the main primary switch turns on and applies onehalf the input voltage across the primary winding, which has 8 turns. The transformer secondary has 2 turns, leading to a 4:1 step-down of the input voltage. For an output voltage of 3.3V the composite duty cycle (D) of the primary switches varies from approximately 75% (low line) to 35% (high line). The secondary employs synchronous rectification controlled by the LM5035C. During soft-start, the sync FET body diodes act as the secondary rectifiers until the main transformer energizes the gate drivers. The DLY resistor programs the nonoverlap timing for the sync FETs to maximize efficiency while eliminating shoot through current. The Sync FET control signals are sent across the isolation boundary using a digital isolator. 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 LM5035C greatly increases the achievable loop bandwidth. The capacitance effect (and associated pole) of the optocoupler is reduced by holding the voltage across the optocoupler constant. The LM5035C voltage mode controller pulse width modulates the error signal with a ramp signal derived from the line voltage (feedforwarding) to reduce the response time to input voltage changes. A standard “type III” network is used for the compensator. The evaluation board can be synchronized to an external clock with a recommended frequency range of 420KHz to 500KHz. The LM5035C evaluation board is designed to provide the design engineer with a fully functional power converter based on the Half Bridge topology to evaluate the LM5035C controller. The LM5035C is a functional variant of the LM5035B Half-Bridge PWM Controller. The amplitude of the SR control signals are 5V instead of the VCC level. 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 • Output current: 0 to 30A • Measured efficiency: 89% at 30A, 92% at 15A • Frequency of operation: 400kHz • Board size: 2.28 x 1.45 x 0.5 inches • Load Regulation: 0.2% • Line Regulation: 0.1% • Line UVLO (33.9V/31.9V on/off) • Line OVP (79.4V/78.3V off/on) • Hiccup 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. Theory of Operation Power converters based on the Half Bridge topology offer high efficiency and good power handling capability in applications up to 500 Watts. The operation of the transformer causes the flux to swing in both directions, thereby better utilizing the magnetic core. The Half Bridge converter is derived from the Buck topology family, employing separate high voltage (HO) and low voltage LM5035C Evaluation Board LM5035C Evaluation Board AN-2043 © 2010 National Semiconductor Corporation 301195 www.national.com AN-2043 30119501 Powering and Loading Considerations Simplified Half Bridge Converter evaluation board undervoltage impedance and the inrush current. the cabling Loading When applying power to the LM5035C evaluation board certain precautions need to be followed. A misconnection can damage the assembly. An appropriate electronic load, with specified operation down to 1.0V 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 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 cause 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. Powering Up Source Power Using the ON/OFF pin (J2) 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 (J1), 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. 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 (75V) the input current will be approximately 1.5A. Therefore, to fully test the LM5035C evaluation board a DC power supply capable of at least 85V and 5A 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 cause voltage droop during turn-on due to 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 lockout, 2 35A) 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. Over Current Protection The evaluation board is configured with hiccup over-current protection. In the event of an output overload (approximately 30119502 Typical Evaluation Setup former needs to be reset, which imposes duty cycle limitaDigital Isolator