QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 617 ISOLATED SYNCHRONOUS FORWARD CONVERTER - QUARTER BRICK LT3781 and LTC1698 DESCRIPTION Demonstration circuit 617 is an isolated synchronous and overvoltage lockout. Cycling short circuit protec- forward converter featuring the LT3781 and LTC1698 tion minimizes thermal stress. The output overvoltage controllers. DC617 is designed to be a board level circuit provides protection for the load should a fault replacement forquarter-bric DC/DC converters. The occur on the sense lines. The standard footprint al- design can provide an isolated 2.5V at 20A from a lows for immediate on board evaluation by plugging 48V (36V to 72V) input. Isolation voltage is 1500VDC. directly into the modules socket. The circuit features low input capacitance, over tem- Design files for this circuit board are available. Call perature protection, soft start with input undervoltage the LTC factory. Table 1. Performance Summary. T =25C, V =48V, full load, ON/OFF and TRIM pins open, +SENSE shorted to +VOUT, -SENSE A IN shorted 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 P-P Output Voltage 2.40 2.50 2.60 V Output Regulation Line 0.1 % Load 0.2 % Output Current 20 A Output Current Limit 24 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 P-P Efficiency 86.1 % 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 2.70 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 617 ISOLATED SYNCHRONOUS FORWARD CONVERTER - QUARTER BRICK PARAMETER CONDITION MIN TYP MAX UNITS Thermal Shutdown At RT1 100 C Isolation Voltage 1500 VDC Isolation Resistance 10 M Isolation Capacitance 2200 pF OPERATING PRINCIPLES CIRCUIT OVERVIEW Secondary side bias supply power is provided by T3, a small inexpensive gate drive transformer. This steps This two-transistor forward converter operates at a up the voltage across the output inductor L3 produc- nominal switching frequency of 240kHz. Pulse width ing about 7V. Feedback control of the output voltage modulation control is done by U3, the LT3781 syn- and synchronous drive is done using U2, the chronous forward controller. Galvanic isolation is met LTC1698. The LTC1698 synchronizes with the LT3781 with transformers T1, T2 and optocoupler ISO1. C10 via T2, a small pulse transformer. The LTC1698 in- is used as a local bypass to reduce common mode cludes an error amplifier and optocoupler drive currents. buffer, eliminating the output feed-forward path asso- The main switching power path is comprised of C2 ciated with 431 type references. U2 also provides and C3 feeding the primary winding of T1, with Q1 output overvoltage protection. The margin pin allows and Q3 as the primary switches. MOSFETs Q4, 5, 6, the output voltage to be adjusted 5%. and 7 are the secondary synchronous rectifiers. L3 During an output short circuit condition, the primary and C4-7 are the secondary output filter. Power is bias supply at Vcc collapses. This results in the con- transferred during the on cycle of Q1 and Q3. D1 and verter harmlessly cycling on and off, keeping power D2 recover energy stored in the leakage inductance of dissipation to a minimum. The cycling rate is nomi- T1 during the off cycle. The input inductor L1 was nally 1Hz with 48V input. When the short is removed, chosen for its relatively low parallel impedance, the converter returns to normal operation. which helps to damp the input filter. C1 bypasses the input terminals. For large values of input inductance, The demo board uses all surface mount devices and an external 12F 100V low ESR aluminum electrolytic will deliver 15A at room temperature with no airflow. capacitor will damp the input filter and provide ade- To prevent thermal shutdown, utilize the curve in Fig- quate stability. See Linear Technologys application ure 11. The thermal shutdown at RT1 is set for note AN19 for a discussion on input filter stability 100C. To run at 20A out with 50C ambient will re- analysis. quire at least 200LFM of airflow. Figure 6 through Figure 10 show other component temperature rise. When the primary switches turn off, the transformer Building the circuit on a multi-layer system board can voltage reverses, with D1 and D2 conducting to reset provide improved thermal performance. Typical effi- the transformer during normal operation. A startup or ciency is shown in Figure 5. transient to no load can cause the pulse width modu- lation to narrow, with insufficient energy to force the For 48V inputs requiring hot swap capability, the reset diodes into conduction. When this occurs, the LT4250H negative voltage hot swap controller pro- charge on C20 gets depleted and the top gate drive vides a seamless interface. shuts off. This will result in the converter cycling on and off. To overcome this, Q10 provides a return path OPTIONAL FAST START CIRCUIT to refresh the top gate boost capacitor C20. When power is first applied, Vcc must rise to 15V for the LT3781 to turn on. The bias supply turn on 2