The power electronics devices make the building blocks of a variety of power electronics converters for changing the ac and dc voltage level or changing the frequency at the interface of the power mains and the load.
The dc-dc converter is needed to change voltage for load equipment or for charging and discharging a battery in the dc power distribution system. Since such a converter achieves the desired function by switching the dc power on and off (chopping the power) at high frequency, it is also known as the dc chopper or switch-mode power converter.
BUCK CONVERTER ON SHIP ELECTRICAL SYSTEM
The buck converter steps down the input voltage; hence, it is also known as the step-down converter.
The switching device used in this converter may be the bipolar junction transistor (BJT), metal-oxide semiconducting field effect transistor (MOSFET), or insulated gate bipolar transistor (IGBT). The switch is turned on and off periodically at high frequency, typically in tens of kilohertz.
Since the buck converter is widely used to charge the battery, it has yet another name, the battery charge converter. It is required to buck (step down) the dc bus voltage to the battery voltage during charging.
In the buck converter, we expect the source side at higher voltage to draw lower current than the load side at lower voltage.
BOOST CONVERTER ON SHIP ELECTRICAL SYSTEM
The boost converter steps up the input voltage to a higher output voltage. When the transistor switch is on, the inductor is connected to the input voltage source, the inductor current increases linearly while the diode is reverse biased, and the capacitor supplies the load current.
When the switch is off, the diode becomes forward biased, the inductor current flows through the diode and the load, and the inductor voltage adds to the source voltage to increase the output voltage.
The output voltage of the boost converter is derived again from the volt-second balance principle in the inductor.
BUCK-BOOST CONVERTER ON SHIP ELECTRICAL SYSTEM
Combining the buck and boost converters in a cascade gives a buckboost converter, which can step down or step up the input voltage. A modified buckboost converter often used for this purpose.
The buck-boost converter is capable of a four-quadrant operation (i.e., both V and I can be positive or negative), making it suitable in variable-speed drives for a dc motor with regenerative braking. The converter is in the step-up mode during the generating operation and in the step-down mode during the motoring operation.
FLYBACK CONVERTER (BUCK OR BOOST) ON SHIP ELECTRICAL SYSTEM
Topology of the flyback converter, which can buck or boost the voltage depending on the turn ratio Nin/Nout of the coupled inductor.
The energy is stored in the primary side of the inductor during on time. When the switch is turned off, the primary side energy is inductively transformed—gets kicked or flyover—to
the secondary side and delivered to the load via diode. The polarity marks of the two
coils are of essence in this circuit.
The magnetic flux in air only—as opposed to flux in a magnetic core—can store magnetic energy. Therefore, the energy storage inductor must have air in its magnetic
flux path. The inductor core in the converter is therefore made with an air gap in one lump or distributed throughout the core, such as in a potted magnetic metal powder core with nonmagnetic binder interleaved that is magnetically equivalent to air. The energy is stored in distributed air gap of the core.
TRANSFORMER-COUPLED FORWARD CONVERTER ON SHIP ELECTRICAL SYSTEM
The converter circuit employs a transformer with suitable turn ratio Nin/Nout in the conventional buck converter such that the output voltage can be below or above the input voltage. The polarity marks are of essence. A bleeding resistor is required in parallel with the load to keep the converter working with a certain minimum load. Its voltage relation is identical to that for the buck converter modified by the transformer turn ratio.
PUSH-PULL CONVERTER ON SHIP ELECTRICAL SYSTEM
The push-pull converter is again a buck-boost converter. The converter topology uses a center-tapped transformer, which gets square wave excitation.
It is seldom used at low power levels but finds applications in systems dealing with several tens of kilowatts and higher power. It can provide a desired output voltage by setting the required transformer turn ratio. The voltage ratio and the efficiency are the same as those given by Equations.
INDUCTOR-COUPLED BUCK CONVERTER ON SHIP ELECTRICAL SYSTEM
The inductor-coupled converter is also known as a Cuk converter after its inventor.
All converters presented previously need an inductor-capacitor (L-C) filter to control the ripple in the output voltage. The buck converter needs a heavier filter than the boost converter. Since ripples are present in both the input and output sides, it is possible to cancel them by coupling the ripple current slopes of two sides with matched inductors. The ripple magnitudes are matched by using the required turn ratio, and the polarities are inverted by winding the two inductors in magnetically opposite directions. The net ripple on the output side is zero at a certain air gap, below which it changes the polarity.
A gap-adjusting screw is needed to tune the gap for a precise match.
