QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479 ISOLATED SYNCHRONOUS FORWARD CONVERTERQUARTER BRICK LT3781 and LTC1698 DESCRIPTION Demonstration circuit 479 is an isolated synchronous lockout. Cycling short circuit protection minimizes ther- forward converter featuring the LT3781 and LTC1698 mal stress. The output overvoltage circuit provides pro- controllers. DC479 is designed to be a board level re- tection for the load should a fault occur on the sense placement forquarter-bric DC/DC converters. The de- lines. The standard footprint allows for immediate on sign can provide 3.3V at 15A from an isolated 48V (36V board evaluation by plugging directly into the modules to 72V) input. Isolation voltage is 1500V DC. The circuit socket. features low input capacitance, over temperature protec- Design files for this circuit board are available. Call tion, soft start with input undervoltage and overvoltage the LTC factory. Table 1. Performance Summary. T =25C, V =48V, full load, ON/OFF and TRIM pins open, +SENSE shorted to +VOUT, SENSE shorted A IN to VOUT, unless otherwise specified. PARAMETER CONDITION MIN TYP MAX UNITS Input Voltage Range 36 48 72 V Maximum Input Current V = 36V, Full Load 1.6 A IN 2 Inrush Transient V = 72V 0.2 A s IN Reflected Ripple Current 100 mA PP Output Voltage 3.24 3.30 3.36 V Output Regulation Line 0.1 % Load 0.2 % Output Current 15 A Output Current Limit 18 A Output Short Circuit Cycling, Auto-restart 1000 ms Output Ripple and Noise RMS 15 mV RMS Peak-to-peak (5Hz to 20MHZ) 40 60 mV PP Efficiency 88.5 % Dynamic Response Peak Deviation 50 100 mV Load Step 50% to 100% Settling Time (to within 10mV of set point) 100 200 s Output Voltage Trim V = 3.3V 4 5 6 % TRIM V = 0V 6 5 4 % TRIM Output Overvoltage 3.65 V On/Off Control Logic Low Voltage: Off 0 0.6 V Logic High Voltage: On 1.0 V Logic Low Current : Off 0.2 mA Quiescent Current: Off 1.4 mA Start-up Inhibit Period 7.5 ms Turn on Time 10 15 ms 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 479 ISOLATED SYNCHRONOUS FORWARD CONVERTERQUARTER BRICK PARAMETER CONDITION MIN TYP MAX UNITS Thermal Shutdown At RT1 100 C Isolation Voltage 1500 V DC Isolation Resistance 10 M Isolation Capacitance 2200 pF OPERATING PRINCIPLES CIRCUIT OVERVIEW chronizes with the LT3781 via T2, a small pulse trans- former. The LTC1698 includes an error amplifier and op- This two-transistor forward converter operates at a tocoupler drive buffer, eliminating the output feed- nominal switching frequency of 240 kHz. Pulse width forward path associated with 431 type references. U1 modulation control is done by U3, the LT3781 synchro- also provides output overvoltage protection. The margin nous forward controller. Galvanic isolation is met with pin allows the output voltage to be adjusted 5%. transformer T1 and optocoupler ISO1. C10 is used as a local bypass to reduce common mode induced current. During an output short circuit condition, the primary bias supply at Vcc collapses. This results in the con- The main switching power path through T1 is comprised verter harmlessly cycling on and off, keeping power dis- of L1, C2 and C3 as the input filter, with Q1 and Q3 as sipation to a minimum. The cycling rate is nominally 1Hz the primary switches. MOSFETs Q4, 5, 6, and 7 are the with 48V input. When the short is removed, the con- secondary synchronous rectifiers. L3 and C4-7 are the verter returns to normal operation. secondary output filter. Power is transferred during the on cycle of Q1 and Q3, and integrated by the output fil- The demo board uses all surface mount devices and will ter, just as in a buck regulator. D1 and D2 recover energy deliver the full rated current at room temperature. With stored in the leakage inductance of T1 during the off cy- elevated temperature operation, airflow is required for cle. The input filter component values for L1, C2 and C3 full rated load. The demo board features thermal over- are optimal and should not be changed without careful load protection. evaluation. C1 bypasses the input terminals. For large For 48V inputs requiring hot swap capability, the values of input inductance, an external aluminum elec- LT4250H negative voltage hot swap controller provides a trolytic capacitor will damp the input filter and provide seamless interface. adequate stability. See Linear Technologys application note AN19 for a discussion on input filter stability analy- OPTIONAL FAST START CIRCUIT sis. When power is first applied, Vcc must rise to 15V for the When the primary switches turn off, the transformer LT3781 to turn on. The bias supply turn on threshold voltage reverses, with D1 and D2 conducting to reset the and hysteresis are set internally by U3. R8 and 9 charge transformer during normal operation. A startup or tran- the 100F capacitor C25, and are gated by Q9. With sient to no load can cause the pulse width modulation to 200 resistance, the charge time is 7.5ms at 48V in. narrow, with insufficient energy to force the reset diodes The values for R8 and R9 can be adjusted in order to into conduction. When this occurs, the charge on C20 change the turn on delay. Values lower than 100 for gets depleted and the top gate drive shuts off. This will each resistor will result in abnormally high peak power, result in the converter cycling on and off. To overcome and possible component failure. Once the LT3781 turns this, Q10 provides a return path to refresh the top gate on, the 5Vref charges C12 causing Q11 to turn off Q9. boost capacitor C20. Bias supply power is delivered through L2 by a winding Feedback control of the output voltage and synchronous on T1. In the event of an output short circuit, the volt- drive is done using U1, the LTC1698. The LTC1698 syn- age on the transformer bias winding collapses. Restart 2