QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 541A 36-72VIN TO 3.3V 40A AND 2.5V 10A, ISOLATED PUSH-PULL CONVERTER LTC3723-1, LTC3901, LT3710 and LT1431 DESCRIPTION Demonstration circuit 541A is an isolated synchro- Design files for this circuit board are available. Call nous push-pull converter featuring the LTC3723-1, the LTC factory. LTC3901, LT1431 and LT3710. The design provides an isolated 3.3V at 40A and 2.5V at 10A from 48V LTC is a trademark of Linear Technology Corporation (36V to 72V) input. Isolation voltage is 1500VDC. The circuit features low input capacitance, input under- voltage lockout and short circuit cycling protection to minimize thermal stress. Table 1. Performance Summary (T = 25C) V =48V, full load, unless otherwise specified. A IN PARAMETER CONDITION VALUE Minimum Input Voltage 36V Maximum Input Voltage 72V Output Voltage (V V = 36V to 72V, I = 0A to 40A 3.3V2% OUT) 3.3V IN OUT Output voltage (Vout) 2.5V V = 36V to 72V, I = 0A to 10A 2.5V2% IN OUT Maximum Input Current V = 48V, 3.3Vat 40A, 2.5V at 10A 3.58A IN 2 Inrush Transient V = 72V 0.5 A s IN Maximum Output Current for 3.3Voutput 40A Maximum Output Current for 2.5Voutput 10A Maximum Output Current for 3.3Voutput With 2.5V output disabled 50A Nominal Switching Frequency 180kHz Output Short Circuit Period Cycling, Auto-restart at 48V, 3.3V shorted 160ms Dynamic Response Peak Deviation with 10A to 30A load step 300mV Settling Time (to within 10mV of set point) 100s Efficiency V = 48V, I = 40A 90% Typical IN OUT Output Ripple 3.3V output V = 48V, I = 40A (100MHz BW) 60mV ripple, 150mV spikes IN OUT Output Ripple 2.5V output V = 48V, I = 10A (100MHz BW) 30mV ripple, 100mV spikes IN OUT Isolation Voltage 1500 VDC Isolation Resistance 10 M Isolation Capacitance 2200 pF 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 541A 36-72VIN TO 3.3V 40A AND 2.5V 10A, ISOLATED PUSH-PULL CONVERTER OPERATING PRINCIPLES CIRCUIT OVERVIEW Figures 6 and 7 detail the temperature rise for the hottest components in the design with and without This push-pull converter operates at a nominal airflow. Based on these measurements and assuming switching frequency of 180 kHz. Pulse width modula- a 50C ambient, it is recommended that the output tion control is done by U1, the LTC3723-1 controller. power be limited to 85W without airflow and 180W Galvanic isolation is met with transformer T1, T2 and with 300 linear feet per minute of airflow. ISO1. C30 is used as a local bypass to reduce com- mon mode currents. When input voltage is applied, R29 provides trickle charge current to C14, resulting in a turn on delay of The primary side power path is comprised of T1, C3, approximately 600ms at 36Vin. C4, C5 Q8 and Q11 as the primary switches. Power is transferred during the on time of Q8 and Q11. MOS- Figure 5 shows efficiency. FETs Q12-Q15, Q23 and Q24 are the secondary syn- chronous rectifiers. L6, and C32-C34 form the secon- SAFETY AND ISOLATION dary output filter. L5, C3, C4 and C5 form the primary The demo board is designed to meet the require- rd input filter. C2 bypasses the input terminals. For large ments of UL 60950, 3 edition for basic insulation in values of input inductance, an external 47uF alumi- secondary circuits. The transformer is designed to num electrolytic capacitor will damp the input filter meet the basic insulation requirement with an isola- and provide adequate stability. See Linear Technology tion voltage of 1500VDC. Application Note AN19 for a discussion on input filter stability analysis. CONDUCTED EMI Tests for conducted emissions were performed for An auxiliary winding on T1 provides bias voltage to the demo board. An external filter using a 47F alu- the LTC3723-1. U1, the LTC3901, synchronizes with minum electrolytic capacitor, 1mH Common Mode the LTC3723-1 via T2, a small pulse transformer, to inductor and 10F film capacitor were used for the provide gate drive to secondary switching MOSFETs. CISPR 22 class B limit. No tests for radiated RFI were During an output short circuit, the primary bias sup- performed. Proper grounding and layout technique ply collapses. This results in the converter harmlessly must be observed to minimize radiation. See Figure 4 cycling on and off, reducing power dissipation to a for EMI test setup. For EMI graphs see Figures 8 and minimum. The cycling rate is nominally 6.25Hz with 9. 48V input. When the short is removed, the converter returns to normal operation. DC541A relies on the pcb area and 300 linear feet per RELIABILITY minute of airflow to provide full load operation to Reliability prediction for the circuit has been calcu- 50C ambient without the use of a heat sink. The lated using the Telcordia (formerly Bellcore) SR-332. maximum output power is primarily limited by com- The black box technique was used. The calculation ponent temperature rise. For example, for continued was made assuming a grounded, fixed, controlled reliability, temperature should be kept below 110 C environment and quality level II. A 50% electrical and the magnetics temperature rise should be limited stress at 40C yields an MTBF (mean time between to 60C. Assuming 50C ambient, this corresponds to failures) of 1.5 million hours. 60C surface mount component temperature rise. 2