How to Invert Voltages

Power converters are commonly used to generate a regulated supply voltage for a circuit from an existing voltage rail. In most applications, a voltage is lowered with a buck-converting switching regulator. Sometimes, though, a voltage is increased, which often happens with an upconverting boost converter. These two topologies, buck and boost, are used in a wide range of applications.

However, there are times when a voltage needs to be inverted. In most cases, a positive supply voltage must be converted into a negative voltage. This is necessary to supply sensors in the signal path to process a signal with positive and negative voltage deflection.

For example, +5 V and –5 V are used as the power supply for an op amp to process a signal voltage. Negative voltages are required in some applications to safely switch off the gates of MOSFETs.

To generate a negative voltage from an existing positive voltage, an inverting topology is suitable (Fig. 1). A positive voltage is transformed into a negative voltage. The circuit is simple, doesn’t require a transformer, and can be built with a standard buck switching-regulator IC. The output voltage of the buck regulator is connected to the system ground, and the GND pin voltage results in a negative voltage that can be adjusted with resistor divider Rfb1 and Rfb2.

Converting to a Positive Voltage

Sometimes the need arises to convert a negative voltage into a positive voltage. A common application is in the field of telecommunications, where –48 V must be converted to +48 V. There are also applications in the industrial sector.

An elegant solution for this requirement is available, and it doesn’t need a transformer. Figure 2 shows an inverting boost converter, which generates a positive voltage from a negative voltage. It doesn’t require a buck regulator, but rather a boost regulator IC. The power level must be designed in such a way that the respective body diodes of the switches are conducting in the right direction.

To convert a negative voltage into a positive voltage efficiently and with many features, additional components are necessary. For example, a level conversion is needed for different setting pins, such as a soft-start function or a clock synchronization function. The allowable voltage range on these pins depends on the GND pin of the switching-regulator IC, which is the negative input voltage.

For comfortable switching, even at higher power, a dedicated switching-regulator IC is suitable for voltage conversions from a negative voltage to a positive voltage. One such IC is the LTC7899, a switch-mode power-supply boost converter controller that’s specially designed for converting negative voltages to positive voltages. It uses external switches, works synchronously, and offers a high voltage range of up to 135 V between the input and output.

Figure 3 shows the LTC7899 in an inverting circuit in the simulation program LTspice, with which the circuit can be simulated.

Conclusion

Negative and positive voltages can also be easily inverted without a transformer. Many applications require such inversion. There are simple solutions and a wide selection of suitable power-converter ICs. However, to design a high-performance, easy-to-use circuit, special switching controllers, such as the LTC7899, are suitable.