DEMO C IR C U IT 1 1 4 3 Q U IC K S TA R T GLT3825 U IDE LT3 8 2 5 Is o la te d F ly b a c k C o n v e rte r w ith S y n c h ro n o u s R e c tific a tio n DESCRIPTION Demonstration circuit 1143 is a 20 W att Isolated Flyback Converter w ith Synchronous R ectification and Primary-Side R egulation featuring the LT3825. cation helps to attain efficiency exceeding 88%. Isolation voltage is 1500VDC. This circuit w as designed to demonstrate the high levels of performance, efficiency, and small solution size attainable using this part in a flyback pow er supply. It operates at 250kH z and produces a regulated 3.3V, 6A output from an input voltage range of 36 to 72V:suitable for telecom and other applications. It has a footprint area that is 1.0 inch square. Synchronous rectifi- Design files for this circuit board are available. Call the LTC factory. P ER F O R M A N C E S U M M A R Y SYM BO L VIN VOUT IOUT FSW VOUT P-P IREG POUT/PIN PAR AM ETER Input Supply R ange O utput Voltage O utput Current R ange Sw itching (Clock) Frequency O utput R ipple O utput R egulation Efficiency (see Figure 2) , LTC, LTM , LT, Burst M ode, O PTI-LOO P, Over-The-Top and PolyPhase are registered trademarks of Linear Technology Corporation. Adaptive Pow er, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Sw ap, LinearView , µM odule, M icropow er Sw itcherCAD, M ultimode Dimming, No Latency , N o Latency Delta-Sigma, N o R SEN SE, O perational Filter, PanelProtect, Pow erPath, Pow erSOT, SmartStart, SoftSpan, Stage Shedding, Sw itcherCAD, ThinSOT, UltraFast and VLDO are trademarks of Linear Technology Corporation. O ther product names may be trademarks of the companies that manufacture the products. Specifications are at TA = 25°C CO N DITIO N S M IN 36 VIN = 36 –72V 0 TYP M AX 72 3.3 VIN = 48V, IOUT = 6A (20M H z BW ) Line and Load (36-72V, 0-6A) VIN =48V, IOUT = 6A 6 250 20 ±1.2 88.5 U N ITS V V A kH z mVP–P % % O P ER A T IN G P R IN C IP L ES The LT3825 Synchronous Flyback PW M Controller is used on the primary and drives a secondary-side M O SFET through a pulse transformer to provide a synchronous rectified output. W hen an input voltage is applied, an undervoltage circuit keeps the LT3825 in its quiescent state w hile a resistor charges Cvcc (C10) to 15V. The controller is then enabled, and start-up commences. The primary circuit operates from the charge stored in Cvcc until the housekeeping w inding of T1 starts to support Vcc. W hen a heavy overload or short-circuit prevents T1 supporting Vcc, the converter operates in ‘burpmode’, cutting off w hen Vcc declines to 10V, maintaining low pow er dissipation in the circuit. The LT3825 provides a synchronous rectifier gate drive signal w hich is passed to the secondary through T2 and subsequently buffered. R egulation is attained by observing the voltage on the housekeeping w inding of T1 during the Flyback time, and Pulse W idth M odulating (PW M ) the Primary G ate drive (PG ) and Synchronous G ate drive (SG ). The 1 LT3825 LT3825 is programmed to compensate for circuit resistance that is outside of the control loop. O ptional LC filter stages on the input and output facilitate low noise. Q U IC K S T A R T P R O C ED U R E Demonstration circuit 1143 is easy to set up to evaluate the performance of the LT3825. R efer to Figure 1 for proper measurement equipment setup and follow the procedure below : NOTE. W hen measuring the output voltage ripple, care must be taken to avoid a long ground lead on the oscilloscope probe. M easure the output voltage ripple by touching the probe tip and ground ring directly across the last output capacitor (C29) as show n in Figure 1. 1. Set an input pow er supply that is capable of 36V to 72V to 48V. Then turn off the supply. 2. W ith pow er off, connect the supply to the input terminals +Vin and –Vin. a. Input voltages low er than 36V can keep the converter from turning on due to the undervoltage lockout feature of the LT3825. b. If c. 4. Check for the proper output voltage of 3.3V. Turn off the pow er at the input. 5. O nce the proper output voltages are established, connect a variable load capable of sinking 6A at 3.3V to the output terminals +Vout and –Vout. Set the current for 0A. a. If efficiency measurements are desired, an ammeter or a resistor shunt that is capable of handling 6Adc can be put in series w ith the output load in order to measure the DC1143A’s output current. b. A voltmeter w ith a capability of measuring at least 3.3V can be placed across the output terminals in order to get an accurate output voltage measurement. efficiency measurements are desired, an ammeter capable of measuring 1Adc or a resistor shunt can be put in series w ith the input supply in order to measure the DC1143A’s input current. 6. Turn on the pow er at the input. A voltmeter w ith a capability of measuring at least 72V can be placed across the input terminals in order to get an accurate input voltage measurement. 7. O nce the proper output voltage is again established, adjust the load w ithin the operating range and observe the output voltage regulation, ripple voltage, efficiency and other desired parameters. 3. Turn on the pow er at the input. 2 NOTE. M ake sure that the input voltage never exceeds 72V. NOTE. If there is no output, temporarily disconnect the load to make sure that the load is not set too high. LT3825 Figure 1. Proper M easurem ent Equipm ent Setup 3 LT3825 DC1143A Lt3825 3.3V-6A Efficiency 90 Efficiency in % 86 82 Vin=36V 78 Vin=48V Vin=72V 74 70 66 0.0 1.0 2.0 3.0 4.0 5.0 6.0 Output Curre nt in Am pe re s Figure 2. Efficiency – N ote 88% efficiency over a w ide current range. 3.45 Output Voltage (V) 3.40 3.35 36Vin 3.30 48Vin 72Vin 3.25 3.20 3.15 0 0.6 1.2 1.8 2.4 3 3.6 4.2 4.8 5.4 Load Current (A) Figure 3. Regulation – N ote regulation band of 1.5% 4 6 LT3825 Figure 4. Output Ripple at 48Vin and 6Aout (20M H z) - 2uS and 20m V /div Figure 5. Transient R esponse W aveform at 48Vin and 5 - 6Aout – 200uS, 5A, and 200m V /div 5 LT3825 6