The Challenge of Pre-Biased Loads and the Definition of a New Operating Mode for DC-DC Converters

The ever-increasing demand for higher efficiency step-down switching regulators in numerous industrial applications has made Synchronous Rectification a staple in power supplies for more than 2 decades. A more modern requirement is that power supplies do not affect a load during power-up if the initial load voltage is other than zero. This is generally referred to as the capability to monotonically start-up into pre-biased loads. Various practical pre-biased start-up scenarios are discussed, suggesting that this is increasingly becoming a mainstream requirement. To support pre-biased start-up the synchronous rectifier FET must be off when the power supply is powering up. To avoid an output voltage transient spike, modern controllers gradually increase the on-time duty cycle of the synchronous FET until the operation becomes "fully synchronous". This method is analogous to a soft-start. A by-product of this method is a new DC-DC operating mode, transition conduction mode (TCM). TCM can degenerate into DCM (discontinuous conduction mode), but otherwise it has unique gain characteristics and potentially useful control implications. The present discussion is restricted to analysis, equations and experimental results particular for TCM in the step-down (buck) converter, but can be extended to the larger class of DC-DC converters.