Design Idea DI-69 DPA-Switch™ 15 W Multi-Output DC-DC Converter Application Device Power Output Input Voltage Output Voltage Topology VoIP Phone DPA424P 15 W 36-75 VDC 5 V / 7.5 V / 20 V Forward Design Highlights • Low component count • Built-in accurate OV/UV with single resistor programming • High efficiency (88%) using synchronous rectification • Multiple outputs with good cross-regulation • Built-in output overload, open loop and thermal protection • 400 kHz switching frequency • No primary current sense resistor required Operation Resistor R1 programs the input under/over voltages to 33 V and 86 V. It also linearly reduces the maximum device duty cycle (DCMAX) with increasing input voltage, helping to prevent core saturation during transients. Capacitor C1, C2 and L1 provide line filtering. Resistor R2 programs the U1 current limit. The Q22 reflected gate capacitance provides optimum reset for transformer T1. Zener VR1 clamps the peak drain voltage to a safe level during transient conditions. Resistors R21, R22, C21 and MOSFETs Q21 and Q22 form a capacitively coupled direct driven synchronous rectifier for the main (5 V) output. Zener diode VR21 acts both as a clamp and as a diode to quickly recharge the Q21 drive capacitor C21. Resistor R23 holds Q21 off when no switching signal is present. The 20 V output is generated from a flyback winding on the L2 inductor, which is rectified and filtered by diode D41 and C41. Zener VR31 and diode D32 provide a pre-load on the 7.5 V output to enhance regulation at light load. D41 BAV19WS J1-1 36-75 VDC L1 1 µH 2.5 A 1 D31 20CJQ060 8 + 3 DC Input Voltage R1 619 kΩ 1% Q22 Si4804 7 4 5 C2 1 µF 100 V L D DPA-Switch U1 DPA424P CONTROL VR1 SMBJ 150 RTN J1-2 DI-69 S 7 6 X C4 220 nF R2 13.3 kΩ 1% C31 100 µF 10 V C25 C22-C24 R4 1 µF 100 µF 5 V 160 Ω 10 V 5 8 7 J2-3 5 V, 2.4 A J2-2 2 D6 BAV 19WS RTN J2-1 C6 4.7 µF 20 V U2 PC357N1T C F J2-4 7.5 V, 0.4 A Q21 VR21 D21 Si4804 SL13 15 V R23 10 kΩ T1 C1 1 µF 100 V 3 L2 16 µH 4 A C21 2.2 nF R21 10 Ω R22 10 Ω 6 4 20 V, 10 mA C41 4.7 µF, 35 V VR31 D32 6.8 V IN4148 U2 R12 150 Ω D11 BAV19WS R3 1.0 Ω C5 47 µF 10 V R16 10.0 kΩ 1% R11 10 kΩ C11 2.2 µF 10 V U3 LM431AIM3 C12 100 nF R13 11 Ω C13 68 nF R14 1 kΩ R15 10.0 kΩ 1% PI-3799-082404 Figure 1. 15 W DC to DC Converter using DPA424P . www.powerint.com August 2004 DI-69 Key Design Points • Zener VR1 safely limits the DRAIN voltage below BVDSS. • At zero load, maximum input voltage, the bias voltage across C6 should be >8 V (12 V to 15 V under normal conditions). • The 5 V output is the regulated output. All other outputs are derived from it. Therefore, tight transformer secondary coupling and low PC board trace impedance from the 5 V output to the 7.5 V output are crucial to achieve good cross-regulation. • The transformer must reset completely under all line and load conditions to prevent core saturation. The gate capacitance of Q22 (with other parasitic circuit capacitances), determines the minimum reset period. The transformer core reset voltage also drives the secondary synchronous rectifier Q22. Efficiency is maximized when Q22 conduction period is maximized. This Q22 conduction period can be extended by adding more capacitance (an R-C snubber across the 5 V winding). The reset period should be set to give as long a reset as possible when operating at low-line (maximum duty cycle) while still guaranteeing complete reset. • Good layout practices should be followed: - Locate C4, C5 and R3 close to U1 with grounds returned to the SOURCE pin. - Minimize the primary and secondary loop areas to reduce parasitic leakage inductance. - Locate D21 and Q22 return connections very close to each other to provide best cross-regulation between 5 V and 7.5 V. VOUT 5 7.5 7 Voltage Range (VDC) 36-72 36-72 36-72 Load Cross-Regulation (%) Range (%) -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 20-100 0-100 100 Table 1. Worst Case Output Cross-Regulation - All Outputs Taken from Minimum to Maximum Load. TRANSFORMER PARAMETERS Core Material Ferroxcube P/N: PTS14/8-3F3, ungapped Bobbin 8 pin P1408 surface mount B&B B-096 or equivalent Winding Details Primary: 8T + 7T, 27 AWG, 5 V 4T, 4 x 28 AWG 7.5 V 2T, 4 x 28 AWG Winding Order (pin numbers) Primary-1 (4-3), 7.5 V (7-8), 5 V (5-6) Primary-2 (3-1) Primary Inductance 434 µH ±25% @ 100 kHz Primary Resonant 3.8 MHz (minimum) Frequency Leakage Inductance 1 µH (maximum) Table 2. Transformer Design Parameters. INDUCTOR PARAMETERS Core Material Bobbin Winding Details Winding Order (pin numbers) Inductance Epcos N87, P/N: B65755-J-R87, Gap for AL = 160 nH/T2 8 pin P1408 surface mount B&B B-096 or equivalent Bias 26T, 34 AWG 5 V 6T, 2 x 28 AWG 7.5 V 12T, 28 AWG 20 V 40T, 34 AWG 7.5 V (7-6), 5 V (8-5), 20 V (1-2), Bias (4-3) Pin (5-8): 16 µH ±10% @ 100 kHz Table 3. Inductor Design Parameters. For the latest updates, visit www.powerint.com Power Integrations may make changes to its products at any time. Power Integrations has no liability arising from your use of any information, device or circuit described herein nor does it convey any license under its patent rights or the rights of others. POWER INTEGRATIONS MAKES NO WARRANTIES HEREIN AND SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS. The products and applications illustrated herein (including circuits external to the products and transformer construction) may be covered by one or more U.S. and foreign patents or potentially by pending U.S. and foreign patent applications assigned to Power Integrations. A complete list of Power Integrations' patents may be found at www.powerint.com. The PI logo, TOPSwitch, TinySwitch, LinkSwitch and EcoSmart are registered trademarks of Power Integrations. PI Expert and DPA-Switch are trademarks of Power Integrations. Copyright 2004, Power Integrations Power Integrations MAIN PHONE NUMBER +1 408-414-9200 B 8/04 5245 Hellyer Avenue APPLICATIONS HOTLINE +1 408-414-9660 APPLICATIONS FAX +1 408-414-9760 www.powerint.com San Jose, California 95138 For a complete listing of worldwide sales offices, please visit www.powerint.com