### Inductor Selection for SEPIC Designs

```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
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
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)
3
1.5 Typ.
Ref.
Schematic
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
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Coiltronics Brand product line of power magnetics continually expands to satisfy shifts in technology and related market needs. Standard Product
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Please visit http://www.cooperbussmann.com/datasheets/elx to see data sheets on the wide variety of inductor solutions we have to offer.
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