The Easy Way to Convert Negative Voltages to Positive

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Generating negative voltages from a positive supply rail is pretty standard. There are many applications that need a negative voltage, for example, in the power industry they turn MOSFETs on and off more efficiently. There are fewer applications where a positive voltage must be generated from a negative rail, but they do exist, and that market is underserved, especially at higher power levels

Frederik Dostal, Subject Matter Expert for Power Management at Analog Devices explains, “generating a positive output voltage from a negative source is not as common because there is lack of applications that only have a negative voltage available. That’s not to say that there are none. Telecom systems, such as base stations, are attached directly to grid power and typically run from a positive input voltage. However, battery backup is required if the power fails, and usually the battery backup is built to provide a negative voltage because of corrosion issues. Telecoms companies don't want to have a big, bulky, expensive transformer, so power conversion is preferred. There are also other applications, such as in the industrial field, where the system might only be supplied with a negative voltage rail, but require a positive voltage for some of the electronics, like sensors. Similar situations are often found in avionics and military applications.”

He continues, “So, the demand does exist, but a lot of designers aren't really aware of how to accomplish the task. Negative to positive conversion can be achieved very similarly to a positive to negative voltage conversion, but in this case, a boost converter is required. Instead of having the positive voltage on the input side, it is attached to the system ground, and then the output of the boost converter will become the positive voltage and the negative voltage would be held to the ground voltage of the converter. That does work, but there are a few things you have to watch out for. For example, when the conversion starts, the positive output voltage hasn't been established. The ground voltage will be adjusted to the negative input voltage, but that can change up and down, and we have to ensure that the converter accepts the positive output voltage that it's sensing, and that the over voltage or the maximum value on the feedback are not violated. This can be done with external components, but that makes the circuit more complex and expensive.”

Analog Devices has recently launched a converter that is specifically targetted at negative to positive conversion at power levels of up to 50W and voltages up to 140V. The LTC7899 incorporates all of the circuitry necessary to accurately convert and needs only two resistors to set the output voltage and it remains accurate, even with a varying input voltage.

Dostal expands, “the LTC7899 is a very flexible controller that it was specifically designed to convert negative voltages to positive, but it can also be used as a positive boost regulator with a large voltage range. It uses external MOSFETs that are controlled with an up drive and a down drive for optimized dead times and improved conversion efficiency. When a boost converter is used to convert negative voltage into a positive voltage, typically the feedback pin needs adjusted to get the correct voltage, so level shifting is needed. An integrated level shifter provides the negative to positive voltage conversions through the feedback pin. That integration brings three advantages – there’s no need for additional development time, the solution is compact, and the regulation of the output voltage is more accurate. It has low noise operation, and spread spectrum frequency modulation to reduce EMI, which can be turned on or off, depending on the use case. Everything has been figured out so that it's really easy to use.”

https://www.analog.com/media/en/technical-documentation/data-sheets/ltc7899.pdf