The buck converter topology used in microcontroller-based chargers works well in most applications but when the input voltage is almost equal to the output voltage this solution cannot be used. For example, as shown in figure 2, it is very difficult to charge a 4.2 V Li-Ion battery from a 5 V supply using buck converter topology due to voltage drops caused by the protection diode and other components. One approach could be to minimize the drop voltages by using low drop components such as low drop diodes. However, this solution is not universally applicable and it is also expensive Another option would be to use a buck-boost converter, but this type of converter provides an inverted output, and therefore is unsuitable. There are techniques to make non-inverting converters, but these converters in general require two inductors, increasing the cost of the solution. This paper describes a simple technique for implementing a microcontroller (MCU) based non-inverting buck- boost converter, which needs only one inductor. The circuit described requires only an extra PWM signal as com- pared to the simple buck converter circuit, making it a low cost solution.
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