tions. Further, during a sudden switch-off of the power There is a total of four crossing of the isolation boundary; the converter, the DC restorer capacitor on the secondary of the power transformer, the feedback and control of the two syngate drive transformer does not have a quick discharge path. chronous MOSFETs. Usually an opto-coupler is used for This will keep SR FET's turned on, resulting in a non-monoisolation of the feedback signal since this a relatively slow tonic decay of the output voltage. analog signal. Most opto-couplers are too slow to use for the These limitations can be addressed using a digital isolator. synchronous MOSFET gate drive. There are fast opto-couThe digital isolators are CMOS devices that use an RF couplers available but there is a big cost premium. Historically, pler to transmit digital information across the isolation barrier. the most common approach has been to use gate drive transThe isolation capability is up to 2500 VRMS. In simple words, formers to provide isolation for the synchronous gate drive the digital isolators are similar to an opto-coupler. While, the signals. The transformers can be used to directly drive the opto-couplers modulate light to transmit electrical signals, the MOSFET gates or the transformers can be used to just isolate digital isolators modulate an RF signal across a semiconducthe control signal which is then applied to a gate driver IC on tor barrier. Furthermore, the digital isolators have lower propthe secondary side. Gate drive transformers have their chalagation delay than the gate drive transformers and do not lenges and limitations. Transformers cannot pass DC. A given suffer volt-second limitations. size transformer can only pass a finite voltage & time product across the isolation boundary. After each on-time, the trans- 3 www.national.com AN-2043 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. AN-2043 Performance Characteristics TURN-ON WAVEFORMS When applying power to the LM5035C 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 startup 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 change from 15A to 22.5A. The upper trace shows minimal output voltage droop and overshoot during the sudden change in output current shown by the lower trace. 30119506 Conditions: Input Voltage = 48VDC Output Current = 30A Bandwidth Limit = 20MHz Trace 1: Output Ripple Voltage Volts/div = 20mV Horizontal Resolution = 1µs/div FIGURE 3. Figure 3 shows typical output ripple seen across the output terminals (with standard 10µF and 1µF ceramic capacitors) for an input voltage of 48V and a load of 30A. This waveform is typical of most loads and input voltages. Figures 4 and 5 show the drain voltage of Q1 with a 5A load. Figure 4 represents an input voltage of 36V and Figure 5 represents an input voltage of 72V. Figure 6 shows the gate voltages of the synchronous rectifiers. The deadtime provided by the 20kΩ DLY resistor is difficult to see at this timescale. 30119504 Conditions: Input Voltage = 48VDC Output Current = 5A Trace 1: Output Voltage Volts/div = 500mV Horizontal Resolution = 0.5ms/div FIGURE 1. 30119507 Conditions: Input Voltage = 36VDC Output Current = 5A Trace 1: Q1 drain voltage Volts/div = 10V Horizontal Resolution = 1µs/div 30119505 Conditions: Input Voltage = 48VDC 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 = 0.5ms/div FIGURE 4. FIGURE 2. www.national.com 4 AN-2043 30119508 30119509 Conditions: Input Voltage = 72VDC Output Current = 5A Trace 1: Q2 drain voltage Volts/div = 10V Horizontal Resolution = 1µs/div Conditions: Input Voltage = 48VDC Output Current = 5A Upper Trace: SR1, Q4 gate Volts/div = 5V Middle Trace: HS, Q2 drain Volts/div = 20V Lower Trace: SR2, Q6 gate Volts/div = 5V Horizontal Resolution = 1µs/div FIGURE 5. FIGURE 6. 