DEMO MANUAL DC1739B-C LTC3765/LTC3766 120W Isolated Forward Converter with Synchronous Rectification DESCRIPTION Demonstration circuit 1739B-C is a 120W isolated forward nous rectification helps to attain efficiency exceeding 94%. converter with synchronous rectification featuring the Secondary-side control eliminates complex opto-coupler LTC 3765/LTC3766 chip set. feedback, providing fast transient response with minimum output capacitance. For other output requirements, see the This circuit was designed to demonstrate the high level of LTC3766 data sheet or contact the LTC factory. performance, efficiency, and small solution size attainable using this chip set in an active-clamp-reset forward con- Design files for this circuit board are available at verter power supply. It operates at 240kHz and produces DEMO MANUAL DC1739B-C OPERATING PRINCIPLES The LTC3765 active clamp forward controller and gate buck converter. Secondary control eliminates delays, tames driver is used on the primary and provides start-up, gate large-signal overshoot, and reduces output capacitance drive, and protection functions. Once start-up is accom- needed to meet transient response requirements. plished, the LTC3766 high efficiency, secondary-side An optional LC filter stage on the input lowers rms input synchronous forward controller takes over, and provides current. The filter must have output impedance that is the LTC3765 with timing information and bias power less than the converter input impedance to assure stabil- through a small pulse transformer. ity. This may require a damping impedance. (See Linear When input voltage is applied, the LTC3765 commences Technology Application Note 19 for a discussion of input soft-start of the output voltage. When the secondary bias filter stability.) A source with a 170m or higher ESR source reaches the undervoltage threshold, the LTC3766 at the filter resonant frequency (~35kHz) is one way of comes alive and takes control by sending encoded PWM providing damping for the filter elements provided on the gate pulses to the LTC3765 through T3. These pulses DC1739B-C. For bench testing, an electrolytic capacitor also provide primary bias power efficiently over a wide has been added at the input terminals to provide suitable input voltage range. damping and ripple current capability. The values selected have a filter resonant frequency that is below the converter The transition from primary to secondary control occurs switching frequency, thus avoiding high circulating cur- at some fraction of the nominal output voltage. From then rents in the filter. on, operation and design is simplified to that of a simple QUICK START PROCEDURE Demonstration circuit 1739B-C is easy to set up to evalu- b. If efficiency measurements are desired, an ammeter ate the performance of the LTC3765/LTC3766. Refer to capable of measuring 10ADC or a resistor shunt can Figure 1 for proper measurement equipment setup and be put in series with the input supply in order to follow the procedure below: measure the DC1739B-Cs input current. Note: When measuring the output voltage ripple, care must c. A voltmeter with a capability of measuring at least be taken to avoid a long ground lead on the oscilloscope 36V can be placed across the input terminals in order probe. Measure the output voltage ripple by touching the to get an accurate input voltage measurement. probe tip and ground ring directly across the last output 4. Turn on the power at the input. capacitor as shown in Figure 1. Note: Make sure that the input voltage never exceeds 36V. 1. Set an input power supply that is capable of 9V to 36V 5. Check for the proper output voltage of 12V. Turn off to 9V. Then turn off the supply. the power at the input. 2. Direct an airflow of 200lfm across the unit for sustained 6. Once the proper output voltages are established, con- operation at full load. nect a variable load capable of sinking 10A at 12V to 3. With power off, connect the supply to the input terminals the output terminals +VOUT and VOUT. Set the current +VIN and VIN. for 0A. a. Input voltages lower than 9V can keep the converter from turning on due to the undervoltage lockout feature of the LTC3765 / LTC3766. dc1739bcf 2