Multiple Output Isolated Power Supply Achieves High Efficiency with Secondary Side Synchronous Post Regulator Design Note 299 Charlie Zhao and Wei Chen Introduction The newly released LT®3710, a secondary side synchronous post regulator controller, provides tight regulation for all outputs of multiple output isolated power supplies. Solutions that use the LT3710 are efficient, use minimal space and provide fast transient response. controller uses the LT3781; a current mode 2-switch forward controller with built-in MOSFET drivers. On the secondary side a synchronous rectifier controller, the LTC1698, provides the voltage feedback for the main 3.3V output as well as the gate drive for the synchronous MOSFETs. The LT3710 circuit precisely regulates the 2.5V output. Total load and line regulation for the 2.5V output is better than 0.2%. From a 48V input, the overall efficiency is about 87% with full load on both outputs. The LT3710 is a voltage mode controller with leading edge modulation, programmable current limit and dual MOSFET drivers. It generates a tightly regulated second output directly from the transformer secondary winding, thus minimizing the size of the output inductor and capacitor of the main output stage. The use of synchronous MOSFETs significantly improves efficiency, making it suitable for low output voltage applications. Conclusion The LT3710 is a high efficiency secondary side synchronous post regulator controller that is designed to generate a tightly regulated secondary output for multiple output isolated power supplies. It can be used with any buck derived single-ended or dual-ended isolated topologies, such as forward, push-pull, halfbridge and full-bridge converters. Design Example Figures 1a and 1b show a dual output high efficiency isolated DC/DC power supply with 36V to 72V input range and two outputs of 3.3V/10A and 2.5V/10A. The basic power stage topology is a 2-switch forward converter with synchronous rectification. The primary side FROM TRANSFORMER SECONDARY WINDING SEE FIGURE 1b C45 10pF R58 10k 5 7 C34 0.01μF VCCS C47 1μF 13 4 C37 680pF 14 6 C35 180pF R57 10k 1 SYNC SS GBIAS TGATE VCC CSET C44 0.1μF 16V C36 4.7μF 16V BOOST LT3710 SW BGATE PGND BGS ILCOMP CL+ VFB 10 C38 0.033μF R50 3.3k C39 330pF D14 CMDSH-3 16 2 Q13 Si7892DP R56 10Ω 3 15 D15 Q14 CMDSH-3 Si7892DP 9 11 12 CL– VAOUT L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. L4 1.8μH R49 SUMIDA 0.006Ω CEP125-IR8 1% + B340A C43 0.01μF C40 + 680μF 4V POSCAP C42 4700pF 8 R51 3.01k 1% R53 220Ω R52 1.4k 1% DN299 F01a Figure 1a. 36V to 72V DC to 3.3V/10A and 2.5V/10A Dual Output Isolated Power Supply (Part 1 of 2: Post Regulator Circuit) 11/02/299_conv VOUT2 2.5V/10A C41 680μF 4V POSCAP Data Sheet Download www.linear.com Linear Technology Corporation FAX: (408) 434-0507 ● www.linear.com OFF ON FZT 853 4.7μF 0.1μF 10k 1N4148 270k 1.24k 73.2k 1 2 13 20 1nF 1μF 5VREF FSET 0.1F 5 6 52.3k 1% SHDN 5VREF OVLO VCC VBST BAS21 DO1608C-105 1.5μF 100V 1.5μF 100V 19 18 15 11 82pF 3 7 0.1μF 4 8 1k 3.3nF 3k 10Ω 10 9 PGND 12 SG 14 BAS21 THERM SYNC SGND SS VC VFB LT3781 TG BSTREF BG SENSE 10k BAS21 BAT54 ZVN3310F BAT54 300pF B2100 5 7 6 5VREF 8 MOC207 4 3 2 1 t8 7 T2 PULSE P2033 4 220pF 1t BAT54 0.020Ω 1/2W Q2 Si7456DP 4 2 3 1μF 1 VCCS 12 4.7μF FZT690B 11 2.2nF 250VAC Si7892DP ×2 5 16 2 6 VOUT1 0.01μF 14 VAUX 0.1μF 3 4 10 PGND GND PWRGD ICOMP MARGIN 1μF 13 7 + B0540W Si7892DP 470μF 4V POSCAP 2.5μH SUMIDA CEP125-2R5 + 1.24k 1% 2.43k 1% 1.78k 1% 3.01k 1% UNLESS OTHERWISE SPECIFIED: ALL CAPS 25V ALL RESISTORS 0.1W, 5% 0.22μF B0540W 22nF 470Ω 1nF CMPZ5240B 10V 10Ω 1nF 100V 2k 1nF 100V 10Ω TO POST REGULATOR CIRCUIT SEE FIGURE 1a VDD ISNS ISNSGND FG CG VCOMP 15 8 SYNC VFB 1k 5 9 OPTODRV OVPIN LTC1698 4.7nF 470Ω B2100 T1 PULSE Q1 Si7456DP PA0191 1 7 t Figure 1b. 36V to 72V DC to 3.3V/10A and 2.5V/10A Dual Output Isolated Power Supply (Part 2 of 2: Main Converter) 11V MMSZ5241B 20k B0540W VIN– 36V–72V VIN+ 1.2μH COILCRAFT D01813P-122HC t ● t (408) 432-1900 For applications help, call (408) 432-1900 dn299f_conv LT/TP 1102 351.5K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 © LINEAR TECHNOLOGY CORPORATION 2002 470μF 4V POSCAP DN299 F01b VOUT1 TRIM VOUT RTN VOUT1+ 3.3V AT 10A