35 Watt Isolated DC/DC Converter Replaces Modules at Half the Cost by Robert Sheehan Introduction The choice between building or buying an isolated DC/DC converter can be a complex decision. If you use an off-the-shelf, module you are constrained by what the module makers offer in their catalogs. In many cases, this may not precisely meet the requirements for a particular project. Also, while simple to use, the cost of these modules can be significantly higher than the cost of “rolling your own.” The complexity of the DC/DC design can be daunting and leads many to the decision to buy. Demonstration circuit DC227 provides a DC/DC solution that can serve the needs of many “standard” module applications and offers the designer the option of customizing the design to suit any slightly unusual system requirements. The power supply now becomes merely another collection of parts in the system. DESIGN IDEAS 35 Watt Isolated DC/DC Converter Replaces Modules at Half the Cost ................................................... 30 Demonstration circuit DC227 is a board level replacement for “halfbrick” DC/DC converters. It can provide 5V or 3.3V at up to 7A from an isolated 48V (36V to 72V) input. The isolation voltage is 500VDC with an option for 1500VDC. The circuit has low input capacitance, fast turn-on time, low shutdown power consumption and overtemperature protection. Continuous short-circuit protection eliminates any restriction on maximum capacitive load. The output overvoltage circuit provides protection for open or short circuits on the output power or sense lines. The standard footprint allows the circuit to fit Robert Sheehan Comparator Circuit Provides Automatic Shutdown of the LT1795 High Speed ADSL Power Amplifier ................................................... 33 Tim Regan SMBus Controlled CCFL Power Supply ..................... 35 Jim Williams Triple Output TFT-LCD Bias Supply Uses All Ceramic Capacitors ....... 36 Gary Shockey Low Noise Boost VCO Power Supply for Portable Applications .................. 37 Ted Henderson Features directly into the module’s socket. Figure 1 shows a typical layout for a 2.28" by 2.40" circuit board. DC227A-A is designed for 500VDC isolation and lowest cost; it uses a standard Coiltronics VERSA-PAC™ transformer and a Pulse Engineering inductor for the output filter. DC227AB has 1500V isolation and uses a semicustom transformer, also from Coiltronics. DC227A-C has 500VDC isolation and achieves the highest efficiency using a Panasonic type PCCS1 inductor for the output filter. The efficiency curves in Figures 2–5 are quite competitive, reaching 85% for the DC227A-C with a 5V output. The efficiency at 3.3V out is somewhat lower, due to the fixed losses of the output rectifier. Circuit Description This single-ended forward converter operates at a nominal switching frequency of 200kHz. Referring to the schematic in Figure 6, pulse width modulation is controlled by U1, an LT1247 current mode PWM controller. Transformer T2 and optocoupler Q7 provide galvanic isolation. C2 is a VERSA-PAC is a trademark of Coiltronics, Inc. Figure 1. Control (left) and power component (right) views of demonstration circuit DC227, a complete 35W DC/DC converter in a 2.28" by 2.40" footprint 30 Linear Technology Magazine • September 1999 VIN = 36V VIN = 72V EFFICIENCY (%) 80 75 VIN = 48V 70 65 60 55 50 0 1 2 3 4 IOUT (A) 5 6 7 Figure 2. DC227A-C 5V output efficiency (typical) local bypass cap to reduce common mode–induced current. To achieve fast start-up time, a hysteretic buck regulator is used for the bias supply power. U2, an LT1431 shunt voltage regulator, provides control for this function, with Q1 acting as the switch element; L2 and C21 provide output filtering. Q2 and Q4 protect the circuit during a hot plug, making this a very robust design; it is also impervious to output short circuits. The input surge voltage is limited to 80V by the rating of Q1–Q4. The main switching power path through T2 comprises L1 and C18 as the input filter, Q6 as the primary switch, D7 as the secondary rectifier and L3 and C14, C16, C17 and C20 as the secondary filter. Transient volt90 85 VIN = 36V EFFICIENCY (%) 80 VIN = 72V 75 70 VIN = 48V 65 90 85 VIN = 36V 80 EFFICIENCY (%) 85 age suppressor D8 is used to protect Schottky diode D7 during large-signal transient conditions. Power