QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A ISOLATED DC/DC POWER CONVERTER LT1952 DESCRIPTION Demonstration circuit 968A-A is isolated input to high current output 1/8th Brick footprint converter featuring the LT®1952 switching controller. The DC968 converts isolated 36V to 75V input to 3.3V output and provides over 30A of output current. The converter operates at 300kHz with efficiency greater than 91%. The DC968 can be easily modified to generate output voltages in the range from 0.6V to 28V. The output currents are limited by total output power of up to 200W. The other available versions of DC968A are: DC968A-B 18-36Vin to 5V@20A DC968A-C 36-75Vin to 12V@12A DC968A-D 36-75Vin to 28V@4A Also, the DC968 can be modified for other input voltages like 8V-36V, 18V-75V, 44V-52V, and so on. The wider input voltage range will decrease Table 1. Performance the converter efficiency. Therefore, narrow input voltage range will be more desirable. The DC968 circuit features soft-start which prevents output voltage overshoot on startup or when recovering from overload condition. The DC968 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. Design files for this circuit board are available. Call the LTC factory. LT is a trademark of Linear Technology Corporation Summary PARAMETER CONDITION VALUE Minimum Input Voltage IOUT = 0A to 30A 36V Maximum Input Voltage IOUT = 0A to 30A 75V VOUT VIN = 36V to 75V, IOUT = 0A to 30A 3.3V ±3% Typical Output Ripple VOUT VIN = 36V to 75V, IOUT = 0A to 30A 50mVP–P Nominal Switching Frequency 300kHz QUICK START PROCEDURE Demonstration circuit 968 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 directly across the Vin or Vout and GND terminals. See Figure 2. for proper scope probe technique. 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A ISOLATED DC/DC POWER CONVERTER 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 DC968 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 75V. 3. Check for the proper output voltage. Vout = 3.3V. If there is no output, temporarily disconnect the load to make sure that the load is not set too high. Figure 1. Proper Measurement Equipment Setup 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A ISOLATED DC/DC POWER CONVERTER Figure 2. Scope Probe Placement for Measuring Input or Output Ripple CHANGING THE OUTPUT VOLTAGE To get higher than 3.3V output voltages transformer with higher turns ratio will be required. Also, output MOSFETs with higher voltage ratings may be required. Please contact LTC factory for details. DC968-A 3.3V @30A Output Efficiency Efficiency To set the output voltage lower than 3.3V, change the bottom voltage divider resistor connected to LT4430 FB pin (see the schematic on page 5). For example, to get 2.5V output, change R38 resistor value to 26.1k. 92% 91% 90% 89% 88% 87% 86% 85% 84% 5 15 25 35 Iout [A] PRIMARY MOSFET DRIVER LTC4440 The DC968 has an LTC4440 MOSFET driver U2. The LTC4440 can be disabled if the efficiency is not important. If U2 is disabled, the DELAY pin of LT1952 should be readjusted to optimize the efficiency. Please contact LT factory for assistance. OPTIONAL RESET CIRCUIT The DC968 is equipped with an optional active reset circuit that is disabled on DC968AA. In most applications the circuit is not required because there is no benefit from active reset. However, if the active reset is required, adding few resistors can enable the circuit. Please consult LT factory for assistance. Figure 3. High efficiency of DC968A-A allows the board to be used in thermally critical applications OUTPUT LOAD STEP RESPONSE The load step response of DC968A-A is very fast even though relatively small amount of output capacitance is present (200uF 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. 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A ISOLATED DC/DC POWER CONVERTER DEBUGGING AND TESTING The DC968 can easily be tested and debugged by powering the bias circuit separately from the main power circuit. To place DC968 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. Figure 4. Fast transient response of DC968 is superior to many competing power modules without the additional output capacitors. SOFT START FUNCTION The DC968 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. To activate the secondary side MOSFET driver (LTC3900) 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. The delay from turn on of FG and gate of Q1 should be in the range from -100ns to +100ns. By slowly increasing the +Vin from 0V to 48V 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. Figure 5. The LT4430 opto coupler driver produces monotonic output voltage rise at startup without output voltage overshoot. 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A ISOLATED DC/DC POWER CONVERTER R1 +Vin 1.5mH Vu1 2.2R D3 BAS516 +Vr2 36V to 72V R11 82k Cin1 Cin2 Cin3 2.2uF, 100V -Vin L1 PA1671.650 1 7 6 10 +Vout Q2 HAT2165 Q1 PHM21NQ15 U2 LTC4440 Vcc 2 GND TG Inp TS 4 1 Vfb 2 Q12 BC857BF R15 115k 3 R42 0R 4 SS C10 0.1u 5 6 R22 7 13.3k 8 R32 332k R28 442k +Vr2 +Vb C2 1n, 200V D7 B0540W D8 B0540W Comp Sout FB=1.23V Vin Rosc Out Sync PGND MaxDC Delay Vr=2.5V SD GND OC Isense Blank 16 14 13 12 11 R33 158k Vu1 10 D10 C9 D6 B0540W Cu1 1uF R20 72.8K 100p R18 10k C5 0.22u, 25V R23 910 1 2 R21 10k Q9 BCX55 Vaux 3 CG 4 R10 1k R19 38.3K U5 LTC3900 CS+ Sync CS- Timer CG GND Vcc FG C7 220p 8 Sout C12 470p 7 6 5 C8 100p FG R17 560R T2A PE-68386 C14 1u, 10V D9 PDZ7.5B 9 SS R29 22.1K Sout 15 Vaux R16 33k C6 0.1u U1 LT1952-1 R14 33K R8 2.2R C4 0.1u 5 BAT760 R13 22K GND 6 Rcs1 3 Boost CG 5 0.015R 1 C3 1u Csys 470uF, 6.3V + 2 +Vin Vu1 Q3 HAT2165 D5 PDZ10B FG R5 10R +Vo1 100uF, X7R +Vb PA0861.004 Co1 Co2 +Vin T1 D2 BAS516 L2 Q10 PZTA42 C13 .47u R24 39K D11 R26 220R U6 PS2801-1 Vfb BAS516 Vaux R31 10R Vu1 C16 .22u, 10V C23 1uF 1 2 3 R30 1.2k R27 270R Vcc Opto GND Comp SS C19 1uF, X5R C15 2.2nF, 2kV U4 LT4430 FB C17 15n R34 1.96k 6 5 R35 82.5k 4 +Vo1 C18 10p C24 47p R38 18.2K 5 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-A ISOLATED DC/DC POWER CONVERTER 6