Quasi-Isolated Flyback; 12V Car Accessory Plug to 5V @ 1.2A Reference Design PRD1205 FEATURES • • • • • Input - Output partially isolated to prevent ground loops Direct primary-seconday feedback through differential op amp provides accurate Vout control Better than 82% efficiency Frequency foldback for short circuit ruggedness Withstands brief 40V load dump input. DESCRIPTION This is a 10V - 14 input isolated flyback converter which uses the ADP1621 controller and an AD8691 operational amplifier. The target application of this converter is for powering a portable media player from the accessory power outlet of an automobile. The input-output isolation (-4V to +14V on audio output ground relative to input power ground) is provided to prevent ground loops between the power accessory plug and the connection from the media player to the car audio system. The AD8691 op amp operates as a 0.25 gain differential amplifier for feedback. This amplifier tolerates this small isolation voltage with tight output voltage regulation from 0 mA to full load, but without the disadvantages that accompany an optocoupler-based feedback scheme. If the differential voltage between input ground and output ground exceeds the design limits, the op amp common mode input range can be violated which would cause the output to lose regulation. To prevent damage caused by common mode voltages beyond the design specification, zener diode D4 shuts down the converter via the ADP1621 SDSN pin during excessive audio ground excursions in the negative direction, and D5 accomplishes the same for positive excursions. Resistors R31 and R32 are available to bring the common mode range into symmetry. Placing R31 = R32 = 27.4K 1% will improve the common mode range to make it approximately -12V to +12V. Diodes D4 and D5 will need to be adjusted to afford appropriate protection. For the widest common mode range, replace U2 with a RRIO type such as AD8515 and use R31 = R32 = 13.3K. When the converter is running normally, U2 output turns on Q2 to cause the ADP1621 to operate at 500 KHz. During output short circuit conditions, low U2 output turns off Q2 which lowers the switching frequency to 100 KHz so as to provide "frequency foldback" to reduce the output current for short circuit ruggedness. The schematic diagram includes two alternative snubber designs. C9, D6, R11, R12 form a damped RCD clamp, which is the more-conventional approach. Instead of these parts, C2 = 10 nF and R20 = 10 ohms 1206 can be used as a SEPIC-related snubber. This provides a slight advantage in efficiency and BOM cost but it couples some additional common mode noise into the output. C3 should be increased to at least 33 nF or so in order to bypass common mode output current if the SEPIC snubber option is used. Rev. 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Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2009 Analog Devices, Inc. All rights reserved. Reference Design Table 1. PRD1205 Target Specifications (0 to 55 degrees C) Rail Volts Min Volts Nom Volts Max Current +5V 4.9 5 5.1 0 - 1.2 Amps DC Vinput 30 45 60 ---- TABLE OF CONTENTS Features ......................................................................................................................................................................1 Description .................................................................................................................................................................1 Revision History.........................................................................................................................................................2 TABLE OF FIGURES Figure 1. Schematic Diagram of Flyback Converter.............................................................................................3 Figure 2. Foldback Current Limiting (measured using SEPIC snubber) ..............................................................6 REVISION HISTORY 02/23/2010—Revision A: Tested Prototype Rev. A | Page 2 of 7 Reference Design Figure 1. PRD1205 Schematic Diagram of Flyback Converter Table 2. Bill Of Materials Sequence Ref Designation Description 1 C2 DNP 2 C3 10 nF 50V X7R 3 C4 1 uF X5R 10V 0603 4 C5 1 uF X5R 10V 0603 5 C6 100 nF X7R 16V 0603 6 C7 2.2 uF GRM31CR71H225KA88 7 C8 2.2 nF X7R 25V 0603 8 C9 100 nF 9 C10 22 uF 10V X5R 1206 10 C11 22 uF 10V X5R 1206 Rev. A | Page 3 of 7 Reference Design PRD1205 Sequence Ref Designation Description 11 C12 1 nF X7R 25V 0603 12 C15 DNP 13 C16 DNP 14 C19 10 nF 50V X7R 15 D1 PDS1040L 16 D2 BAV70 17 D3 MMSD4148 18 D4 MMBZ5231 19 D5 MMBZ5246 20 D6 MMSD4148 21 L1 MSD1278 4.7 uH 22 Q1 SUD23N06 Vishay 23 Q2 MMBT3904 24 Q3 MMBTA06 25 Q4 MMBT3904 26 R1 Res 0.033 ohms 5% RL1220T-R033-J 27 R2 Res 40.2K 0603 1% 28 R3 Res 40.2K 0603 1% 29 R4 Res 237 0603 1% 30 R5 Res 10.0K 0603 1% 31 R6 Res 10.0K 0603 1% 32 R7 Res 221 ohms 1% 33 R8 Res 10.0K 0603 1% 34 R9 Res 10K 0603 5% 35 R11 Res 10 ohms 1206 36 R12 Res 1K 1206 5% 37 R13 Res 1.6K 1206 5% Rev. A | Page 4 of 7 Reference Design PRD1205 Sequence Ref Designation Description 38 R14 Res 200K 0603 1% 39 R15 Res 20K 0603 5% 40 R16 Res 1K 0603 5% 41 R17 Res 10K 0603 5% 42 R18 Res 4.7K 0603 5% 43 R19 Res 33 Ohms 0603 5% 44 R20 DNP 45 R21 Res 20K 0603 5% 46 R25 Res 1K 0603 5% 47 R27 Res 47.5K 0603 1% 48 R29 DNP 49 R30 DNP 50 R31 DNP 51 R32 DNP 52 U1 ADP1621 53 U2 AD8691 Rev. A | Page 5 of 7 Reference Design Figure 2. PRD1205 Foldback Current Limiting (measured using SEPIC snubber) Measured Limit Current 3.000E+00 Measured Limit Current 2.500E+00 Ilimit 2.000E+00 1.500E+00 1.000E+00 5.000E-01 0.000E+00 0.00E+00 1.00E+00 2.00E+00 3.00E+00 4.00E+00 5.00E+00 6.00E+00 Vout Table 3. Spot Efficiency Measurement Vin Amps Input Watts Input 1.207E+01 6.040E-01 7.290E+00 Watts Output Efficiency Vout Amps Output 4.997E+00 1.200E+00 5.996E+00 82.26% Rev. A | Page 6 of 7 Reference Design PRD1205 NOTES ©2009 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. Error! Unknow Rev. A | Page 7 of 7