DC-DC Power Converters

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introduction

Dc-dc power converters are employed in a variety of applications, including power supplies for
personal computers, office equipment, spacecraft power systems, laptop computers, and
telecommunications equipment, as well as dc motor drives. The input to a dc-dc converter is an unregulated
dc voltage Vg. The converter produces
a regulated output voltage V, having a
magnitude (and possibly polarity) that
differs from Vg. For example, in a
computer off-line power supply, the
120 V or 240 V ac utility voltage is
rectified, producing a dc voltage of
approximately 170 V or 340 V,
respectively. A dc-dc converter then
reduces the voltage to the regulated 5 V
or 3.3 V required by the processor ICs.


Converter circuit topologies
A large number of dc-dc converter circuits are known that can increase or decrease the magnitude
of the dc voltage and/or invert its polarity [1-5]. Figure 4 illustrates several commonly used dc-dc converter
circuits, along with their respective conversion ratios. In each example, the switch is realized using a power
MOSFET and diode; however, other semiconductor switches such as IGBTs, BJTs, or thyristors can be
substituted if desired.


Analysis of converter waveforms
Under steady-state conditions, the voltage and current waveforms of a dc-dc converter can be
found by use of two basic circuit analysis principles. The principle of inductor volt-second balance states
that the average value, or dc component, of voltage applied across an ideal inductor winding must be zero.
This principle also applies to each winding of a transformer or other multiple winding magnetic devices. Its
dual, the principle of capacitor amp-second or charge balance, states that the average current that flows
through an ideal capacitor must be zero. Hence, to determine the voltages and currents of dc-dc converters
operating in periodic steady state, one averages the inductor current and capacitor voltage waveforms over
one switching period, and equates the results to zero.