QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1032A 36V-72VIN, ISOLATED SYNCHRONOUS FORWARD LTC3725/LTC3726 DESCRIPTION Demonstration circuit 1032A is a high power isolated output voltages see DC1031A-A/B/C (2.5V/3.3V/5V at synchronous forward converter featuring the 20A). For higher powers see DC888A-A/B/C (3.3V at LTC3725 and LTC3726 When powered from a 36- 50A / 5V at 35A / 12V at 20A) which also support 72V input, a single DC1032A provides an isolated polyphase operation for even higher powers. The 12V at 12A in a eight-brick footprint. The converter simple architecture can be easily modified to meet operates at 300kHz and achieves efficiency up to different input and output voltage requirements. 94.0% with synchronous output rectifiers. Secon- Design files for this circuit board are available. dary-side control eliminates complex optocoupler Call the LTC factory. feedback, providing fast transient response with a , LTC and LT are registered trademarks of Linear Technology Corporation. minimum amount of output capacitance. For other 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 12A 12V OUT IN OUT Maximum Output Current 200LFM 12A Typical Output Ripple V V = 48V, I = 12A, 300kHz 100mV OUT IN OUT PP Output Regulation Over All Input Voltages and Output Currents 1% (Reference) Peak Deviation with 0A to 12A Load Step (10A/us) 600mV Load Transient Response Settling Time 50us Nominal Switching Frequency 300kHz Efficiency V = 48V, I = 12A 93% Typical IN OUT Isolation BASIC 1500VDC Approximate Size Component Area x Top Component Height 2.3 x 0.9 x 0.34 OPERATING PRINCIPLES SINGLE PHASE The LTC3726 secondary side controller is used on the PWM gate pulses to the LTC3725 primary driver via secondary and the LTC3725 smart driver with self- signal transformer, T2. The LTC3725 then operates as starting capability is used on the primary. When an a simple driver receiving both input signals and bias input voltage is applied, the LTC3725 (U1 in Figure power through T2. The transition from primary to 14), which is powered through R29 and Q28, begins secondary control occurs seamlessly. From that point a controlled soft-start of the output voltage by switch- on, operation and design simplifies to that of a simple ing MOSFETs Q8 and Q11. As the output voltage be- buck converter. The LTC3726 regulates by observing gins to rise, the LTC3726 secondary controller is the output voltage directly resulting in superior output powered up via Q27. The LTC3726 then assumes voltage regulation and transient response. control of the output voltage by sending encoded 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1032A 36V-72VIN, ISOLATED SYNCHRONOUS FORWARD QUICK START PROCEDURE Demonstration Circuit 1032A is easy to set up to phase can be put in series with the input supply evaluate the performance of the LTC3725 and in order to measure the DC1032As input cur- LTC3726. Refer to Figure 1 for proper equipment rent. setup. Follow the procedure below: c. A voltmeter with a capability of measuring 72V NOTE: When measuring the input or output voltage can be placed across the input to get an accurate ripple, care must be taken to avoid a long ground lead input voltage measurement. on the scope probe. Measure the output (or input) 4. Turn on the power at the input. voltage ripple by touching the probe tip and probe NOTE: Make sure that the input voltage 72V. ground directly across the +Vout and Vout (or +Vin and Vin) terminals. See 0 for proper scope probe 5. Check for the proper output voltage of 12V. technique. 6. Turn off the power at the input. 1. The optional input LC filter stage (C2/L1) lowers ac 7. Once the proper output voltages are established, input rms current. A power supplys complete in- connect a variable load capable of sinking 12A at put filter must have output impedance that is less 12V to the output terminals +Vout and Vout. Set than the converter input impedance to assure sta- current to 0A. bility. This may require a damping impedance. (See Linear Technology Application Note AN19 for a a. If efficiency measurements are desired, an am- discussion of input filter stability.) A source with a meter or a resistor current shunt that is capable 50mOhm or higher ESR at the filter resonant fre- of handling at least 12Adc can be put in series quency is one way of providing damping for the fil- with the output load in order to measure the ter elements provided on the DC1032A. For bench DC1032As output current. testing, adding an 82uF electrolytic capacitor such b. A voltmeter with a capability of measuring at as a Sanyo 100ME82AX to the input terminals will least 12V can be placed across the output termi- provide suitable damping and ripple current capa- nals in order to get an accurate output voltage bility. The values selected have a filter resonant measurement. frequency that is below the converter switching frequency, thus avoiding high circulating currents 8. Turn on the power at the input. in the filter. NOTE: If there is no output, disconnect the load to 2. Set an input power supply to a voltage of 36V. verify that the load is not set too high. Make sure that it is capable of 36V to 72V at a cur- 9. Once the proper output voltage is established, ad- rent supplying capability of at least 5A. Then, turn just the load within the operating range and ob- off the supply. serve the output voltage regulation, ripple voltage, 3. With power off, connect the supply to the input efficiency and other desired parameters. terminals +Vin and Vin. a. Input voltages lower than 36V can keep the con- verter from turning on due to the undervoltage lockout feature of the LTC3725. b. If efficiency measurements are desired, an am- meter capable of measuring at least 5Adc per 2