QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1031A-C 36V-72VIN, SYNCHRONOUS FORWARD CONVERTER LTC3725 / LTC3726 DESCRIPTION Demonstration circuit 1031A-C is a 36V-72Vin, syn- trol of the supply eliminating the need for an opto- chronous forward converter featuring the coupler, self-starting architecture, input undervoltage LTC3725/LTC3726. This circuit was designed spe- lockout, and output overvoltage protection. Design files for this circuit board are available. cifically to attain a high current, low ripple, synchro- Call the LTC factory. nously rectified forward to efficiently power 5.0V loads at up to 20A from a typical telecom input volt- , LTC and LT are registered trademarks of Linear Technology Corporation. age range. This circuit features secondary-side con- Table 1. Performance Summary (T = 25C) A PARAMETER CONDITION VALUE Minimum Input Voltage 36V Maximum Input Voltage 72V Output Voltage V V = 36V to 72V, I = 0A to 20A 5.0V OUT IN OUT Maximum Output Current 200LFM Airflow 20A Typical Output Ripple V V = 72V, I = 20A 100mV OUT IN OUT PP Size Component Area x Top Component Height 2.3 x 0.9 x 0.394 Peak Deviation with Load Step of 10A to 20A (10A/us) 200mV Load Transient Response Settling Time 40us Nominal Switching Frequency 300kHz Efficiency V = 48V, I = 20A 91.5% Typical IN OUT OPERATING PRINCIPLES The LTC3726 controller is used on the secondary and The transition from primary to secondary control oc- the LTC3725 driver with self-starting capability is curs seamlessly at a fraction of the output voltage. used on the primary. When an input voltage is ap- From that point on, operation and design simplifies to plied, the LTC3725 begins a controlled soft-start of that of a simple buck converter. Secondary sensing the output voltage. As this voltage begins to rise, the eliminates delays, tames large-signal overshoot and LTC3726 secondary controller is quickly powered up reduces output capacitance while utilizing off-the- via T1, D1, and Q27. The LTC3726 then assumes shelf magnetics and attaining high efficiency. control of the output voltage by sending encoded For large values of input inductance, a 100V, 47uF elec- PWM gate pulses to the LTC3725 primary driver via trolytic capacitor can be added across the input termi- the small signal transformer, T2. The LTC3725 then nals to damp the input filter and provide adequate stabil- operates as a simple driver receiving both input sig- ity. See Linear Technology Application Note AN19 for a nals and bias power through T2. discussion on input filter stability analysis. A recom- mended part is the Sanyo 100MV39AX. 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1031A-C 36V-72VIN, SYNCHRONOUS FORWARD CONVERTER QUICK START PROCEDURE Demonstration circuit 1031A-C is easy to set up to 3. Turn on the power at the input. evaluate the performance of the LTC3725/LTC3726. NOTE: Make sure that the input voltage never ex- Refer to Figure 1 for proper measurement equipment ceeds 72V. setup and follow the procedure below: 4. Check for the proper output voltage of 5.0V NOTE: When measuring the input or output voltage 5. Turn off the power at the input. ripple, care must be taken to avoid a long ground lead on the oscilloscope probe. Measure the output (or 6. Once the proper output voltages are established, input) voltage ripple by touching the probe tip and connect a variable load capable of sinking 20A at probe ground directly across the input or output ca- 5.0V to the output terminals +Vout and Vout. Set pacitor. See Figure 2 for proper scope probe tech- the current for 0A. nique. a. If efficiency measurements are desired, an am- 1. Set an input power supply that is capable of 36V to meter or a resistor current shunt that is capable 72V at a current of at least 3.5A to a voltage of 36V. of handling at least 20Adc can be put in series Then, turn off the supply. with the output load in order to measure the DC1031A-Cs output current. 2. With power off, connect the supply to the input terminals +Vin and Vin. b. A voltmeter with a capability of measuring at least 5.0V can be placed across the output ter- a. Input voltages lower than 36V can keep the con- minals in order to get an accurate output voltage verter from turning on due to the undervoltage measurement. lockout feature of the LTC3725/LTC3726. 7. Turn on the power at the input. b. If efficiency measurements are desired, an am- meter capable of measuring 3.5Adc can be put NOTE: If there is no output, temporarily disconnect in series with the input supply in order to meas- the load to make sure that the load is not set too ure the DC1031A-Cs input current. high. c. A voltmeter with a capability of measuring at 8. Once the proper output voltage is established, ad- least 72V can be placed across the input termi- just the load within the operating range and ob- nals in order to get an accurate input voltage serve the output voltage regulation, ripple voltage, measurement. efficiency and other desired parameters. 2