(R) Design Idea DI-53 DPA-SwitchTM 50 W DC-DC Dual Output Converter Application Device Power Output Input Voltage Output Voltage Topology Telecom DPA425R 50 W 36-75 VDC 5 V & 3.3 V Forward Sync. Rect. recirculate the transformer leakage energy during normal operation, with Zener VR1 providing absolute clamping for transient conditions. Design Highlights * High efficiency: 90% at 36 VDC using synchronous rectification * Dual output with tight cross-regulation 4% from zero to full load on both outputs * Output overload, open loop and thermal protection * 300 kHz switching frequency to allow sufficient transformer reset time with sync rectification * 3.85 x 2.25 x 0.6 inch (~9.62 W/in3) Capacitor C21 charges the gate of Q2, the forward synchronous rectifier MOSFET of the 5 V output. Resistor R21 limits gate oscillation and R22 provides gate pull-down. Zener diode VR20 limits the Q2 gate voltage during conduction and also reverse charges C21 during the Q2 off time. The same drive technique is used for the forward synchronous rectifier MOSFET (Q4) of the 3.3 V output (with C22, R24, R25, and VR21). Operation DPA-Switch greatly simplifies the design compared to a discrete implementation. This design uses a coupled output inductor and synchronous rectification to achieve excellent cross-regulation and high efficiency. MOSFETs Q1 and Q3 are driven via resistors R23 and R26 from the transformer (T1) reset voltage and operate only when Q2 and Q4 are off. Diodes D20 and D21 provide a conduction path for the output inductor (L4) current when the transformer reset is complete. Resistor R1 programs the under/over voltages and linearly reduces the maximum duty cycle with input voltage to prevent core saturation during load transients. Components D1, D2, C9 and L2 implement a resonant clamp circuit to catch and T1 + VIN 36-75 VDC A winding on the coupled inductor L4, along with diode D4 and capacitor C9, provide the DPA-Switch bias voltage. L4 3.5 H 9,10 7,8 R23 10 8,9 L1 1 H 2.5 A Q1 D20 R26 10 4,5 D1 ES1D R20 3.3 2,3 10 C9 150 pF 200 V R25 10 k C21-C22 4700 pF R21 10 R24 10 Q4 C23-C26 100 F 10 V 6,7 5 U1 DPA425R D C9 4.7 F 16 V VR1 SMBJ 150 A X F R3 15 k 1% C5 0.22 F R4 1.0 C6 68 F 10 V RTN R12 15.9 k 1% C16 100 nF D3 BAV19WS R6 150 R12 5.1 C D2 ES1D S R10 8.66 k 1% U2 U2 PC267 N1T L CONTROL C28 1 F D4 R7 10 k DPA-Switch C27 1 F 6 C7 1 nF 1.5 kV C1-C3 1 F 100 V -VIN D21 VR21 15 V C20 Q2 R22 VR20 3.3 nF 10 k 15 V L2 220 H 3.3 V, 6 A (8 A pk) 3,4 Q3 1 R1 619 k 1% 1,2 5 V, 6 A (8 A pk) R9 220 C15 10 F 10 V C14 1 F U3 LM431 AIM3X R11 10.0 k 1% PI-3561-072203 Figure 1. DPA425R - 50 W, 5 V, 6 A and 3.3 V, 6 A DC-DC Converter. . DI-53 www.powerint.com July 2003 DI-53 Key Design Points TRANSFORMER PARAMETERS * Capacitors C20, CQ1gs and CQ3gs will all load transformer reset. Choose values to ensure sufficient reset at low line and safe maximum drain voltage at high line. Also use 300 kHz operation for longest reset time. * Capacitors C21 and C22 will capacitively drive MOSFET gate capacitances CQ2gs and CQ4gs (respectively). C21 and C22 should be chosen to ensure that gate drive voltage attains turn-on threshold of MOSFET (VgTH), at worst case conditions (low line for forward MOSFET). * Reduce transformer leakage inductance by filling each winding layer across the entire width of the bobbin. * Higher efficiency (+1%) can be acheived by using a DPA426R and increasing R3 to reduce the internal current limit. PI-3562-061603 95 Efficiency (%) 90 Core Material Ferroxcube P/N: EFD25-3F3, ungapped Bobbin 10-pin EFD25 surface mount bobbin Winding Details Primary 11T, 4 x 28 AWG 3.3 V 2T, 2 x 4 x 26 AWG 5 V 3T, 2 x 4 x 26 AWG Winding Order and Pin Numbers 5 V (6, 7-8, 9), Primary (1-10), 3.3 V (4.5-2.3) Primary Inductance 250 H 25% at 300 kHz Primary Resonant Frequency 3.8 MHz (minimum) Leakage Inductance 0.8 H (maximum) Table 1. Transformer Design Parameters. INDUCTOR PARAMETERS 85 80 75 36 VDC 54 VDC 72 VDC 70 Core Material Ferroxcube P/N: EFD25-3F3 ungapped Bobbin 10-pin EFD20 surface mount bobbin Winding Details 5 V 6T, 2 x 4 x 26 AWG 3.3 V 4T, 2 x 4 x 26 AWG Bias 12T, 1 x 30 TIW Winding Order and Pin Numbers 5 V (9, 10-7, 8), 3.3 V (1, 2-3, 4), Bias (FL1-FL2) Inductance Pin (1, 2-3, 4): 3.5 H 10% at 300 kHz 65 60 2 4 6 8 10 IOUT1 + IOUT2 (A) Figure 2. Efficiency vs. Output Power. A 7/03 12 Table 1. Inductor Design Parameters. www.powerint.com