The specifications shown in Table 1 are the basis for a design comparison. The first design uses a coupled inductor and the second uses two separate inductors. This design is typical of an automotive input voltage range with an output power of 64 W.
Table 1: Prototype SEPIC electrical specification
Equation 1 determines the coupled inductor requires an inductance of 12 μH, with a combined current rating of 13 A (based on IIN plus IOUT). This design poses a particular challenge because of the limited selection off-the-shelf inductors. Therefore, a custom inductor from RENCO was specified and designed.
This inductor was wound on a split-bobbin to intentionally introduce leakage inductance to minimize circulating currents which can induce losses. These losses are due to the AC capacitor ripple voltage being imposed across the leakage inductance. For designs of lower power, coupled inductors from Coilcraft (MSS1278 series) and Coiltronics® (DRQ74/127 series) offer good off-the-shelf alternatives. For the separate inductor design, a 33 μH Coilcraft SER2918 was used for L1 and a 22 μH Coiltronics HC9 was selected for L2.
Each was chosen based on winding resistance, current rating and size. Care must be taken when selecting the inductors because core and AC winding losses must also be considered. These losses reduce the inductor’s allowable DC current, but not all vendors provide adequate information to calculate this. Failure to properly calculate this could greatly increase core temperature beyond the typical 40°C rise, decrease efficiency, and hasten premature failure.
Figure 2 show the prototype SEPIC schematic with a coupled inductor. To implement the separate inductors in the design, the coupled inductor was simply replaced with two inductors on the same PWB. Figure 3 shows both prototype circuits, with L1 occupying the space of the coupled inductor and L2 in the upper right corner.
Figure 2: A 16V/4A SEPIC converter schematic with coupled inductor
(click here to enlarge).
Figure 3: Coupled inductor (left) and dual-inductor (right)
SEPIC converter prototypes.
As expected, both circuits operated in a nearly identical fashion, with the switching voltage and current waveforms being essentially the same. But there were several key differences in performance. While the control loop for the coupled inductor design was quite benign, the separate inductor design was initially unstable.
Measurement of the loop gain determined that a high-Q, low-frequency resonance was the culprit, requiring the addition of a damping R/C filter in parallel with the AC capacitor. The resonant frequency, while greatly simplified, appeared to be approximately 1/2π√[Cac×(L1+L2)]. The SEPIC circuit has a quite complex control loop characteristics, necessitating the use of mathematical tools for detailed analysis because the analytical results are often difficult to interpret. Adding this R/C damping filter (220 μF/2 Ω) adds cost, circuit area, and losses.
This is in addition to the 10% area premium that two inductors require over a single, coupled inductor. Figure 4 shows the measured efficiency for both circuits. It can be seen that there is an across the board boost in efficiency of up to 0.5% for the coupled-inductor design.
This is likely due to lower overall core losses in the coupled inductor design, since its DC wiring losses were actually higher than that of the dual inductor design. L2 uses a powered iron core material, which tends to have higher losses than the ferrite material used for L1 and the custom coupled inductor (Reference 3). While ferrite material for L2 could have been used, it would have resulted in a larger area.
Figure 4: Both coupled and separate Inductors
achieve good efficiency.
The SEPIC converter can be successfully implemented with either dual inductors or a single coupled inductor. Improved efficiency, reduced circuit area, and more benign control loop characteristics are benefits realized in the prototype hardware when using a properly wound, custom-coupled inductor. While custom components are less desirable than off-the-shelf parts, many coupled inductors are readily available, albeit in smaller sizes. If time-to-market is critical, separate inductors provide greater flexibility to the designer.
1. Balance volt-microseconds for L1 and L2:
L1: DVin = (1-D)(Vcap+Vout-Vin)
L2: (1-D)Vout = DVcap
Vout = VcapD/(1-D)
Substitute L2 equation into L1 equation:
DVin = (1-D)(Vcap+VcapD/(1-D)-Vin)
DVin = (1-D)Vcap+VcapD -(1-D)Vin
DVin = (1-D)Vcap+VcapD -Vin+DVin
Vin = (1-D)Vcap+VcapD
Vin = Vcap-DVcap+VcapD
Vin = Vcap
2. John Betten, “SEPIC Converter Benefits from Leakage Inductance,” PowerPulse.Net, Design Features, May 27, 2010.
3. Robert Kollman, “Don’t get burned by inductor core loss,” Power Management Designline, July 13, 2009.
To learn more about this SEPIC converter and other power solutions, visit: www.ti.com/power-ca.
About the Author
John Betten is an Applications Engineer and Senior Member of Group Technical Staff at Texas Instruments, and has more than 25 years of AC/DC and DC/DC power conversion design experience. John received his BSEE from the University of Pittsburgh and is a member of IEEE. You can reach John at email@example.com.
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