QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-C ISOLATED DC/DC POWER CONVERTER LT1952 DESCRIPTION Demonstration circuit 968A-C is isolated input to high current output 1/8th Brick footprint converter featuring the LT®1952 switching controller. The DC968-C converts isolated 36V to 75V input to 12V output and provides over 12A of output current. The converter operates at 300kHz with efficiency up to 95%. With proper amount of airflow, the DC968-C converter can generate up to 15A of output current. 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-A 36-75Vin to 3.3V@30A DC968A-B 18-36Vin to 5V@20A 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. 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 the converter 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 12V ±3% Typical Output Ripple VOUT VIN = 36V to 72V, IOUT = 0A to 12A 100mVP–P Nominal Switching Frequency 300kHz QUICK START PROCEDURE Demonstration circuit 968 is easy to set up to evaluate the performance of LT1952-1 circuit. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: 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. NOTE: When measuring the input or output voltage ripple, care must be taken to avoid a long ground lead 1. With power off, connect the input power supply to Vin and GND. Make sure that the input power sup- 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-C ISOLATED DC/DC POWER CONVERTER ply has sufficient current rating at minimum input voltage for the required output load. 2. Turn on the power at the input. NOTE: Make sure that the input voltage does not exceed 75V. 3. Check for the proper output voltage. Vout = 12V, +/-3%. 4. Once the proper output voltage is established, ad- just the load within the operating range and observe the output voltage regulation, ripple voltage, efficiency and other parameters. 5. The DC968 is equipped with an output capacitor CSYS (150uF) that approximates typical system rail capacitance. If system board already has capacitance of similar value CSYS can be removed. 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-C ISOLATED DC/DC POWER CONVERTER Figure 2. Scope Probe Placement for Measuring Input or Output Ripple CHANGING THE OUTPUT VOLTAGE To get higher than 12V output voltages transformer with higher turns ratio may be required. Also, output MOSFETs with higher voltage ratings may be required. Please contact LTC factory for details. 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 DC968A-C. 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. LT1952 DC968-C 12V @12A output 96% 95% Efficiency To set the output voltage lower than 12V, change the bottom voltage divider resistor connected to LT4430 FB pin (see the simplified schematic on page 5). For example, to get 9V output, change R38 resistor value to 24.9k. However, keep in mind that changing the transformer as well may increase the efficiency. The schematic on page 6 shows all of the components on the PCB. The schematic on page 6 can be used as a reference if optional circuits need to be implemented. 94% 93% 92% 91% 90% 0 2 4 6 8 10 12 14 Iout [A] Figure 3. High efficiency of DC968A-C allows the board to be used in thermally critical applications with outputs up to 15A. OUTPUT LOAD STEP RESPONSE The load step response of DC968A-C is very fast even though relatively small amount of output capacitance is present (33uF ceramic and 150uF 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-C ISOLATED DC/DC POWER CONVERTER DEBUGGING AND TESTING The DC968 can easily be tested and debugged by powering the bias circuit with a separate power source. 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) connect a 10V, 100mA power source to collector of Q9 via 1A, 40V diode. That way the current coming from DC968 bias supply will not be able to flow back to 10V supply once the converter is running. 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 correct delay from turn on of FG and gate of Q1 is usually in the range from -100ns to +100ns. By slowly increasing the +Vin from 0V to 48V the output voltage and input current can be monitored. The input current should not exceed 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-C ISOLATED DC/DC POWER CONVERTER +Vb R1 +Vin L2 Vu1 DO1607B-155ML-C Q10 PA1393.302 7 R11 82k Cin1 Cin2 Cin3 2.2uF, 100V 6 D3 BAS516 D5 PDZ10B Q2 HAT2244 10 Vu1 1 C3 1u 2 3 C21 220p 1 33K 2 R15 133k 3 4 C10 0.1u 5 6 R22 7 +Vr2 442k 8 R28 442k 13.3k R32 BC857BF Q12 R14 +Vb TG Inp TS 5 L3 DO1608C-684ML-C C2 820p 5 4 Comp Sout FB=1.23V Vin Rosc Out Sync PGND MaxDC Delay Vr=2.5V OC SD Isense GND Blank 16 Sout 14 Rcs1 0.010R C9 12 11 10 R23 Vu1 R9 47R 100p R20 68.1k 2k C11 2.2n 9 R26 220R C13 1uF 4 R30 1.2k C15 CG GND Vcc FG Vaux R17 560R C7 220p 8 Sout 7 C8 100p 6 5 T2A PE-68386 FG R19 C14 1uF C12 470p 38.3K D11 BAS516 U6 PS2801-1 Vfb Sync CS- Timer 3 CG U5 LTC3900 CS+ 2 R31 10R C16 .22uF C23 1uF 1 R21 10k D9 PDZ7.5B R24 82K 158K R18 10k R10 1k Vu1 R8 2.2R R16 47k Q9 BCX55 Vaux Cu1 4.7u D10 D7 BAS521 D6 BAS521 C5 0.22u, 25V 15 13 CG GND BAS516 R33 R29 22K GND BAT760 R13 22K Vfb Boost 6 Csys 150uF, 16V, APXE + 2 Q1 PHM21NQ15 R3 57 D1 U1 LT1952-1 Vcc Q3 HAT2244 PA0423 +Vin U2 LTC4440 L1 T1 FG R5 10R +Vo1 +Vout BAS516 +Vr2 -Vin 1 PZTA42 2.2R 36V to 72V D2 Co1 33uF, 16V +Vin C19 1uF 1 2 3 U4 LT4430 R27 270R Vcc Opto GND Comp SS FB R38 18.2K 2.2nF, 2kV 5 C17 3.3n R34 8.45k 6 5 R35 348K 4 C18 10p +Vo1 C24 10p QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-C ISOLATED DC/DC POWER CONVERTER 6