Inductor Selection for SEPIC Designs Introduction/Basic Operation The single ended primary inductor converter (SEPIC) allows the output voltage to be greater than, less than, or equal to the input voltage in DCDC conversion. Some typical applications include digital cameras, cellular phones, CD/DVD players, PDA’s and GPS systems.During the switch (SW) ON time the voltage across both inductors is equal to Vin. When the switch is ON capacitor Cp is connected in parallel with L2. The voltage across L2 is the same as the capacitor voltage, -Vin. Diode D1 is reverse bias and the load current is being supplied by capacitor Cout. During this period, energy is being stored in L1 from the input and in L2 from Cp. L1 V in + Cp Dual Winding Inductor Solutions D1 V out SDQ Series DRQ Series Switch L2 C out Figure 1: Simple SEPIC Circuit During the switch (SW) OFF time the current in L1 continues to flow through Cp, D1 and into Cout and the load recharging Cp ready for the next cycle. The current in L2 also flows into Cout and the load, ensuring that Cout is recharged ready for the next cycle. During this period the voltage across both L1 and L2 is equal to Vout. The voltage across Cp is equal to Vin and that the voltage on L2 is equal to Vout, in order for this to be true the voltage at the node of Cp and L1 must be Vin + Vout. The voltage across L1 is (Vin+Vout) – Vin = Vout. Inductor Selection Procedures Case 1: Two Separate Inductors Application Conditions: • Input voltage (Vin) – 2.8V – 4.5V • Output (Vout & Iout) – 3.3V, 1A • Switching Frequency (Fs) – 250kHz • Efficiency - 90% Step 1. Calculate The Duty Cycle: D = Vout/(Vout + Vin) The worst case condition for inductor ripple current is at maximum input voltage D = 3.3/(3.3 + 4.5) = 0.423. The output inductor is sized to ensure that the inductor current is continuous at minimum load and that the output voltage ripple does not affect the circuit that the converter is powering. In this case we will assume a 20% minimum load thus allowing a 40% peak-to-peak ripple current in the output inductor L2. Dual Winding Schematics Dual Inductor 1 2 L1 3 Parallel Mode Series Mode 1 L2 2 L1 4 3 2 1 L1 L2 4 L2 3 Step 2. Calculate The Value of L2 V = L di/dt • V is the voltage applied to the inductor • L is the inductance • di is the inductor peak to peak ripple current • dt is the duration for which the voltage is applied L = V.dt/di • dt = 1/Fs x D • dt = 1/(250 x 103) x 0.423 = 1.69µ-Sec • V = Vin during the switch ON time so; • L2 = 4.5 x (1.69 x 10-6/0.4) • L2 = 19µH Result: Using the nearest preferred value would lead to the selection of a 22 µH inductor. It is common practice to select the same value for both input and output inductors in SEPIC designs although when two separate parts are being used it is not essential. 4 Case 2: Coupled Inductor Step 1. Perform Step 1 and The I rms Portion of Step 3 from the Two Separate Inductor Selection The application information listed for the two inductor selection will be used. Step 2. Calculate The Inductance Value SDQ Series DRQ Series Step 3. Calculate RMS and Peak Current Ratings for Both Inductors Input Inductor L 1 • Irms = (Vout x Iout)/(Vin (min) * efficiency) • Irms = (3.3 x 1)/(2.8 x 0.9) = 1.31A • Ipeak = Irms + (0.5 x Iripple) • Iripple = (V.dt)/L • Iripple = (2.8 x 2.2 x 10-6)/22 x 10-6 = 0.28A • Ipeak = 1.31 + 0.14 =1.45A Although worst case ripple current is at maximum input voltage the peak current is normally highest at the minimum input voltage. Result: 22µH, 1.31Arms and 1.45Apk rated inductor is required. For example the Coiltronics® DR73-220 which has 1.62Arms and 1.67Apk current ratings. Output Inductor L2 • Irms = Iout = 1A L = V.dt/di From our earlier example the output ripple current needs to be 0.4Apk-pk, so now we calculate for 0.8A as the ripple current is split