Power Tips: Don’t be negative on negative supplies!

Most power supply designers understand how to go from higher voltages to lower voltages (buck converter) or from lower voltages to higher voltages (boost converter).  But, what if you need to generate a voltage of different polarity?  These power supply designs are less common, but still very necessary for a range of industrial, Audio and RF applications.

There are a couple different ways to generate a negative output voltage from a positive. You could use any kind of  isolated converter (flyback, forward, etc.) or a buck-boost converter.  

When using an isolated converter, the GNDs are isolated and the designer is free to connect the load to meet the needs of the design.  A buck-boost converter (Figure 1) is an easy way to generate this negative voltage using a non-isolated topology.  

 simple schematic of a buck-boost converter

Figure 1 - simple schematic of a buck-boost converter

The challenge of a non-isolated topology is how to relate the negative output voltage signal back to the control signals.  An amplifier or transistor can be used to create a level shifter, but there is an easier and lower cost way.  Utilizing any generic buck converter IC, you can hook the ic up in a configuration to solve this challenge. 

Figure 2 - 

The idea for this configuration is to hook up the output inductor to GND (instead of VOUT in a buck converter).  The ground connection on the control IC is connected to –VOUT which allows the IC to float negative with the output voltage.  Having the IC at the same reference level –VOUT makes it easy to connect the feedback for the output voltage regulation.

A few considerations need to be taken when using this approach to ensure that the control IC and power stage components are not damaged.  The control IC and power devices need to be voltage rated for the full range of the input plus the negative output. The power devices and inductor also need to be rated for higher current than just the output or input current.  Also, it's worth noting that almost any buck converter IC can be used in this topology: synchronous, non-synchronous, integrated FETs or external FETs.

Because these designs are used less often than typical buck or boost converters, they have been wildly popular on PowerLab.  Below are a few of our favorite designs for generating voltage of different polarity:

  • PMP2768:  11 to 15V Input, -3.3V@9A, Synchronous Buck-Boost
  • PMP3143:  10 to 18V Input, -15V@0.5A, Synchronous Buck-Boost
  • PMP6867:  9 to 32V Input, -28V@1.5A, LED Driver
  • PMP7330:  8 to 14V, -12V@0.1A, Integrated FET
  • PMP8298:  8 to 14V, -4.35V@15A, High Power Synchronous Buck-Boost

In summary:  Don’t be negative!  These negative output voltage supplies can be handled with some simple modifications to a regular buck converter.