DEMO MANUAL DC1739A-B LTC3765/LTC3766 150W High Efficiency Active Clamp Forward Converter DESCRIPTION Demonstration circuit 1739A-B is a 150W isolated forward Synchronous rectification helps to attain efficiency ex- converter with synchronous rectification featuring the ceeding 94%. Secondary-side control eliminates complex LTC3765/LTC3766. opto-coupler feedback, providing fast transient response with minimum output capacitance. The proprietary Direct This circuit demonstrates a high level of performance, ef- Flux Limit scheme guarantees no transformer saturation. ficiency, and small solution size attainable using these parts For other output requirements, see the LTC3766 data sheet in an active-clamp-reset forward converter power supply. or contact the LTC factory. It operates at 240kHz and produces a regulated 12V, 12.5A output from an input voltage range of 18V to 72V, making Design files for this circuit board are available at it well suited for telecom, industrial, and other applica- DEMO MANUAL DC1739A-B OPERATING PRINCIPLES The transition from primary to secondary control occurs Technology Application Note AN19 for a discussion of at some fraction of the nominal output voltage. From then input filter stability.) A source with a 300m or higher ESR on, operation and design is simplified to that of a simple at the filter resonant frequency (~100kHz) is one way of buck converter. Secondary control eliminates delays, tames providing damping for the filter elements provided on the large-signal overshoot, and reduces output capacitance DC1739A-B. For bench testing, an electrolytic capacitor needed to meet transient response requirements. has been added at the input terminals to provide suitable damping and ripple current capability. The values selected An optional LC filter stage on the input lowers RMS input have a filter resonant frequency that is below the converter current. The filter must have output impedance that is switching frequency, thus avoiding high circulating cur- less than the converter input impedance to assure stabil- rents in the filter. ity. This may require a damping impedance. (See Linear QUICK START PROCEDURE Demonstration circuit 1739 is easy to set up to evalu- 4. Turn on the power at the input. ate the performance of the LTC3765/LTC3766. Refer to NOTE. Make sure that the input voltage never exceeds 75V. Figure 1 for proper measurement equipment setup and 5. Check for the proper output voltage of 12V. Turn off the follow the procedure below: power at the input. NOTE. When measuring the output voltage ripple, care must 6. Once the proper output voltages are established, con- be taken to avoid a long ground lead on the oscilloscope nect a variable load capable of sinking 12.5A at 12V to probe. Measure the output voltage ripple by touching the the output terminals +V and V . Set the current probe tip and ground ring directly across the last output OUT OUT for 0A. capacitor as shown in Figure 1. a. If efficiency measurements are desired, an ammeter or 1. Set an input power supply that is capable of 18V to 72V a resistor shunt that is capable of handling 12.5ADC to 18V. Then turn off the supply. can be put in series with the output load in order to 2. Direct an airflow of 200LFM across the unit for sustained measure the DC1739A-Bs output current. operation at full load. b. A voltmeter with a capability of measuring at least 12V 3. With power off, connect the supply to the input terminals can be placed across the output terminals in order +V and V . to get an accurate output voltage measurement. IN IN 7. Turn on the power at the input. a. Input voltages lower than 18V can keep the converter from turning on due to the undervoltage lockout NOTE. If there is no output, temporarily disconnect the feature of the LTC3765/LTC3766. load to make sure that the load is not set too high. b. If efficiency measurements are desired, an ammeter 8. Once the proper output voltage is again established, capable of measuring 10ADC or a resistor shunt can adjust the load within the operating range and observe be put in series with the input supply in order to the output voltage regulation, ripple voltage, efficiency measure the DC1739A-Bs input current. and other desired parameters. c. A voltmeter with a capability of measuring at least 72V can be placed across the input terminals in order to get an accurate input voltage measurement. dc1739afb 2