Used for: generating a reverse polarity voltage of any value


  • Steps positive input up to any negative value or vice versa
  • Simple circuit
  • Current limiting/short circuit protection relatively easy because of series switch
  • Requires high-side gate drive for both switches (not for D1 if used)


  • High output ripple - can be > output DC current so need large, low ESR output caps
  • Only negative output available (for +ve input) or vice versa


  • SW1 (Q1) and SW2 (D1) are alternately on and off at high frequency
  • SW1 and SW2 are never both on together
  • Vout = -D Vin/(1- D) where D is the switching duty cycle (0 -1)
  • Input ripple current is low, set by inductor L
  • Output ripple current is high and is sunk into C1
  • SW2 can be a diode - the action of SW1 turning off automatically forward biases the diode causing it to conduct. Action of SW1 turning on reverse biases the diode causing it to block current


There is a version of a buck-boost called the four transistor buck boost which gives a positive output for a positive input, lower or higher than the output, using one inductor.

This is simply a circuit which can be physically switched between classic buck or boost. It is more expensive because it has redundant switches and is more lossy as it has an extra switch element in the output path as a buck converter and in the input path as a boost converter. There are some control ICs like LM5175 that can do the switch selection automatically.

The inductor can’t be optimum for both modes though. Why not just use a SEPIC?

Can be useful at low power when the transistors are integral to the control IC