SUB-SYSTEM BOARD 5509 Oceanside (MAXREFDES9 ): 3.3V to 15V Input, 15V (12V) Output, Isolated Power Supply Oct 25, 2012 Abstract: This document details the Oceanside (MAXREFDES9 ) subsystem reference design, a 3.3V to 15V input, 15V (12V) output, isolated power supply. The Oceanside design includes a high-efficiency step-up controller, a 36V H-bridge transformer driver for isolated supplies, a wide input range, and a pair of adjustable output low dropout (LDO) linear regulator. Test results and hardware files are included. Introduction The Oceanside design (MAXREFDES9 ) uses a step-up controller (MAX668), a 36V H-bridge transformer driver (MAX13256), and a pair of low dropout (LDO) linear regulators (MAX1659 x2) to create a 15V (12V) output isolated power supply from a wide range of input voltages. This general purpose power solution can be used in many different types of isolated power applications, but is mainly More detailed image (PDF, 1.8MB) targeted for industrial sensors, industrial automation, process control, and medical applications. Figure 1. The Oceanside subsystem design block diagram. Features Applications Isolated power Industrial sensors 15V (12V) outputs Process control Small printed-circuit board (PCB) area Industrial automation Pmod -compatible form factor Medical Detailed Description of Hardware The Oceanside subsystem reference design operates from a 3.3V to 15V DC power source. The MAX668 high- Page 1 of 8efficiency step-up controller boosts the input voltage to 16V and connects to the input of the H-bridge transformer driver. The MAX13256 H-bridge transformer driver switches at 425kHz and drives the primary side of the 1:1 turns ratio, with the use of TGMR-501V6LF transformers from Halo Electronics. The full-wave bridge rectifiers convert the AC outputs of the Halo transformers secondary coils into DC outputs. The LDOs regulate the voltages to 15V. The Zener diodes (D3 and D4) protect the LDOs by keeping their input voltages below 16.5V. The output current of the H-bridge transformer driver is limited to 300mA by the R5 2.2k resistor, which protects the LDO from overcurrent. The following equation sets the current limit for the H-bridge transformer driver. R5 (k) = 650/ILIM (mA) Having a step-up controller before the H-bridge transformer driver gives users the advantage of not needing to change the transformers to get a different turns ratio when a different input supply voltage is applied. The input power is selectable by JU1. Place the shunt on the 2-3 position to power the board by an external power supply connected to the VIN and GND1 connectors. Place the shunt on the 1-2 position to power the board by the J1 Pmod-compatible connector. To change the output voltage of this subsystem reference design, simply change the feedback resistors (R7R10) of the LDOs (U3 and U4), and also change the feedback resistors (R3 and R4) of the step-up controller (U1). The output voltage of the MAX1659 LDO is set by the following equation: V = V (1 + R7/R9) for U3 OUT SET V = V (1 + R8/R10) for U4 OUT SET Where V = 1.21V SET The output voltage of the MAX668 step-up controller is set by the following equation: V = V (1 + R3/R4) OUT SET Where V = 1.25 SET For example, for the 12V output application, change R7 and R8 to 90.9k, and change R9 and R10 to 10.2k. The output of the LDOs becomes 12V when their inputs (outputs of the transformers and the full-wave bridge rectifiers) are higher than 12V plus the dropout voltage. Next, change R3 to 100k and R4 to 10.5k to set the output of the step-up controller to 13.15V (slightly higher than 12V plus the dropout voltage and two diode voltage drops from the full-wave bridge rectifier). It is important to not set this voltage too much higher than the output of LDOs, because the extra power would be lost in the LDOs and thus efficiency would be lower. In applications that are sensitive to output voltage ripple, a lowpass LC pi filter can be added in front of the LDO input It is recognized by UL 60950 and The isolation transformer in this design has an isolation voltage of 5000V RMS EN 60950 and falls into thefunctiona insulation class. Quick Start Required equipment: Oceanside (MAXREFDES9 ) board 3.3V to 15V power supply Page 2 of 8