No need to fret about negative power!

Most power supply designers know how to convert a higher voltage to a lower voltage (a buck converter) or a lower voltage to a higher voltage (a boost converter). But what if you want to generate voltages of different polarities? This type of power supply design is not common, but it is necessary for a variety of industrial, audio, and RF applications.

There are several different ways to generate a negative voltage from a positive terminal. You can use any type of isolated converter (flyback, forward, etc.) or a buck-boost converter .

When using an isolated converter , GND is isolated and the designer is free to connect the load as required by the design. When using a non-isolated topology to generate this negative voltage, a buck-boost converter (Figure 1) is most convenient.

Figure 1 : Simple schematic of a buck-boost converter 

The challenge of non-isolated topologies is how to relate the negative output voltage to the control signal. You can use amplifiers or transistors to create a level shifter, but there are cheaper and more convenient ways. You can use any common buck converter  IC and connect it in a certain configuration to solve this challenge.

Figure 2  — 

The idea behind this configuration is to connect the output inductor to GND (instead of VOUT in a buck converter). The ground connection on the control IC can be connected to -VOUT, allowing the IC to float negative with the output voltage. Providing the IC at the same reference level –VOUT allows for easy connection of feedback for output regulation.

There are some things to keep in mind when using this approach to protect the control IC and power stage components from damage. The control IC and power components should be rated for voltages no less than the sum of the input and negative output voltages. Also, the power devices and inductors should be rated for currents greater than the input or output. It is worth mentioning that any buck converter IC can be used for this topology: synchronous, non-synchronous, integrated FET, or external FET.

 These designs have always been popular at PowerLab because they are used less frequently than typical buck or boost converters.  Here are some of our favorite designs for generating voltages of different polarities:

• PMP2768 : 11 to 15V input, -3.3V at 9A  , synchronous buck-boost
• PMP3143 : 10 to 18V input, -15V at 0.5A  , synchronous buck-boost
• PMP6867 : 9 to 32V Input, -28V at 1.5A, LED Driver
• PMP7330 : 8 to 14V, -12V at 0.1A, Integrated FET
• PMP8298: 8 to 14V, -4.35V at 15A, high power synchronous buck-boost

In summary, no need to be uncomfortable with negative supplies! These negative output voltage supplies can be handled by simply scaling a normal buck converter.