between the two windings L = 4.5 x (1.69 x 10-6/0.8) = 9.5µH • A coupled inductor has the current flowing in one inductor and if the two windings are closely coupled the ripple current will be split equally between them. • Using a coupled inductor reduces the required inductance by half. • Since the two winding are on the same core they must be the same inductance value. Step 3. Calculate the Peak Current Continuing with the example using an inductance value of 10µH we now need to calculate the worst case peak current requirement. The RMS current in each winding is already known. • Input inductor RMS current = 1.31A • Output inductor RMS current = 1A • Ipeak = Iin + Iout + (0.5 x Iripple) • Iripple = (2.8 x 2.2 x 10-6)/10 x 10-6 = 0.62A -6 -6 • Iripple = (4.5 x 1.69 x 10 )/22 x 10 = 0.346A • Ipeak = 1.31 + 1 + 0.31 = 2.62A @ minimum input voltage • Ipeak = 1 + 0.173 = 1.173A Result : A 22µH, 1Arms and 1.173Apk rated inductor is required, which for simplicity could be the same DR73-220 inductor used for L1 Result: A 10µH coupled inductor with 2.31Arms and 2.62Apk current ratings is required, for example the Coiltronics® DRQ74-100. Using a coupled inductor takes up less space on the PCB and tends to be lower cost than two separate inductors. It also offers the option to have most of the inductor ripple current flow in either the input or the output. By doing this the need for input filtering can be minimized or the output ripple voltage can be reduced to very low levels when supplying sensitive circuits. Typical Applications Using Inductors for SEPIC Designs Mobile Phones PDAs Digital Cameras Servers Laptop Computers Display Backlighting Flat-Screen Televisions DRQ Series Part Number DRQ73-1R0-R DRQ73-2R2-R DRQ73-3R3-R DRQ73-4R7-R DRQ73-100-R DRQ73-220-R DRQ73-330-R DRQ73-470-R DRQ73-680-R DRQ73-101-R DRQ73-221-R DRQ73-331-R DRQ73-471-R DRQ125-1R0-R DRQ125-1R5-R DRQ125-2R2-R DRQ125-3R3-R DRQ125-4R7-R DRQ125-100-R DRQ125-220-R DRQ125-330-R DRQ125-470-R DRQ125-680-R DRQ125-101-R DRQ125-221-R DRQ125-331-R DRQ125-471-R Rated OCL Inductance (μH) 1.00 2.20 3.30 4.70 10.0 22.0 33.0 47.0 68.0 100 220 330 470 1.00 1.50 2.20 3.30 4.70 10.0 22.0 33.0 47.0 68.0 100 220 330 470 +/-20% (μH) 0.992 2.070 3.540 4.422 10.30 22.65 34.41 48.62 68.91 101.4 223.3 325.5 465.8 0.894 1.478 2.208 3.084 5.274 9.654 22.36 33.74 47.47 67.91 102.7 216.8 332.6 473.1 Parallel Ratings Irms Isat Amps Amps Peak 5.25 7.97 4.11 5.52 3.31 4.22 3.09 3.78 2.08 2.47 1.62 1.67 1.31 1.35 1.08 1.14 0.89 0.96 0.73 0.79 0.52 0.53 0.42 0.44 0.35 0.37 15.0 23.6 13.8 18.3 10.9 15.0 9.26 12.7 7.18 9.71 5.35 7.17 3.70 4.71 3.28 3.84 2.71 3.24 2.22 2.70 1.78 2.20 1.19 1.51 1.06 1.22 0.87 1.02 DCR Ω OCL Typ. +/-20% (μH) 3.968 8.280 14.16 17.69 41.20 90.60 137.6 194.5 275.6 405.6 893.2 1302 1863 3.576 5.912 8.832 12.34 21.10 38.62 89.44 135.0 189.9 271.6 410.8 867.2 1330 1892 0.0103 0.0167 0.0259 0.0297 0.0656 0.107 0.166 0.241 0.358 0.527 1.05 1.59 2.36 0.0024 0.0029 0.0045 0.0063 0.0105 0.0189 0.0396 0.0505 0.0740 0.101 0.170 0.384 0.482 0.718 Series Ratings Irms Isat Amps Amps Peak 2.63 3.99 2.06 2.76 1.66 2.11 1.55 1.89 1.04 1.24 0.811 0.83 0.653 0.68 0.542 0.57 0.444 0.48 0.367 0.39 0.260 0.27 0.211 0.22 0.173 0.18 7.51 11.8 6.89 9.15 5.46 7.50 4.63 6.35 3.59 4.86 2.67 3.59 1.84 2.36 1.64 1.92 1.35 1.62 1.11 1.35 0.892 1.10 0.594 0.755 0.530 0.610 0.434 0.510 DCR Ω Typ. 0.0411 0.0669 0.1035 0.1188 0.2623 0.429 0.665 0.965 1.43 2.11 4.20 6.36 9.44 0.0096 0.0114 0.0182 0.0253 0.0420 0.0757 0.159 0.203 0.297 0.440 0.682 1.54 1.93 2.87 Note: DRQ 74 and DRQ127 not shown. For full product information and a listing of all available inductor values, see http://www.cooperbussmann.com/datasheets/elx, Data Sheet number 4311. DRQ73 Dimensions - mm Top View Bottom View 0.60 • 1 DRQ73 ### 2 Recommended Pad Layout Side View 6.1 