QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 894C-B 36V-72VIN, SYNCHRONOUS FLYBACK LT3825 DESCRIPTION Demonstration circuit 894C-B is a 36V-72Vin, ulation without the need of an optocoupler, synchronous flyback converter featuring the self-starting architecture and input undervolt- LT3825. This circuit was designed specifically age lockout. to attain a high current, low ripple, synchro- Design files for this circuit board are avail- nously rectified flyback to efficiently power 5.0V able. Call the LTC factory. loads at up to 8A from a typical telecom input , LTC and LT are registered trademarks of Linear Technology Cor- voltage range. This circuit features synchro- poration. nous rectifier drive outputs, output voltage reg- 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 8A 5.0V OUT IN OUT Maximum Output Current 8A Output Ripple V V = 48V, I = 8A < 20mV (typical) OUT IN OUT PP Output Regulation Over All Input Voltages and Output Currents 1.0% (typical) Peak Deviation with Load Step of 6A to 8A (10A/us) 100mV (< 2%) Load Transient Response Settling Time 100us Nominal Switching Frequency 200kHz Efficiency V = 48V, I = 8A 92% (typical) IN OUT OPERATING PRINCIPLES The LT3825 controller exhibits a self-starting Modulated (PWM) in order to keep the output capability. When an input voltage is applied, a voltage constant. The synchronous gate drive trickle charge resistor, R8, charges C10 (See signal is transmitted to the secondary via the Figure 9) to power Vcc. Then, the IC begins a small signal transformer, T2. The output of T2 controlled soft-start of the output voltage. As then drives a discreet gate drive buffer, R26, this voltage begins to rise, Vcc power is quickly Q12, and Q13 in order to achieve fast gate taken over by T1, D2, and R7. When the soft- transition times, hence a higher efficiency. start period is over, the LT3825 then regulates The two-stage input filter, C25, L1, and C30 and output voltage by observing the pulses across output filter, C1, C2, C5, L2, and C29 are the the auxiliary winding of T1 during the flyback reasons that this flyback has exceptionally low time. The Primary Gate drive (PG) and Syn- conducted emissions. chronous Gate (SG) drive is then Pulse Width 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 894C-B 36V-72VIN, SYNCHRONOUS FLYBACK QUICK START PROCEDURE Demonstration circuit 894C-B is easy to set up 3. Turn on the power at the input. to evaluate the performance of the LT3825. Re- NOTE: Make sure that the input voltage never fer to Figure 1 for proper measurement equip- exceeds 72V. ment setup and follow the procedure below: 4. Check for the proper output voltage of 5.0V NOTE: When measuring the input or output volt- 5. Turn off the power at the input. age ripple, care must be taken to avoid a long ground lead on the oscilloscope probe. Meas- 6. Once the proper output voltages are estab- ure the output (or input) voltage ripple by touch- lished, connect a variable load capable of ing the probe tip and probe ground directly sinking 8A at 5.0V to the output terminals across the +Vout and Vout (or +Vin and Vin) +Vout and Vout. Set the current for 0A. terminals. See Figure 2 for proper scope probe a. If efficiency measurements are desired, an technique. ammeter or a resistor current shunt that is 1. Set an input power supply that is capable of capable of handling at least 8Adc can be 36V to 72V at a current of at least 2A to a put in series with the output load in order to voltage of 36V. Then, turn off the supply. measure the DC894C-Bs output current. 2. With power off, connect the supply to the in- b. A voltmeter with a capability of measuring put terminals +Vin and Vin. at least 5.0V can be placed across the out- put terminals in order to get an accurate a. Input voltages lower than 36V can keep the output voltage measurement. converter from turning on due to the un- dervoltage lockout feature of the LT3825. 7. Turn on the power at the input. b. If efficiency measurements are desired, an NOTE: If there is no output, temporarily dis- ammeter capable of measuring 2Adc can connect the load to make sure that the load is be put in series with the input supply in or- not set too high. der to measure the DC894C-Bs input cur- 8. Once the proper output voltage is estab- rent. lished, adjust the load within the operating c. A voltmeter with a capability of measuring range and observe the output voltage regula- at least 72V can be placed across the input tion, ripple voltage, efficiency and other de- terminals in order to get an accurate input sired parameters. voltage measurement. 2