Field of the Invention
This application relates to voltage converters and more particularly to a time based approach for more efficient switched mode power conversion in DC-DC converters.
Description of the Related Art
Traditional DC-DC converters typically employ one of three operating modes: a) buck, b) boost, or c) buck-boost. FIG. 1 shows a DC-DC converter 100 including switches S1, S2, S3, and S4 that may be configured for all three operating modes by varying the switch setting.
In a buck mode of the DC-DC converter, switches S1 and S4 are closed (S2 and S3 open) during the charge cycle (TON) for the inductor 101. During the discharge cycle (TOFF), S2 and S4 are closed (S1 and S3 open). FIG. 2A shows current flowing through the inductor 100 during the charge and discharge cycles. The inductor current reaches its peak at Ip and then the discharge portion of the cycle begins. The discharge cycle ends when the inductor current reaches zero. The cycle then repeats.
In a boost mode, switches S1 and S3 are closed (S2 and S4 open) during the charge cycle (TON) for the inductor. During the discharge cycle (TOFF), S1 and S4 are closed (S2 and S3 open). FIG. 2B shows current flowing through the inductor during the charge and discharge cycles. The inductor current reaches its peak at Ip and then the discharge portion of the cycle begins.
In a buck-boost mode, switches S1 and S3 are closed (S2 and S4 open) during the charge cycle (TON) for the inductor. During the discharge cycle (TOFF), S2 and S4 are closed (S1 and S3 open). FIG. 2C shows current flowing through the inductor during the charge and discharge cycles. The inductor current reaches its peak at Ip and then the discharge portion of the cycle begins.
The above conventional modes all suffer from drawbacks in the increasingly popular pulse frequency modulation (PFM) configuration having multiple outputs sharing a single inductor. A particularly important drawback of buck and boost methods is poor response when the output voltage 103 operates close to the input voltage (battery 105) causing unacceptably long inductor charging or discharging times. The buck-boost method shown in FIG. 2C improves this response time, but at the price of significant loss in energy efficiency due to the need to operate at high switching frequency. The high frequency is needed because less energy is transferred to the load during a charge/discharge cycle. That can be seen graphically as the area under the curves in FIG. 2C is smaller than in 2A or 2B for any particular cycle. An additional disadvantage of the above traditional methods is the need for a user to manually set the operating mode i.e. buck, boost, or buck-boost.
Accordingly, improvements in DC-DC converters is desirable to overcome the disadvantages described above.