0.73 3.55 Max 1.73 1.00 4 2 3 3 1 4 4 3 1 2 7.6 Max. 7.6 Max. 1.00 0.40 0.40 4 3 1 2 1.73 0.80 7.9 7.9 Dual Inductor Mode Series Mode DRQ125 Dimensions - mm Top View Bottom View Recommended Pad Layout Side View 6.00 Max 2.05 3.85 0.50 3.85 • DRQ125 ### wwlly y R 1 2 4 3 1 2.00 2 3 1 4 Dual Inductor 1 L1 12.5 Max 12.5 Max 4 2 3 2.50 1 4 2 3 0.50 13.80 13.80 Dual Inductor Mode Series Mode 10.0 Schematic 3 2.50 Parallel Mode Series Mode 2 1 4 3 L2 L1 2 1 4 3 2 L1 L2 L2 4 DRQ Series SDQ Series Part Number SDQ12-1R0-R SDQ12-2R2-R SDQ12-3R3-R SDQ12-4R7-R SDQ12-100-R SDQ12-220-R SDQ12-330-R SDQ12-470-R SDQ25-1R0-R SDQ25-2R2-R SDQ25-3R3-R SDQ25-4R7-R SDQ25-100-R SDQ25-220-R SDQ25-330-R SDQ25-470-R SDQ25-680-R SDQ25-101-R SDQ25-221-R SDQ25-331-R SDQ25-471-R Rated Part OCL Inductance (μH) 1 2.2 3.3 4.7 10 22 33 47 1 2.2 3.3 4.7 10 22 33 47 68 100 220 330 470 Marking +/-20% (μH) 0.81 2.25 3.61 4.41 9.61 22.09 32.49 47.61 0.97 2.31 2.89 5 9.8 22.47 33.8 47.43 69.19 98.57 223.1 329.7 472.4 B D E F J L M N C E F G K M N O P R T U V Parallel Ratings Irms Isat Amps Amps DCR Ω OCL Typ. 2.49 1.60 1.28 1.12 0.831 0.548 0.439 0.401 3.15 2.67 2.50 1.96 1.53 1.01 0.812 0.749 0.603 0.499 0.326 0.292 0.243 0.0403 0.0977 0.1527 0.1990 0.3620 0.8332 1.29 1.55 0.0252 0.0351 0.0399 0.0653 0.1068 0.2431 0.3795 0.4461 0.6865 1.00 2.36 2.93 4.25 +/-20% (μH) 3.24 9.00 14.44 17.64 38.44 88.36 130.0 190.4 3.87 9.25 11.55 20.00 39.20 89.89 135.2 189.7 276.8 394.3 892.4 1318.7 1889.6 3.38 2.03 1.60 1.45 0.981 0.647 0.533 0.441 4.09 2.65 2.37 1.80 1.29 0.849 0.692 0.584 0.484 0.405 0.269 0.222 0.185 Series Ratings Irms Isat Amps Amps DCR Ω 1.25 0.800 0.640 0.560 0.416 0.274 0.220 0.201 1.58 1.34 1.25 0.98 0.765 0.507 0.406 0.374 0.302 0.249 0.163 0.146 0.121 0.1611 0.3908 0.6106 0.7959 1.45 3.33 5.18 6.21 0.1007 0.1402 0.1595 0.2612 0.4273 0.9724 1.52 1.78 2.75 4.02 9.42 11.71 16.99 Typ. 1.69 1.01 0.800 0.724 0.490 0.323 0.267 0.220 2.05 1.32 1.18 0.900 0.643 0.425 0.346 0.292 0.242 0.203 0.135 0.111 0.093 Note: For full product information and a listing of all available inductor values, see http://www.cooperbussmann.com/datasheets/elx, Data Sheet number SDQ Series. SDQ12 and SDQ25 Dimensions - mm Top View Bottom View Side View Pin #1 identifier 4 1 3 2 2 5.2 Max 1 2.975 1 Part marking (Note A) 4 PAD LAYOUT 2 PAD LAYOUT 3 1.5 Typ. Ref. Schematic Recommended Pad Layout 1.5 typ ref 5.95 1 4 2 3 1 4 1 2 3 2 4 4 2 3 2.975 5.950 TRANSFORMER PARALL EL 3 SERIES R2.250 1.02 2.975 R2.250 4 2.975 5.950 2.575 5.2 Max 5.15 SDQ12 = 1.2mm Max SDQ25 = 2.5mm Max SDQ Series The Cooper Bussmann Coiltronics® brand of magnetics specializes in standard and custom solutions, offering the latest in state-of-the-art low-profile high power density magnetic components. We remain at the forefront of innovation and new technology to deliver the optimal mix of packaging, high efficiency and unbeatable reliability. Our designs utilize high frequency, low core loss materials, and new and custom core shapes in combination with innovative construction and packaging to provide designers with the highest performance parts available on the market. The Coiltronics Brand product line of power magnetics continually expands to satisfy shifts in technology and related market needs. Standard Product Categories include: • Shielded Drum Inductors • Unshielded Drum Inductors • High Current Inductors • Toroidal Inductors • Specialty Magnetics • Custom Magnetics Please visit http://www.cooperbussmann.com/datasheets/elx to see data sheets on the wide variety of inductor solutions we have to offer. For techncial inquiries e-mail [email protected] Order samples online - www.cooperbussmann.com © 2008 Cooper Bussmann St. Louis, MO 63178 636-394-2877 www.cooperbussmann.com Reorder # 4030 1008 PDF Only

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