QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-E ACTIVE RESET ISOLATED DC/DC POWER CONVERTER LT1952 DESCRIPTION Demonstration circuit 1317A-E is isolated input to high current output 1/8th Brick footprint converter featuring the LT(R)1952 switching controller with Active Reset circuit. The Active Reset circuit can improve the efficiency in wide input voltage applications. Also, the Active Reset allows the implementation of self-driven synchronous secondary rectifiers in some applications. The DC1317A-E converts isolated 36V to 72V input to 5V output and provides over 12A of output current. The converter operates at 260kHz with the peak efficiency greater than 94% at 8A load. The DC1317 can be easily modified to generate output voltages in the range from 0.6V to 48V. The output currents are limited by total output power of up to 150W. The other available versions of DC1317A are: DC1317A-F, 9-36Vin to 3.3V, 20A DC1317A-G, 9-36Vin to 12V, 5A DC1317A-H, 9-36Vin to 48V, 1.5A The DC1317 circuit features soft-start which prevents output voltage overshoot on startup or when recovering from overload condition. The DC1317 has precise over-current protection circuit that allows for continuous operation under short circuit conditions. The low power dissipation under short circuit conditions insures high reliability even during short circuits. The LT1952 can be synchronized to an external clock of up to 400kHz. Please refer to LT1952 data sheet for design details and applications information. DC1317A-A, 36-72Vin to 3.3V, 30A DC1317A-B, 18-72Vin to 5V, 25A DC1317A-C, 18-72Vin to 12V, 10A Design files for this circuit board are available. Call the LTC factory. DC1317A-D, 18-72Vin to 24V, 5A LT is a trademark of Linear Technology Corporation Table 1. Performance Summary PARAMETER CONDITION VALUE Minimum Input Voltage IOUT = 0A to 12A 36V Maximum Input Voltage IOUT = 0A to 12A 72V VOUT VIN = 36V to 72V, IOUT = 0A to 12A 5V 3% Typical Output Ripple VOUT VIN = 36V to 72V, IOUT = 0A to 12A 50mVP-P Nominal Switching Frequency 260kHz QUICK START PROCEDURE Demonstration circuit 1317 is easy to set up to evaluate the performance of LT1952 circuit. 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 input or output voltage ripple by touching the probe tip di- 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-E ACTIVE RESET ISOLATED DC/DC POWER CONVERTER rectly across the Vin or Vout and GND terminals. See Figure 2. for proper scope probe technique. If there is no output, temporarily disconnect the load to make sure that the load is not set too high. 1. With power off, connect the input power supply to Vin and GND. Make sure that the input power supply has sufficient current rating at minimum input voltage for the required output load. 4. Once the proper output voltage is established, adjust the load within the operating range and observe the output voltage regulation, ripple voltage, efficiency and other parameters. 2. Turn on the power at the input. 5. The DC1317 is equipped with an output capacitor CSYS (470uF) that approximates typical system rail capacitance. If system board already has capacitance of similar value CSYS can be removed. NOTE: Make sure that the input voltage does not exceed 72V. 3. Check for the proper output voltage. Vout = 5V. Figure 1. Proper Measurement Equipment Setup 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-E ACTIVE RESET ISOLATED DC/DC POWER CONVERTER Figure 2. Scope Probe Placement for Measuring Input or Output Ripple PRIMARY MOSFET DRIVER LTC4440 The DC1317 has an optional LTC4440 MOSFET driver U2. The LTC4440 can be used if much bigger MOSFETs are used. Please contact LT factory for assistance. ACTIVE RESET CIRCUIT The DC1317 is equipped with an active reset circuit that consists of Q13 and C25. The capacitor C25 charges to the appropriate transformer reset voltage. The MOSFET Q13 applies the C25 reset voltage across the primary of transformer T1. The C25 reset voltage varies with the duty cycle and automatically adjusts to provide volt-second balance for transformer magnetizing current. OUTPUT LOAD STEP RESPONSE The load step response of DC1317A-E is very fast even though relatively small amount of output capacitance is present (100uF ceramic and 470uF electrolytic). This is thanks to fast error amplifier of LT4430, optimal amount of current slope compensation of LT1952, fast opto coupler and fast error amplifier of LT1952. If higher load steps need to be handled more output capacitance can be added in order to keep the voltage transients at the desired level. The load step transients are shown in Figure 4. Efficiency LT1952 Efficiency (DC1317A-E) Self Driven SR, 5V output 95% 94% 93% 92% 91% 90% 89% 88% 87% 86% Figure 4. Fast transient response of DC1317A-E is superior to many competing power modules without the additional output capacitors. 