How to reverse the polarity of the voltage

Power converters are commonly employed to generate stable supply voltages for circuits from existing voltage rails. In most applications, voltage is reduced via step-down switching regulators. However, on occasion, voltage elevation is required, typically achieved using boost converters. Both step-down and boost topologies find application across numerous domains.

Nevertheless, voltage inversion is occasionally required. Most frequently, a positive supply voltage must be converted to a negative voltage. This is necessary to power sensors within signal paths, enabling them to process signals with both positive and negative voltage deflections. For instance, +5V and -5V are employed as operational amplifier supplies to handle signal voltages. Certain applications demand negative voltages to safely turn off MOSFET gates. To generate a negative voltage from an existing positive supply, the inverting topology illustrated in Figure 1 provides a suitable solution. Here, the positive voltage is converted into a negative voltage. This circuit structure is straightforward, requires no transformer, and can be implemented using standard buck switching regulator ICs. The buck regulator's output voltage is connected to the system ground terminal. The voltage at the GND pin generates the negative voltage, which can be adjusted via the resistor divider network comprising Rfb1 and Rfb2.

Figure 1. Switching Power Supply Converter Generating Negative Voltage from Positive Voltage

Beyond this application, it is sometimes necessary to convert negative voltage into positive voltage. A common application is within the telecommunications sector, where -48 V must be converted to +48 V. Furthermore, applications exist within the industrial sector. For this requirement, an efficient solution does indeed exist that does not require a transformer. Figure 2 illustrates an inverting boost converter capable of generating positive voltage from negative voltage. It requires not a buck regulator but a boost regulator IC. The power supply level must be appropriately designed so that the corresponding body diode of the switch conducts in the correct direction.

Figure 2. Switching Power Supply Converter Generating Positive Voltage from Negative Voltage

To efficiently convert negative voltage to positive voltage whilst achieving multiple characteristics, additional components are required. For instance, different configuration pins necessitate level shifting to enable functions such as soft start or clock synchronisation. The permissible voltage range on these pins depends upon the GND pin of the switching regulator IC, namely the negative input voltage.

Figure 3. LTspice schematic diagram of the LTC7899 circuit

To achieve smooth switching even at higher power levels, a dedicated switching regulator IC can be employed to efficiently convert negative voltages to positive ones. The LTC7899 is such an IC. It is a switching power supply boost converter controller specifically designed to convert negative voltages to positive voltages. It utilises an external switch, operates synchronously, and provides an exceptionally wide voltage range of up to 135 V between input and output. Figure 3 displays the LTC7899 inverting circuit implementation in the LTspice® simulation programme, enabling users to perform circuit simulation analysis using this software.

Conclusion

Inverting negative and positive voltages without a transformer is straightforward. Numerous applications require such inversion capability. While multiple simple solutions exist, alongside a wide selection of suitable power converter ICs, employing specialised switch controllers like the LTC7899 proves highly appropriate for designing high-performance, user-friendly circuits.