QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776 36V-72VIN, FORWARD CONVERTER LTC3705 and LTC3706 DESCRIPTION Demonstration circuit 776 is a 36V-72Vin, forward converter featuring the LTC3705 and LTC3706. This circuit was designed specifically to attain a high stepdown ratio power supply in one stage in order to efficiently power 1.5V loads from a typical telecom input voltage range. Isolation voltage is 1500VDC. This circuit features secondary-side control of the supply without the need of an optocoupler, selfstarting architecture, input undervoltage lockout, and output overvoltage protection. Design files for this circuit board are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. Table 1. Performance Summary (TA = 25C) PARAMETER CONDITION VALUE Minimum Input Voltage 36V Maximum Input Voltage 72V Output Voltage VOUT Maximum Output Current Typical Output Ripple VOUT Output Regulation Dynamic Response VIN = 36V to 72V, IOUT = 0A to 45A, 400LFM 1.5V 1% 400LFM 45A 200LFM 40A VIN = 48V, IOUT = 30A (20MHz BW) 30mVP-P Line 0.01% Load 0.18% Peak Deviation 65mV Load Step of 20A to 40A (1A/us min) Settling Time (to within 15mV of set point) Nominal Switching Frequency Efficiency On/Off Control Isolation Voltage 50us 215kHz VIN = 48V, IOUT = 45A, 400LFM 84.75% Typical Logic Low Voltage-Off 1V MAX Logic High Voltage-On 2.5V MIN Basic Insulation 1500VDC OPERATING PRINCIPLES The LTC3706 controller is used on the secondary and the LTC3705 driver with self-starting capability is used on the primary. When an input voltage is applied, the LTC3705 begins a controlled soft-start of the output voltage. As this voltage begins to rise, the LTC3706 secondary controller is quickly powered up via T1, D3, and Q13. The LTC3706 then assumes control of the output voltage by sending encoded 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776 36V-72VIN, FORWARD CONVERTER PWM gate pulses to the LTC3705 primary driver via the small signal transformer, T3. The LTC3705 then operates as a simple driver receiving both input signals and bias power through T3. The transition from primary to secondary control occurs seamlessly at a fraction of the output voltage. From that point on, operation and design simplifies to that of a simple buck converter. Secondary sensing eliminates delays, tames large-signal overshoot and reduces output capacitance. This, while utilizing offthe-shelf magnetics and attaining high efficiency. For large values of input inductance, a 100V, 47uF electrolytic capacitor can be added across the input terminals to damp the input filter and provide adequate stability. See Linear Technology Application Note AN19 for a discussion on input filter stability analysis. A recommended part is the Sanyo 100MV39AX. QUICK START PROCEDURE Demonstration circuit 776 is easy to set up to evaluate the performance of the LTC3705 and LTC3706. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: NOTE: When measuring the input or output voltage ripple, care must be taken to avoid a long ground lead on the oscilloscope probe. Measure the output (or input) voltage ripple by touching the probe tip and probe ground directly across the +Vout and -Vout (or +Vin and -Vin) terminals. See Figure 2 for proper scope probe technique. 1. Set an input power supply that is capable of 36V to 72V to a voltage of 36V. Then turn off the supply. 2. With power off, connect the supply to the input terminals +Vin and -Vin. a. 3. Input voltages lower than 36V can keep the converter from turning on due to the undervoltage lockout feature of the LTC3705. b. If efficiency measurements are desired, an ammeter capable of measuring 3Adc can be put in series with the input supply in order to measure the DC776A's input current. 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. NOTE: Make sure that the input voltage never ex- ceeds 72V. 4. Check for the proper output voltage of 1.5V 1%. Turn off the power at the input. 5. Once the proper output voltages are established, connect a variable load capable of sinking 50A at 1.5V to the output terminals +Vout and -Vout. Set the current for 0A. 6. a. If efficiency measurements are desired, an ammeter or a resistor shunt that is capable of handling 50Adc can be put in series with the output load in order to measure the DC776A's output current. b. A voltmeter with a capability of measuring at least 2V can be placed across the output terminals in order to get an accurate output voltage measurement. Turn on the power at the input. NOTE: If there is no output, temporarily disconnect the load to make sure that the load is not set too high. 7. Once the proper output voltage is again established, adjust the load within the operating range and observe the output voltage regulation, ripple voltage, efficiency and other desired parameters. Turn on the power at the input. 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776 36V-72VIN, FORWARD CONVERTER Figure 1. Proper Measurement Equipment Setup GND VIN Figure 2. Measuring Input or Output Ripple 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776 36V-72VIN, FORWARD CONVERTER MEASURED DATA Figures 3 through 13 are measured data for a typical DC776A. 90 88 Efficiency (%) 86 84 82 80 36VIN 78 48VIN 76 72VIN 74 72 70 5 10 15 20 25 30 35 40 45 50 Current (A) Figure 3. Efficiency (400LFM) 1.565 Regulation (V) 1.545 1.525 1.505 36VIN 1.485 48VIN 1.465 72VIN 1.445 1.425 5 10 15 20 25 30 35 40 45 50 Current (A) Figure 4. Regulation (400LFM) 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776 36V-72VIN, FORWARD CONVERTER Figure 5. Output Voltage Ripple (72Vin and 50A) Figure 6. Load Transient Response (>1A/us) 5 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776 36V-72VIN, FORWARD CONVERTER Figure 7. Temp Data (48Vin, 45A, 400LFM - front) Figure 8. Temp Data (48Vin, 45A, 400LFM - back) 6 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776 36V-72VIN, FORWARD CONVERTER Figure 9. Temp Data (48Vin, 40A, 200LFM - front) Figure 10. Temp Data (48Vin, 40A, 200LFM - back) 7 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776 36V-72VIN, FORWARD CONVERTER Figure 11. Turn-on Waveforms (72Vin and 50A) Figure 12. Turn-off Waveforms (72Vin and 50A) 8 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776 36V-72VIN, FORWARD CONVERTER Figure 13. Control Loop Bode Plot (48Vin and 20A) 9 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776 36V-72VIN, FORWARD CONVERTER 10