DC-DC түрлендіргіштер: Buck-Boost түрлері мен жұмыс принциптері

Type of DC to DC converter
the type of switched mode power supply

The buck–boost converter is a type of DC to DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It is equivalent to a flyback converter using a single inductor instead of a transformer. Two different topologies are called buck–boost converter. Both of them can produce a range of output voltages, ranging from much larger (in absolute magnitude) than the input voltage, down to almost zero. In the inverting topology, the output voltage is of the opposite polarity than the input. This is a switched mode power supply with a similar circuit configuration to the boost converter and the buck converter. The output voltage is adjustable based on the duty cycle of the switching transistor. One possible drawback of this converter is that the switch does not have a terminal at ground; this complicates the driving circuitry. However, this drawback is of no consequence if the power supply is isolated from the load circuit (if, for example, the supply is a battery) because the supply and diode polarity can simply be reversed. When they can be reversed, the switch can be placed either on the ground side or the supply side. When a buck (step down) converter is combined with a boost (step up) converter, the output voltage is typically of the same polarity of the input, and can be lower or higher than the input. Such a non inverting buck boost converter may use a single inductor which is used for both the buck inductor mode and the boost inductor mode, using switches instead of diodes, sometimes called a "four switch buck boost converter", it may use multiple inductors but only a single switch as in the SEPIC and Ćuk topologies.

Like the buck and boost converters, the operation of the buck boost is best understood in terms of the inductor's "reluctance" to allow rapid change in current. From the initial state in which nothing is charged and the switch is open, the current through the inductor is zero. When the switch is first closed, the blocking diode prevents current from flowing into the right hand side of the circuit, so it must all flow through the inductor. However, since the inductor doesn't allow rapid current change, it will initially keep the current low by dropping most of the voltage provided by the source. Over time, the inductor will allow the current to slowly increase by decreasing its own resistance. In an ideal circuit the voltage drop across the inductor would remain constant. When the inherent resistance of wires and the switch is taken into account then the voltage drop across the inductor will also decrease as the current increases. Also during this time, the inductor will store energy in the form of a magnetic field.

The four switch converter combines the buck and boost converters. It can operate in either the buck or the boost mode. In either mode, only one switch controls the duty cycle, another is for commutation and must be operated inversely to the former one, and the remaining two switches are in a fixed position. A two switch buck boost converter can be built with two diodes, but upgrading the diodes to FET switches doesn't cost much extra while efficiency improves due to the lower voltage drop.