How to Invert Voltages
摘要
This power management tip introduces the two different circuits for a positive-to-negative voltage conversion and for a negative-to-positive voltage conversion. It explains how a step-down (buck) regulator circuit may easily be modified to convert a positive voltage into a negative voltage and how a boost converter may easily be modified to convert a negative voltage into a positive voltage.
Introduction
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, however, a voltage is increased. This often happens with an upconverting boost converter. These two topologies, buck and boost, are used in many applications.
However, sometimes 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 be able 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 as shown in Figure 1 is suitable. A positive voltage is transformed into a negative voltage. The circuit is simple, does not 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, which can be adjusted with resistor divider Rfb1 and Rfb2.
In addition to this application, however, there is sometimes the need 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. However, there are also applications in the industrial sector. There is indeed an elegant solution for this requirement that does not require a transformer. Figure 2 shows an inverting boost converter, which generates a positive voltage from a negative voltage. This does not require a buck regulator, but 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 required 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 is specially designed for converting negative voltages to positive voltages. It uses external switches, works synchronously, and offers a very 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.
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