DEMO MANUAL DC1961A LT8309 No-Opto Flyback Converter with Synchronous Rectifier Description Demonstration circuit 1961A features the LT 8309, a configuration allows the synchronous MOSFET to remain secondary synchronous driver in an isolated, no opto- conducting at all times, even when output is shorted coupler, flyback converter. It regulates a 12V, 5A output to ground, so as to provide a very robust short circuit from a 36V to 72V input source. performance. Output regulation is handled on the primary side by the The Performance Summary table summarizes the per- LT3748, a boundary conduction mode flyback controller formance of the demo board at room temperature. For which senses output voltage directly from the primary, re- thermally critical applications, proper amount of air flow sulting in a simple flyback schematic with no opto-coupler. can help to reduce power components temperature rise, therefore greatly improving circuit reliability. The LT8309 synchronous rectifier driver replicates the behavior of a diode by sensing the synchronous MOSFET The LT8309 data sheet gives a complete description of drain-to-source voltage to determine its turn on period. the part, operation and application information. The data By replacing the diode with a N-channel MOSFET, applica- sheet must be read in conjunction with this quick start tions are no longer restricted by the heat constraints of guide for demo circuit 1961A. the rectifier diode. Design files for this circuit board are available at On the DC1961A, the LT8309 is biased from the recti- DEMO MANUAL DC1961A Quick s Dp Demonstration circuit 1961A is easy to set up to evalu- 3. Check for the proper output voltages. The output should ate the performance of the LT8309. Refer to Figure 1 for be regulated at 12V (5%). proper measurement equipment setup and follow the NOTE: The LT3748 requires a minimum load to maintain procedure below: good output voltage regulation. On the DC1961A, in order 1. With power off, connect the input power supply to the to avoid pre-loading, a Zener diode is placed between its + board through V and GND terminals. Connect the V and V to serve as a minimum load. IN OUT OUT + load to the terminals V and V on the board. OUT OUT 4. Once the proper output voltage is established, adjust 2. Turn on the power at the input. the input voltage and load current within the operating range and observe the output voltage regulation, ripple NOTE: Make sure that the input voltage does not exceed voltage, efficiency and other parameters. 72V. To operate the board with higher input/output volt- age, input capacitor, output capacitor and MOSFETs with NOTE: When measuring the input or output voltage ripples, higher voltage ratings are needed. care must be taken to avoid a long ground lead on the oscilloscope probe. Measure the input or output voltage ripple by touching the probe tip directly across the V IN + and GND, or V and V terminals. See Figure 2 for OUT OUT POWER SUPPLY proper scope probe technique. + + V IN I + IN I OUT + + + V LOAD OUT Figure 1. Proper Measurement Equipment Setup INPUT OR OUTPUT CAPACITOR Figure 2. Proper Scope Probe Placement for Measuring Input/Output Ripple dc1961af 2 ure roce tart