is transferred during the on cycle of Q6 and integrated by the output filter, just as in a buck regulator. The input filter component values for L1 and C18 are optimal and should not be changed without careful evaluation. C19 damps the input filter and will provide adequate stability for large values of input inductance. See LTC Application Note 19 for a discussion of input filter stability analysis. Output voltage feedback is controlled using U3, another LT1431 shunt voltage regulator, as an error amplifier. In the event of a fault on the output power or sense lines, Z1/Q5 will override U3 and provide overvoltage protection. R10 and R21 are sized to handle any overvoltage condition. During an output short-circuit condition, the LT1247 is able to decrease the on time of Q6 to less than 200ns. This results in good control of the output short-circuit current, keeping power dissipation to a manageable level. The demonstration circuit uses surface mount devices for Q6 and D7. For elevated temperature operation at the full rated load, TO-220 devices can by mounted on a standard halfbrick heat sink. For –48V inputs that require hot swap capability, the LT1640H negative voltage HotSwap™ controller provides a seamless interface. Demonstration circuit DC223A-B using the LT1640HCS8 is the recommended solution for use with the DC227A. 75 VIN = 72V 70 VIN = 48V 65 60 55 50 0 1 2 3 4 IOUT (A) 5 6 7 Figure 4. DC227A-C 3.3V output efficiency (typical) Conclusion At 35 watts, the topology presented here is one of the most common used by the module manufacturers. This is only one solution for isolated power, and opens up many possibilities for other input and output voltage combinations. For lower power, demonstration circuit DC211 using the LT1425 isolated flyback switching regulator is designed for 10 watts. Demonstration circuit DC259 using the LT1339 adds synchronous rectification, providing a high efficiency solution for 50 watts. See the DC/DC Converter Module section of LTC’s Volume 1 1999 New Products Catalog for additional information. 90 85 80 EFFICIENCY (%) 90 VIN = 36V 75 VIN = 72V 70 65 VIN = 48V 60 60 55 55 50 50 0 1 2 3 4 IOUT (A) 5 6 0 7 1 2 3 4 IOUT (A) 5 6 7 Figure 5. DC227A-A/B 3.3V output efficiency (typical) Figure 3. DC227A-A/B 5V output efficiency (typical) http://www.linear-tech.com/ezone/zone.html Articles, Design Ideas, Tips from the Lab… Linear Technology Magazine • September 1999 31 32 ON/OFF CHASSIS –VIN + C19 12µF 100V +VIN 36V–72V R12 20k D5 BAS21 RT/CT ISENSE FB COMP COILTRONICS VP5-1200; JP1 500VDC ISOLATION HIGH EFFICIENCY DC227A-C (605) 665-1627 (803) 946-0362 (804) 239-6941 (619) 661-6835 (847) 639-6400 COILTRONICS CTX02-14281-X2; JP2 L1: COILCRAFT D01608C-472 L2: COILCRAFT D03316P-105 T1: DALE LPE-3325-A142 C14, C16, C17, C20: AVXTPSE227M010R0100 C18: ITW PAKTRON 225100ST3827T C19: SANYO 100MV12GX VCC VREF 5 6 7 8 R24 4.75k R23 16.2k L3 C5 0.1µF + C4 1µF R5 3.9Ω C21 10µF 25V C18 2.2µF 100V C15 470pF 200V R33 33Ω 1/4W PANASONIC ETQPAF7R2HA PULSE ENGINEERING PE-53663 PULSE ENGINEERING PE-53663 GND OUTPUT 1500VDC ISOLATION T2 AND JUMPER R6 845Ω 4 3 2 1 R25 4.7Ω Q4 FMMTA06 U1 LT1247CS8 RT1 NTC 10k AT 25°C R19 10Ω R13 47k COILTRONICS VP5-1200; JP1 5 8 4 C10 0.01µF DC227A-B C1 4700pF C3 0.01µF R4 8.2k R3 1k GND-S REF RTOP 3 R15 1k L2 1mH 500VDC ISOLATION C6 3300pF R2 1.30k GND-F RMID COMP COLLECTOR V+ Q1 FMMTA56 U2 LT1431CS8 R11 1k DC227A-A D2 BAT54 R7 100k 6 7 2 1 Q2 FMMTA56 C12 1000pF 1000V D4 FMMD914 D3 BAT54 Q3 FMMTA06 R18 100k R16 10k L1 4.7µH 6 4 8 5 D1 FMMD914 9 4 C2 VITRIMON VJ1808Y102KXGAT MURATA GHM3045X7R22K-GC D6 FMMD914 Q7 MOC207 R29 330Ω C8 2200pF L3 9.3µH 7.2A GND-F RMID COMP COLLECTOR V+ GND-S RTOP REF U3 LT1431CS8 C11 0.022µF 5 4 8 3 R30 1k R31 100Ω Z1 5.6V C14, C16, C17, C20 220µF, 10V ×4 R10 100Ω 1/2W R28 7.41k 0.1% JP3 R27 15.4k 0.1% R20 10k C9 0.047µF R22 4.99k 0.1% R26 100Ω R14 220Ω 1/4W NOTES: 1. CHANGE Z1 TO A 3.3V ZENER FOR 3.3V OUT 2. JP3: OPEN = 3.3V; SHORTED = 5V 6 7 2 1 C13 0.022µF Q5 FMMT3904 R17 10Ω, 1/4W D8 1SMB36A C7 1500pF D7 MBRB2545CT 7 (DUAL) 6 R1 Q6 MTB20N20E 22.1Ω T1 T2 VITRIMON VJ1808Y102KXGAT 8 7 JP1 10 3 JP2 11 2 12 1 C2 1000pF, 1000V OR 2200pF SAFETY RECOGNIZED R21 100Ω 1/2W –SENSE TRIM +SENSE –VOUT +VOUT DESIGN IDEAS Figure 6. 35W isolated DC/DC converter schematic diagram Linear Technology Magazine • September 1999