Electronic circuit designs often require power sources with negative output voltages. There are many different ways of producing a negative output voltage from a positive input voltage. One option is the simple and low cost polarity-inverting (buck-boost) converter. The advantages of this topology are that it requires low component count and that it can be built with standard high-side regulator ICs such as those intended for buck regulators. This article discusses the important technical details about the topology including input and output voltage ripple and loop stability requirements, and shows an example circuit.
The polarity inverting (buck-boost) topology
The polarity inverting buck-boost converter is a very basic power conversion topology like the buck topology and the boost topology. Each of these basic topologies is built with one active switch (usually a MOSFET) a passive switch (a diode) and an inductor. Figure 1 shows these three basic voltage conversion topologies. The simplicity of the polarity-inverting topology is one of its biggest advantages. The topology needs very few power stage components thereby reducing the cost and development complexity. The polarity inverting topology reverses the polarity of the input voltage, but allows the absolute output voltage to be higher or lower than the absolute of the input voltage.
Besides providing negative voltages in systems the polarity-inverting topology is very useful to power loads that do not depend on a polarity in reference to a system ground but are being supplied by an input voltage which might be higher or lower than the output voltage. A good example is powering a string of LEDs with a total voltage drop of 16V from a supply voltage in the range between 9V and 30V. In this example, the string of LEDs can accept a negative 16V, since the LEDs may just be turned around by swapping the anode connection with the cathode connection. The polarity inverting topology can power the negative 16V from positive 9V of input voltage as well as from 30V of input voltage.
Figure 1. Most basic switch-mode power supply topologies.
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Compared to the buck and the boost topology, the inverting topology has the main energy storing element, the inductor, connected between the switch node and ground. In the buck topology the inductor is connected to the output node, in the boost topology the inductor is connected to the input node. The inductor always prevents current from changing instantaneously. Thus the inductor silences the noise on a specific node. Therefore the buck topology has a very silent output voltage but is very noisy on the input. The boost topology is very silent on the input side but is very noisy on the output side. The polarity-inverting topology is noisy on the input side as well as on the output side. Noisy nodes require good capacitive filtering to reduce the ripple voltage. In the polarity-inverting topology, both, the input node as well as the output node require good low ESR bypassing capacitors to minimize voltage ripple and noise.