|Thursday, 02 November 2006|
Electrical loads are often required to be powered by one or more direct current (DC) voltages at regulated values that are derived from an available DC voltage source having a different and often unregulated value. In general, a DC (direct current) potential is supplied to the electronic device from either a battery or an alternating current (AC) rectification circuit. The DC potential is generally operating at a level which is either lower than or higher than the voltage level required by the electronic device. DC-to-DC converters provide the capability to convert energy supplied by a power supply from one voltage and current level to another voltage and current level. The converter receives electrical energy from a primary DC source and provides electrical energy output at a precisely controlled voltage or current magnitude. The primary DC source is typically an unregulated voltage source such as a full wave rectifier having a filtered output and an input connected to utility AC power or alternatively the output of an uninterruptible power supply. The basic principle of a DC-DC converter is that the switching element is turned on/off at high frequency, and the ratio of the on-time to off-time, or the duty ratio, is controlled in a variable manner to keep the DC output voltage at a certain level. Usually, the voltage is controlled to a constant level through negative feedback of the output voltage. More specifically, the output voltage is fed back and compared with a reference value by an error amplifier to find the error. A switching control circuit varies the duty ratio of the switching drive pulse sent to the switching element corresponding to the error signal of the error amplifier. DC-DC converters accept input energy from a voltage source at an input voltage and current and provide converted output energy at an output voltage and current, usually to a capacitively filtered output that functions as a voltage sink. When isolation is used, the input voltage is switched on and off at a high frequency and provided to a transformer. The transformer provides both input/output isolation and any required voltage level conversion. However, because the input voltage is typically switched at a high frequency the output voltage and current from the transformer cannot be directly provided to a load in a regulated manner. Typically, a DC-to-DC converter modulates the duty cycle of an output stage by a driving signal, to thereby regulate the output voltage of the converter within a demanded range. In general, a DC-DC power converter is coupled to an input power source with a voltage level either lower than or higher than the voltage level required by an electronic device. Switching regulators indirectly regulate an average DC output voltage to a device or application by switching energy on and off in an inductor. By comparing the output voltage to a reference, the inductor current can be controlled to provide a desired output voltage.
Direct-current to direct current voltage converters are used frequently in electrical and electronic systems to convert one voltage potential to another voltage potential. DC-DC converters typically have some form of regulation that controls an output voltage for the DC-DC converter as the electrical power consumed by an electrical load connected with the DC-DC converter changes. A DC-DC converter is generally composed of switching elements and an inductive element. The DC-DC converter comprises a converter circuit for converting an input voltage into an output voltage having a voltage value different from that of the input voltage in accordance with a switching element turned on and off by a rectangular-wave signal, as well as an output voltage detection circuit for detecting a value of the output voltage to output the detection voltage and a duty-ratio setting circuit for feedback-controlling a duty ratio on the basis of the detection voltage so that the output voltage would be controlled at a predetermined value. This structure allows the output voltage of the DC-DC converter to be controlled at a constant value. When the switching elements are turned on/off, current flows in the inductive element, and, as a result, the stored energy is supplied to the load side. By controlling the on/off timing of the switching elements, it is possible to supply the desired voltage that is different from the power source voltage to the load. A DC-DC converter includes a step-down circuit, which generates a DC output voltage that is lower than a power supply voltage, and a step-up circuit, which generates a DC output voltage that is higher than the power supply voltage. The DC-DC converter further includes a control circuit for receiving a switching signal from an external device to switch the operational mode from a step-up operation to a step-down operation or from a step-down operation to a step-up operation. A direct current (DC) voltage can be converted to another DC voltage via a DC-to-DC converter. DC-to-DC converters may take a variety of forms, such as a full-bridge inverter, a half-bridge inverter, a buck converter, a boost converter, or a flyback converter. Each type is better suited for a specific type of application. A switching DC-to-DC converter regulates a DC voltage source for supplying a DC output voltage with a desired voltage level by appropriately controlling a duty cycle of a power switch transistor. Where the DC output voltage is larger than the DC voltage source, the switching DC-to-DC converter is generally referred to as a boost converter or regulator. On the other hand, the switching DC-to-DC converter is generally referred to as a buck converter or regulator where the DC output voltage is smaller than the DC voltage source. A boost converter is typically used when the desired output DC voltage needs to be greater than the input voltage, while a buck converter is typically used when the output voltage needs to be less than the input voltage. A phase-modulated full-bridge converter (PMC) is a common topology used for DC--DC conversion. The PMC circuit typically includes an inductor and capacitor connected in series across the input terminals of the circuit. The main advantage of the PMC circuit is zero-voltage-switching of the switching elements while still operating at a constant switching frequency, which allows a simple control circuit. A constant on time converter, also known as pulse-frequency modulated (PFM) converter, consists of a control loop which contains an error amplifier, a comparator, and one or more drivers, usually coupled with a synchronous rectifier to improve performance. Another kind of DC to DC converter is referred to as a hysteretic converter, including voltage mode hysteretic converter and current mode hysteretic converter.
DC-to-DC power converters are widely utilized in power supplies to convert an input DC voltage into a specified output DC voltage. For low power digital applications, typically DC-to-DC power converters are required to efficiently convert an unregulated input DC voltage to a substantially constant output voltage. Modern electrical devices such as desktop computers, laptops, digital cameras, cellular phones, personal digital assistants (PDAs) and the like, include DC-DC converters that are required to be powered by regulated DC supply voltages of specific values, when the only electrical power available may be from a different DC voltage source having a voltage level different from the DC supply voltage or voltages desired. For example, in a computer system, due to the power supplies of different voltages for CPU, memory and hard disk drive, the DC-to-DC converter is required to regulate the power source voltage of the computer system to various supply voltages supplied to various operational units of the computer system. Switched mode DC-to-DC power converters are commonly used in the electronics industry to convert an available direct current (DC) level voltage to another DC level voltage. A switched mode converter provides a regulated DC output voltage by selectively storing energy by switching the flow of current into an output inductor coupled to a load. Multi-phase DC-DC converters are commonly used as point-of-load regulators when single-phase converters are insufficient. A single-phase converter may be insufficient due either to physical or economic limitations. One of the economic benefits afforded by multi-phase DC-DC converters is reduction in voltage ripple on the output. Buck converters are employed in applications where the electronic devices require a lower operating voltage than is supplied by the battery or the AC rectification circuit. Boost converters are employed in applications where the electronic devices require a higher operating voltage than is supplied by the battery or the AC rectification circuit. A single switch forward type DC-DC converter is widely used in the middle and low power conversion occasion due to the simplicity of the structure. A voltage step down synchronized rectifier type DC-DC converter is used as a power source for a variety of information equipment. Zero-voltage-switched (ZVS), full-bridge (FB), phase-shifted (PS) converters are commonly used for DC-DC conversion because of the several advantages that they offer over other approaches.