DEMO MANUAL DC1860B LT3748 No-Opto Isolated FlybacK Converter for PSE Applications Description Demonstration circuit 1860B is an isolated flyback con- efficiency, small size solution with improved regulation. verter featuring the LT3748. This demo circuit is designed The part senses the isolated output voltage directly from for Power Sourcing Equipment (PSE) applications with a the primary side flyback waveform during the off time of 52.5V output from a 10V to 15V DC input, with nominal the power switch. No third winding, opto-coupler, or signal input voltage of 12V. The output current is from 0mA to transformer is required for regulation. 700mA, and is capable of a maximum short period overload The LT3748 data sheet gives a complete description of current of 900mA. Table 1 summarizes the performance the part, operation and application information. The data of the demo board. This demo circuit can also be easily sheet must be read in conjunction with this quick start modified for different PSE applications with some pre- guide for demo circuit 1860B. designed transformers. Design files for this circuit board are available at The LT3748 is a high input voltage isolated flyback control- DEMO MANUAL DC1860B Uq ick stArt proceDU Demo circuit 1860B is easy to set up to evaluate the ex-board preload requirement. The efficiency is higher performance of the LT3748. Refer to Figure 1 for proper than 90% without the preload resistors, but the output equipment setup and follow the procedure below. may exceed 57V under no load or transient condition. 1. With power off, connect the input power supply to the 5. Once the proper output voltage is established, adjust board through V (E1) and GND (E2) terminals. the input voltage and load current within the operating IN range and observe the output voltage regulation, ripple + 2. Connect the load to the terminals V (E6) and V OUT OUT voltage, efficiency and other parameters. (E7) on the board. NOTE: When measuring the input or output voltage 3. Turn on the power at the input. Increase V slowly to IN ripples, care must be taken to avoid a long ground lead 10V. on the oscilloscope probe. Measure the input or output NOTE: Make sure that the input voltage does not exceed voltage ripple by touching the probe tip directly across the 15V. If higher operating voltage is required, power com- terminals of the input or output capacitor. See Figure2 for ponents with higher voltage ratings should be used. proper scope probe technique. 4. Check for the proper output voltages. The output should NOTE: A gate resistor of less than 10 can be added be regulated at 52.5V (5%). to the demo circuit to slow down the turning off of the switch, and reduce the switching node voltage spike. A NOTE: The LT3748 requires minimum load to maintain RC snubber can be added as well to the primary winding good output voltage regulation. A zener diode D1 together of the transformer to damp the spike and reduce the EMI. with preload resistors R2, R14, R15 and R16 are placed in the output to clamp the output voltage to ~52.5V without dc1860bfa 2 re