advertisement Secondary Side Dual Output Controller Tightly Regulates Two Isolated Outputs – Design Note 1000 Charlie Zhao and Wei Chen Introduction Many systems with dual output isolated power supplies require tight regulation on both outputs. The LT®3804 makes it possible to satisfy this requirement in a space saving and efficient circuit. The LT3804 is a secondary side dual output controller. It contains an error amplifier and an optocoupler driver to regulate the first output. It also includes a complete PWM controller and dual N-channel MOSFET drivers for the second output. The power to the second output is transferred directly from the transformer secondary winding without going through the first output stage, thus minimizing the size of the output inductor and capacitor of the first output. Accurate output regulation for both outputs is assured—even if the power supply’s outputs are far away from the actual loads—due to true differential remote sensing. Dual Output Isolated Power Supply Figure 1 shows a dual output high efficiency isolated DC/DC power supply with 36V to 72V input range and two outputs of 3.3V/15A and 1.8V/15A. The basic power stage topology is a 2-switch forward converter with synchronous rectification. The primary side is controlled by the LT3781, a current mode 2-switch forward controller with built-in MOSFET drivers. On the secondary side, the LT3804 provides voltage feedback for the 3.3V output and regulation for the 1.8V output. The 3.3V output is monitored by the LT3804 and fed to the LT3781 on the primary side via an optocoupler (MOC207) and the LT3804’s built-in optocoupler driver. An LTC1693-1 high speed dual N-channel MOSFET driver 12/03/1000 provides the gate drive for the synchronous MOSFETs at the 3.3V output stage. The 1.8V output stage is a special synchronous buck converter whose input is a pulsed waveform instead of a DC voltage. The LT3804 precisely regulates the 1.8V output by further reducing and controlling the duty cycle of the switching waveform from the power transformer (T1) secondary winding. The circuit provides 1500V input to output isolation and a switching frequency of 230kHz. The complete circuit fits easily on a standard half brick size PC board. With a 48V input, the overall efficiency is about 87% at full loads for both outputs. LT3804 Versatility The LT3804 can be used to regulate the outputs of any buck derived single-ended or dual-ended isolated topologies, including forward, push-pull, half-bridge and fullbridge converters. The device’s leading-edge modulation allows it to operate with either current mode or voltage mode primary controllers. Conclusion The LT3804 is a high efficiency secondary side switching controller ideal for dual output isolated power supplies. Unlike a traditional auxiliary supply from the secondary circuits, an LT3804-based design can precisely regulate both outputs. Its true differential remote sensing and synchronous rectification features make it suitable for high current applications where the output voltage can be as low as 0.6V. , LTC, and LT are registered trademarks of Linear Technology Corporation. VI P1 +VIN Q1 Si7456DP L1 1.2µH DO1813P-122HC T1 PULSE PAO191 SEC L3 CEP125-1R8 • 1 VO1 • 7 C3 1.5µF 100V ×2 36V TO 72V D2 B2100 D1 B2100 5 2 4 L2 1mH COILCRAFT DO1608C-105 VIN R7 10k VCC D20 B0540W R8 10Ω R26 73.2k 1% R24 270k 0.25W C20 0.1µF 25V D13 BAS21TA 13 VCC 2 OVLD 1 SHDN 5VREF D17 MMBD4148 P9 OFF ON C25 4.7µF 16V R43 10k C7 1µF C28 1000pF 19 VBST R36 1k C19 0.1µF TG 18 15 11 BSTREF BG SENSE PGND SG VFB THERM 5 5VREF 6 SYNC SS R38 1k C17 0.1µF 9 SGND VC 8 7 3 12 R37 10 680Ω 5VREF 4 C26 6.8nF R45 1.24k 1% C31 1µF RT1 100k C32 82pF R59 52.3k 1% R42 2.43k 1% C29 0.01µF C30 0.015µF R46 330Ω 7 6 ISO1 MOC207 4 3 1 5 R33 6.8k D10 MMBZ5240B 10V C21 1µF 16V D9 BAT54 LTC1693-1 8 VCC2 V 1 CC1 IN1 OUT1 3 IN2 OUT2 2 GND1 GND2 R31 470Ω • C16 0.01µF C13 4.7µF 25V R32 6.8k D8 BAT54 6 7 5 4 OPTO R35 1k 2 FROM TRANSFORMER SECONDARY WINDING VCCS C45 10pF VCCS OPTO R5 100k P3 PGOOD 23 19 16 C37 680pF 7 1 2 R13 1.5k C24 R19 10nF 1k R65 10Ω VO1 P4 VO1S+ 3.3V OUTPUT REMOTE SENSE 22 C50 1000pF 18 C51 15nF P5 VO1S– • C15 4.7µF 16V VCCS • C33 0.01µF R16 2k 0.25W R18 100Ω C22 4700pF 8 SEC • R34 470Ω T2 P2033 PULSE ENG R29 1k 14 LT3781 FSET T3 P2033 PULSE ENG R30 1k C1 0.01µF 20 VCCS D6 B340A Q12 FZT690B VCC C14 330pF VOUT C6 4.7µF 25V D12 BAT54S VIN VCC R6 3.3Ω D4 BAT21TA D7 BAT54 Q7 FZT853 D18 B0540W C10 2200pF 250VAC D3 BAT21TA VCC Q10 ZVN3310F D19 5241B 11V 3 + P6 VO1 3.3V 15A P7 VO RTN • R9 0.015Ω 1% 1/2W P2 –VIN Q4 Si7892DP ×2 • Q3 Si7456DP R25 20k C7 470µF 4V POSCAP ×3 Q6 Si7892DP ×2 13 10 28 R14 10k 1% R15 10k 1% R66 2.21k 1% R67 2.21k 1% R57 10k C47 1µF D14 25 16V 4 CMDSH-3 8 SYNC VCC OPTO BOOST GBAIS LT3804 PGOOD TGATE BGS SW BGATE CLIP CL2P CLIN CL2N VAOUT2 VFB2 GNDS1 GNDS2 PGIN1 PGIN2 17 R64 10Ω 26 Q14 Si7892DP ×2 14 12 11 3 SS2 9 C41 + 680µF 2.5V POSCAP ×3 P10 +VO2 1.8V 15A (2R5TPD680M ×3) 15 ILCOMP2 24 L4 CEP125-1R8 21 VFB1 PGND Q13 Si7892DP 5 20 VAOUT1 SS1 D15 CMDSH-3 27 6 CSET ILCOMP1 C36 C44 4.7µF 0.1µF 16V 16V C39 330pF R50 3.3k C38 0.033µF R53 220Ω C42 4700pF R54 3.01k 1% R62 3.01k 1% R60 1.5k 1% R61 1.5k 1% R63 10Ω P6 VO2S+ 1.8V OUTPUT REMOTE SENSE P11 VO2S– C43 0.1µF DN1000 F01 R68 10Ω Figure 1. 36V-72V DC to 3.3V/15A and 1.8V/15A Dual Output Isolated Power Supply Data Sheet Download http://www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900, Ext. 2593 dn1000f LT 1203 • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2003