Output voltage ranging between the minimum and the maximum input voltage
Up to now, we have seen two types of converters, one capable of decreasing the input voltage (buck converter) and the other one capable of increasing it (boost converter). What happens, however, if we need an intermediate output voltage? For example, if we want to power our 5 volt device and sometimes we have a 4.2 V LiPo battery available, and some other ones an external 12 V power supply; or if we have a circuit working at 3.3 volts and we have to power it with a LiPo that supplies 4.2 volts when it is completely charged and 3 volts when it is almost depleted?
An immediate solution is the one to use two converters, and to select by means of a (manual or electronic) commutator the one to be used from time to time; this solution – however – is quite an intricate and problematic one.
Another alternative is the one to use a boost converter, so to bring the input voltage to the maximum accepted value, and then a buck converter in cascade, so to bring it back to 5 volts. In our case, it would be:
Even this solution is an intricate one, it uses two complete converters and doubles the power losses, thus reducing the total efficiency.
There are buck-boost converters that allow to obtain whichever output voltage but, if they are not isolated from a transformer, the voltages are obtained with an inverse polarity with respect to the input one, which is often unacceptable.
Among the various implemented types, one recently emerged: it is named SEPIC, and was mainly encouraged by the arrival of the LiPo batteries that, as we just said, supply a voltage that is 4.2 V when fully charged and at the end-of-charge they even go under 3 volts; it was born in order to power devices that require 3.3 volts, that is to say a voltage that is between these values. Even here, this is the case of an intermediate output voltage among the possible input values, even if the window is smaller.
