Introduction to Switch-mode DC to DC converters
Any type of power Converter whether it is AC to DC or DC to AC converter takes electrical power in one form and converts it in to another form. A DC to DC converter is an electronic circuit which by electronic switching transforms a DC source voltage from one level to another.
Significance of DC to DC conversion
But why DC to DC converters are needed? What is the need to convert DC voltage to DC voltage? It can be explained as follows for example consider electronic switch mode converters used in cell phones and laptop computers. These devices are powered using rechargeable batteries. This battery has to power all the components in the cellular phone or laptop such as display, USB devices e.t.c. Each of these components may be operating at different voltages and current levels. DC to DC converters step up or step down DC voltage level so that it will be suited to the requirement of interest.
Also as the stored power drains with time, the battery voltage declines. By providing feedback from the output stage, DC to DC converters can be made to compensate this lowered battery voltage.
Often the input to these DC to DC converters is an uncontrolled DC voltage forms a diode rectifier which is unregulated. The input to DC to DC converters is unregulated not due to poor rectification of diode rectifier but due to fluctuations in line voltage. These line fluctuations gets reflected in the output which makes it unregulated. The DC to DC converters converts these unregulated DC input voltage into controlled DC output at desired voltage level. They can be thought to be analogous to transformers which step up or step down AC currents or voltages.
Typical block diagram of Switch-mode DC to DC converter
The typical block diagram of Switch-mode DC to DC converter is shown the figure below
A Switch-mode DC to DC converter typically consists of following components
The electronic switch (power MOSFET, IGBT, Thyristor e.t.c) converts input DC to AC signal. The isolation transformer isolates input and output stages. Isolation transformer stage may or may not exist in a DC to DC converter based on requirement. The AC output from switch or isolation transformer secondary is converted into DC using rectifier and filter circuits. The filter and regulator stages contain storage elements which store the input energy temporarily and then releasing that energy to the output at a different voltage
Types of DC to DC converters
There are mainly two types of DC to DC converters
1. DC to DC converters without isolation
These DC to DC converters does not have any isolation transformer between input and output stages. Some of the commonly used DC to DC converters with isolation are
a. Buck converter
b. Boost converter
c. Buck-boost converter
The buck converter is step down converter (input voltage > output voltage) whereas Boost converter is a step up converter (voltage > output voltage). The buck boost converter is derived from step up and step down converters. The buck boost converter can be operated in step up or step down mode base on duty cycle of switch (ton/Ts where ton is the duration for which switch is on and Ts is the switching time period). The Step down and step up converters are basic converter topologies based on which other converters are derived.
2. DC to DC converters with isolation
A transformer is provided in between to isolate the input and output stages. The electrical isolation is an additional feature and is mainly useful in cases where the input voltage level (Vin) and output voltage level (Vout) differs significantly i.e. high or low values of Vout/Vin. The DC to DC converters with isolation is again divided into two types based on polarity of transformer core excitation
Unidirectional core excitation, core is excited with forward currents of only one direction. In these DC to DC converters the isolation transformer core is operated in only the positive part of B-H curve.
Bidirectional core excitation, core is excited with currents in either direction. In these DC to DC converters the isolation transformer core is operated alternatively in positive and negative portions of B-H curve.
Some of the commonly used DC to DC converters with isolation are
a. Cuk converter (can be used in non isolated mode also)
b. Fly back converter
c. Forward converter
d. Full bridge converter
e. Half bridge converter
f. Push-pull converter e.t.c
Applications of DC to DC converters
The DC-DC converter products are used extensively for diverse applications in the healthcare (bio life science, dental, imaging, laboratory, medical), communications, computing, storage, business systems, test and measurement, instrumentation, and industrial equipment industries. They are used in Used in electric motor drives, in Switch mode DC power supplies e.t.c