0 2 4 6 8 10 12 14 Iout [A] Figure 3. High efficiency of DC1317A-E allows the board to be used in thermally demanding applications SOFT START FUNCTION The DC1317 features LT4430 opto coupler driver that has soft start function which produces monotonic startup ramp shown if Figure 5. The rise time of output voltage is controlled by capacitor C19 that is connected to OC (overshoot control) pin of LT4430. 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-E ACTIVE RESET ISOLATED DC/DC POWER CONVERTER components that can be removed. Also, the PCB layout has a common schematic that is used just for the layout. The actual circuit schematic shows component values. In some cases, a resistor is used inplace holder for a capacitor as in case of C4. Please modify the reference designators in your schematic to reflect the actual component used. The following simple PCB layout rules should be helpful. Figure 5. The LT4430 opto coupler driver produces monotonic output voltage rise at startup without output voltage overshoot. DEBUGGING AND TESTING The DC1317 can easily be tested and debugged by powering the bias circuit separately from the main power circuit. To place DC1317 into debug mode remove the resistor R1 and connect 48V, 100mA power source to +Vb node (right side of R1). By doing this, the primary PWM controller LT1952 can be activated without the main primary power being applied to +Vin. To activate the secondary side feedback circuit LT4430 diode OR a 10V, 100mA power source into collector of Q9. Once the primary and secondary controllers are running the MOSFET gate timing can be checked. If the MOSFET gate timing is correct the main power input can be applied to +Vin. By slowly increasing the +Vin from 0V to 48V the output voltage and input current can be monitored. The input current should not exceeded 100mA without the output load. If one of the MOSFETs is damaged, the input current will exceed 100mA. If possible use solid ground planes on layers 2 and NThe ground planes will prevent the switching noise from coupling into sensitive lines. Place sensitive lines on the inner layers that will be shielded by grounds on layers 2 and N-1. Keep the loop formed by Q1, RCS1, Cin and T1 tight. Keep the loop formed by Q2, Q3 and T1 tight. Keep noise sensitive nodes like SD/VSEC, ROSC, FB, COMP, ISENSE, BLANK and DELAY as small as posible by placing the associated components close to the LT1952 and LT4430 chips. Use local vias for all components that connect to ground planes. Do not place any traces on the layers 2 and N-1 to avoid ground planes from being compromised. If the PCB layout has to be done on 2 or 4-layer PCB use the same guidelines as outlined above. Also, maximize the ground connections between components by placing the components tight together. Please contact LT factory for additional assistance. PCB LAYOUT The pcb layout should be carefully planned to avoid potential noise problems. The PCB layout for DC1317A can be used as a guide. Since demo board DC1317A has 8 versions the PCB layout has optional 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-E ACTIVE RESET ISOLATED DC/DC POWER CONVERTER +Vin 2.2R R11 82k Cin1 Cin3 2.2uF, 100V C25 C33 0.1u Q13 Si2325 R13 22K BC856T R14 33K U1 LT1952-1 1 2 Q12 R15 115k 3 R42 C10 0.1u 0R 4 SS 5 6 R22 7 13.3k R32 332k 8 R28 442k +Vr2 +Vb DC1317A-E schematic 9, 10 GND 2 4 Q1 Si7818 Comp Sout FB=1.23V Vin Rosc Out Sync PGND MaxDC Delay Vr=2.5V OC SD Isense GND Blank 16 Vu1 15 14 T1 PA2720 Q2 Q3 FDMC8854 FDMC8854 R8 100R 13 C9 12 100p 11 10 R33 158k R20 75k 9 R24 39K U6 PS2801-1 Vfb D11 BAS516 U4 LT4430 1 R26 220R C23 1uF R23 910 D6 BAS516 Rcs1 0.025R Cu1 4.7u 5V, 12A + D14 BAS516 SS R29 22K 5 +Vo1 +Vout 33n, 200V, 1206 BAS516 Vfb 6, 7 L1 3.1uH R50 3.3R +Vin R45 10k D17 1 D5 PDZ10B -Vin R53 200 D2 BAS516 D3 BAS516 +Vr2 36V to 72V 1.5mH Csys 470uF, 6.3V R1 L2 Q10 PBSS8110 Vu1 +Vb Co1 100uF, X7R +Vin C16 .22u, 10V 2 3 R30 1.2k R27 270R Vcc Opto GND Comp SS C19 0.33u FB 6 5 4 C17 10n R34 5.36k R35 137k R38 18.2K C15 2.2nF, 2kV NOTE: C13 .47u This schematic shows only the components required for operation of -E version of DC1317A demo board. All optional components of DC1317A demo board have been removed. Also, all zero-ohm resistors have been replaced with wires. Please consult the full DC1317A-E schematic to decide if any of the optional components should be included in your design. 5 +Vo1 C24 47p QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-E ACTIVE RESET ISOLATED DC/DC POWER CONVERTER 6 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1317A-E ACTIVE RESET ISOLATED DC/DC POWER CONVERTER 7