DEMO MANUAL DC2199B-C LTC3765/LTC3766 300W Isolated Forward Converter with Synchronous Rectification DESCRIPTION Demonstration circuit 2199B-C is a 300W isolated forward approaching 96%. Secondary-side control eliminates converter with synchronous rectification featuring the complex opto-coupler feedback, providing fast transient LTC3765/LTC3766 chip-set. It produces a regulated 56V, response with minimum output capacitance. For other 5.4A output from an input voltage range of 36V to 60V. output requirements, see the LTC3766 data sheet or contact the LTC sales. This circuit was designed to demonstrate the high levels of performance, efficiency, and small solution size attain- Design files for this circuit board are available at able using these parts in an active-clamp-reset forward DEMO MANUAL DC2199B-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 less the LTC3765 with timing information and bias power than the converter input impedance to assure stability. through a small pulse transformer. This may require a damping impedance, which is provided When input voltage is applied, the LTC3765 commences by R1. (See Linear Technology Application Note 19 for a soft-start of the output voltage. When the output reaches discussion of input filter stability.) R1 is coupled through the RUN threshold, the LTC3766 comes alive and takes a tiny 2mm 2mm inductor L1, and provides damping control by sending encoded PWM gate pulses to the with arbitrarily low source impedance. For bench testing, LTC3765 through T2. These pulses also provide primary an electrolytic capacitor has been added at the input ter- bias power efficiently over a wide input voltage range. minals to provide suitable ripple current capability. The values selected have a filter resonant frequency that is The transition from primary to secondary control occurs below the converter switching frequency, thus avoiding at some fraction of the nominal output voltage. From then high circulating currents in the filter. on, operation and design is reduced to that of a simple QUICK START PROCEDURE Demonstration circuit 2199B-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 DC2199B-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 60V 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 ex- 1. Set an input power supply that is capable of 36V to 60V ceeds 60V. to 36V. Then turn off the supply. 5. Check for the proper output voltage of 56V. Turn off 2. Direct an airflow of 200LFM across the unit for sustained the power at the input. operation at full load. 6. Once the proper output voltages are established, con- 3. With power off, connect the supply to the input terminals nect a variable load capable of sinking 5.4A at 56V to +V and V . IN IN the output terminals +V and V . Set the current OUT OUT a. Input voltages lower than 36V can keep the converter for 0A. from turning on due to the undervoltage lockout a. If efficiency measurements are desired, an ammeter feature of the LTC3765/LTC3766. or a resistor shunt that is capable of handling 5.4ADC can be put in series with the output load in order to measure the DC2199B-Cs output current. dc2199bcfa 2