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| SEPIC versus 4 switch buck-boost DC/DC converter |
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| lordnoxx:
Hi guys, I am recently reading in Maniktalas "Switching Power Supplies A-Z". And the author does a great job describing the whole world of buck-boost and boost-buck topologies. Even though there are some obvious differences in the basic circuits of a SEPIC and a 4 switch buck-boost DC/DC converter both do basically the same task. Well I already know that they differ in terms of switching losses (1 Mosfet for SEPIC versus 4 Mosfets for the 4 switch buck-boost) and that the 4 switch buck-boost is likely more difficult to control around that area where VIN equals VOUT. And I am aware that the 4 switch buck-boost can be controlled in many different ways...such as to only clock the buck side when VIN>VOUT and vise versa. Probably the 4 switch buck-boost can also (depending on the control algorithm or control IC) be made bidirectional in terms of power flow. Both have the same transfer function (VOUT/VIN = D/(1-D)). And so on and so forth. But as already mentioned SEPIC or 4 switch buck-boost...both do basically the same task. So which one to choose for which application? Are there any differences I have not covered? Advantages/Disadvantages? :-// |
| Weston:
Technically you can implement the buck-boost converter with two switches and two diodes. And you might see the SEPIC converter with a second switch replacing the diode for synchronous rectification and higher efficiency. The diode in the SEPIC converter will also experience recovery loss. The biggest difference is that the SEPIC converter requires two inductors. Inductors are often the largest and one of the highest loss component in power converters. Additionally, the switch in SEPIC converter needs a higher rating than the ones in the buck-boost, it sees Vin + Vout and experiences a current of Iout/(1-D) (also can be expressed as Iin/D). The SEPIC converter has control advantages because you only need to drive on switch and from my impression it seems to be used more in lower power designs / integrated switching regulators. Additionally, the SEPIC converter has continuous input current, which makes filtering easier, while the buck-boost converter can have discontinuous input or output current. For the buck-boost converter the Vin~=Vout case is a bit of a hassle to deal with but there are control schemes that add minor additional switching loss. Meanwhile, the switch in the SEPIC converter has to deal with twice the current at Vin=Vout. In high power applications the higher switch rating and the second inductor of the SEPIC converter can be a disadvantage. At high powers (i.e. cost) its less of an issue to add additional switches. I would say that the buck-boost converter has advantages at higher power levels. |
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