5 www.national.com Application Circuit: Input 36 to 75V, Output 3.3V, 30A 30119518 AN-2043 Application Circuit www.national.com 6 Part Description Qty Ref Designator Remark 1 LM5035C Controller MH20 1 U1 NSC LM5035CMH 2 LM5110-1M Dual Driver 1 U2 NSC LM5110-1M 3 LM8261M5 Op Amp SOT23-5 1 U3 NSC LM8261M5 4 LM4041AIM3-1.2 Ref Amp SOT23 1 U5 NSC LM4041AIM3-.12 5 Opto-Coupler PS2811-1M 1 U4 NEC PS2811-1M 6 Digital Isolator IC SOIC-8 1 U6 Silicon Labs SI8420BB-D 7 Cer Cap 47pF 50V COG 0603 1 C21 TDK C1608COG1H470J 8 Cer Cap 150pF 50V COG 0603 1 C26 TDK C1608COG1H151J TDK C1608COG1H471J 9 Cer Cap 470pF 50V COG 0603 1 C34 10 Cer Cap 1000pF 50V X7R 0603 2 C19, C37 TDK C1608X7R1H102K 11 Cer Cap 2000pF 50V COG 0603 2 C27, C32 TDK C1608COG1H222J 12 Cer Cap 6800pF 50V COG 0603 1 C28 TDK C1608COG1H682J 13 Cer Cap 0.022uF 25V COG 0603 1 C35 TDK C1608COG1E223J 14 Cer Cap 0.1uF 50V X7R 0603 3 C2, C33, C36 TDK C1608X7R1H104K 15 Cer Cap 1.0uF 16V X7R 0603 2 C25, C31, C29, C20 TDK C1608X7R1C105K 16 Cer Cap 470pF 50V COG 0805 2 C15, C16 17 Cer Cap 0.1uF 100V X7R 0805 2 C17, C24 TDK C2012X7R2A104K 18 Cer Cap 0.33uF 50V X7R 0805 1 C7 TDK C2012X7R1H334K 19 Cer Cap 2.2uF 16V X7R 0805 2 C1, C22 TDK C2012X7R1C225K 20 Cer Cap 4.7uF 16V X7R 1206 1 C18 TDK C3216X7R1C475K 21 Cer Cap 22uF 6.3V X5R 1206 4 C11–C14 TDK C3216X5R0J226M 22 Cer Cap 2200pF 2000V X7R 1812 1 C38 TDK C4532X7R3D222K 23 Cer Cap 6.8uF 50V X7R 1812 4 C3–C6 TDK C4532X7R1H685M 24 POSCAP 220uF 6.3V 3 C8–C10 Sanyo 6TPE220MI 25 Res 2.8 Ohm 0.1W 1% 0603 1 R12 Vishay CRCW06032R80F 26 Res 10 Ohm 0.1W 1% 0603 2 R17, R35 Vishay CRCW060310R0F 27 Res 100 Ohm 0.1W 1% 0603 3 R25, R27 Vishay CRCW06031000F 28 Res 549 Ohm 0.1W 1% 0603 1 R21 Vishay CRCW06035490F 29 Res 1K Ohm 0.1W 1% 0603 4 R13, R18 Vishay CRCW06031001F 30 Res 1.58K Ohm 0.1W 1% 0603 1 R24 Vishay CRCW06031581F 31 Res 2.0K Ohm 0.1W 1% 0603 1 R31 Vishay CRCW06032001F 32 Res 4.12K Ohm 0.1W 1% 0603 1 R20 Vishay CRCW06034121F 33 Res 5.11K Ohm 0.1W 1% 0603 1 R32 Vishay CRCW06035111F 34 Res 8.06K Ohm 0.1W 1% 0603 1 R22 Vishay CRCW06038061F 35 Res 10K Ohm 0.1W 1% 0603 2 R7, R30 Vishay CRCW06031002F 36 Res 10.2K Ohm 0.1W 1% 0603 1 R26 Vishay CRCW06031022F 37 Res 14.7K Ohm 0.1W 1% 0603 1 R33, R46 Vishay CRCW06031472F 7 KEMT C0805C471M5RAC www.national.com AN-2043 Item AN-2043 Item Part Description Qty Ref Designator Remark 38 Res 15K Ohm 0.1W 1% 0603 1 R29, R41 Vishay CRCW06031502F 39 Res 20K Ohm 0.1W 1% 0603 1 R34 Vishay CRCW06032002F 40 Res 25.5K Ohm 0.1W 1% 0603 1 R23 Vishay CRCW06032552F 41 Res 100K Ohm 0.1W 1% 0603 2 R3, R4 Vishay CRCW06031003F 42 NU 0805 1 R14 43 Res 10 OHM 1/10W 1% 0805 3 R1, R11, R15 Vishay CRCW080510R0F 44 Res 49.9 OHM 1/10W 1% 0805 1 R5 Vishay CRCW080549R9F 45 Res 2K OHM 1/10W 1% 0805 1 R2, R19 Vishay CRCW08052001F 46 Res 10K OHM 1/10W 1% 0805 1 R6 Vishay CRCW08051002F 47 Res 64.9K OHM 1/10W 1% 0805 1 R16 Vishay CRCW08056492F 48 Res 100K OHM 1/10W 1% 0805 2 R10, R36 Vishay CRCW08051003F 49 Res 10 OHM 1% 2010 2 R8, R9 Vishay CRCW201010R0F 50 Schottky, Diode, 75V 150mA SOT23 1 D1 51 Diode, 75V 250mA SOD-323 2 D2, D4 52 Diodes, Rectifier, Bridge, 30V 1 BR1 53 Zener 8.2V 5% SOT23 1 Z1 Central CMPZ4694 54 Zener 11V 5% SOT23 1 Z2 Central CMPZ4698 55 Zener 5.6V, 5% SOT23 1 Z4 Central CMPZ4690 NU BAV70-TP Central CMDD4448 BAT54BRW NU SOT23 1 Z3 56 N-FET 100V 25m ohm 2 Q1, Q2 57 N-FET 30V 3m ohm 4 Q4–7 58 NPN, ZETEX 45V 2A 2 Q3, Q8 ZETEX FCX690B 59 NPN, ON SEMI 45V, 225mW 1 Q10 MMBT6429LT1G 60 NU 1 Q9 NU 61 Inductor 2.2uH 5.4A 1 L1 TDK RLF7030T-2R2M5R4 62 Inductor 1.2uH 37A 1 L2 Coilcraft SER2010-122MX 63 Transformer 8:5:2:2 1 T1 Coilcraft DA2025-AL 64 Current XFR 100:1, 10A 1 T2 Pulse Engr P8208 65 Test Pin, Brick 0.040X0.5 6 J1–3, J5–7 Mill-Max 3104-2-00-80-00-00-080 66 Test Pin, Brick 0.080X0.375 2 J4, J8 Mill-Max 3231-2-00-01-00-00-080 www.national.com 8 NU Vishay Si7456DP Vishay Si7336ADP AN-2043 PCB Layouts 30119510 Top Side 30119511 Bottom Side 9 www.national.com AN-2043 30119512 Layer 1 30119513 Layer 2 www.national.com 10 AN-2043 30119514 Layer 3 30119515 Layer 4 11 www.national.com AN-2043 30119516 Layer 5 30119517 Layer 6 www.national.com 12 AN-2043 Notes 13 www.national.com LM5035C 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 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