13-09-2013, 12:30 PM
DC-DC Converter Basics
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ABSTRACT
A DC-to-DC converter is a device that accepts a DC input voltage and produces a DC output
voltage. Typically the output produced is at a different voltage level than the input. In addition,
DC-to-DC converters are used to provide noise isolation, power bus regulation, etc. This is a
summary of some of the popular DC-to-DC converter topolopgies:
BUCK CONVERTER STEP-DOWN CONVERTER
In this circuit the transistor turning ON will put voltage Vin on one end of the inductor. This
voltage will tend to cause the inductor current to rise. When the transistor is OFF, the current will
continue flowing through the inductor but now flowing through the diode. We initially assume that
the current through the inductor does not reach zero, thus the voltage at Vx will now be only the
voltage across the conducting diode during the full OFF time. The average voltage at Vx will
depend on the average ON time of the transistor provided the inductor current is continuous.
Transition between continuous and discontinuous
When the current in the inductor L remains always positive then either the transistor T1 or the
diode D1 must be conducting. For continuous conduction the voltage Vx is either Vin or 0. If the
inductor current ever goes to zero then the output voltage will not be forced to either of these
conditions. At this transition point the current just reaches zero as seen in Figure 3. During the
ON time Vin-Vou
CONVERTER COMPARISON
The voltage ratios achievable by the DC-DC converters is summarised in Fig. 10. Notice that only
the buck converter shows a linear relationship between the control (duty ratio) and output
voltage. The buck-boost can reduce or increase the voltage ratio with unit gain for a duty ratio of
50%.
Isolated DC-DC Converters
In many DC-DC applications, multiple outputs are required and output isolation may need to be
implemented depending on the application. In addition, input to output isolation may be required
to meet saftey standards and / or provide impedance matching.
The above discussed DC-DC topologies can be adapted to provide isolation between input and
output.
Flyback Converter
The flyback converter can be developed as an extension of the Buck-Boost converter. Fig 14a
shows tha basic converter; Fig 14b replaces the inductor by a transformer. The buck-boost
converter works by storing energy in the inductor during the ON phase and releasing it to the
output during the OFF phase. With the transformer the energy storage is in the magnetisation of
the transformer core. To increase the stored energy a gapped core is often used.
In Fig 14c the isolated output is clarified by removal of the common reference of the input and
output circuits.