The present invention relates generally to electrical power supplies. More particularly, it relates to a step-down power converter with a full bridge circuit connected without isolation to buck output circuits.
Buck converters are commonly used in electronics for changing the voltage or polarity of a power supply. Buck converters typically employ two electronic switches (typically MOSFETs) in combination with an output inductor. The switches are alternately turned on, thereby providing current pulses to the output inductor.
Buck converters are often used to provide power to microprocessors, since they can convert standard 12 volt electrical power to the lower voltage needed. However, in recent years, the power requirements for microprocessors have begun to exceed the capabilities of conventional buck converters. Some microprocessors now in development, for example, will require 100 Amps at 1 volt, with less than 25 mV ripple. It is very difficult to produce this electrical current from the 12 volt power available, especially in combination with demanding efficiency and cost requirements. Conventional converters are expensive, inefficient, and require a large amount of circuit board space.
FIG. 1 shows a multi-phase buck converter of the type commonly used in conventional microprocessor power supplies. The specific multi-phase buck converter of FIG. 1 includes three buck converter circuits connected in parallel, with each converter having two transistor switches (e.g. Q1 and Q2), and an output inductor L. The buck converters are operated in a phased relationship, so that the frequency of the output ripple at the load RL is a multiple of the frequency of each buck converter circuit. The circuit design of FIG. 1 presents several problems when attempts are made to increase output current, reduce output voltage, and reduce output voltage ripple. When solving these problems conventionally, more transistors are added to increase current capability, more buck converter stages are added, the capacitor C is increased in size, and operating frequency is increased. These changes tend to increase cost and circuit board space requirements, since more components are used. Efficiency is reduced because reverse recovery loss of the transistor body diodes is proportional to switching frequency. Also, turn-off loss is increased since it is proportional to both on-state current and operating frequency. Also, if a high voltage step-down ratio is required (e.g. step down from 12 volts to 1 volt), the necessary low duty cycle contributes to large switching losses and reduced efficiency.
With microprocessors having ever-more demanding power requirements (higher current, lower voltage, lower ripple), the conventional multi-phase buck converter approach is becoming impractical. There is an urgent need for voltage step-down power supplies that can provide well-regulated and high current power for advanced microprocessors. It would be particularly beneficial if such a power supply had reduced cost and reduced circuitboard space requirements, and had improved efficiency.
The present invention includes an electrical power step-down converter having a full bridge circuit, a transformer with primary and secondary windings, and a pair of buck output circuits. The transformer primary is connected across legs of the full bridge circuit. The transformer secondary is connected across legs of the full bridge circuit at point downstream from the full bridge (i.e. between the full bridge and buck output circuits). The full bridge circuit and buck output circuits are not isolated; they are directly connected.
In high power embodiments of the invention, the step-down converter can include an additional transformer and an additional pair of buck output circuits. The additional pair of buck output circuits are connected in parallel with the buck output circuits.
Preferably, the transformer has a 1:1 turns ratio. The transformer primary can have a capacitor connected in series to prevent saturation of a magnetic core of the transformer.
The present invention also includes a step-down converter having a full bridge circuit, a pair of primary inductors connected across the full bridge circuit, and a pair of buck output circuits connected to the full bridge circuit. Each buck output circuit has a secondary inductor coupled to one of the primary inductors. The buck output circuits are not isolated; they are directly connected to the full bridge circuit. Also, additional primary inductors and additional buck output circuits can be provided for higher power capability.