dc to dc converters

Guru/DC2DC/Basics/ May 5, 2006 Basic Principle of a dc-to-dc converter 1

DC to DC converters

A dc-to-dc converter is used to change the dc voltage from one level to another. In this

case, the dc input voltage is fixed and the level of the dc output voltage depends upon the

converter’s topology. The dc output voltage can be higher or lower than the input voltage.

Since the advent of diodes, the techniques have been developed to obtain the dc

voltage from the time-varying sinusoidal (ac) supply. The half-wave rectifier and the

bridge rectifier are used to obtain dc voltage from a single-phase time-varying source. To

control the ripple of the rectified output voltage, large capacitor filters are used. These

circuits, now referred to as the linear regulators, operate at the frequency of the ac

voltage, which is usually either 50 Hz or 60 Hz.

Until about two or three decades ago, the linear regulators were the only reliable

methods to meet all dc requirements. Some of the major problems associated with the

linear regulator is its size and weight of its components such as the transformer. The

voltage regulator element in these circuits has a comparatively high voltage across its

terminals and dissipates large amounts of power, which results in low efficiency. For this

very reason, the use of linear regulators is now limited to low power applications.

As the power semiconductor devices became more reliable and efficient in their

operation, the switched mode power supplies came into existence. In the design of these

power supplies, the semiconductor devices are either switched on or switched off. Due to

the low voltage drop across the semiconductor device when it is on, its power

consumption is low. For this reason, the switched mode power supplies are highly

efficient. Since the switching action, which simply means to turn a power semiconductor

device either on or off, is usually done at high frequencies, the relative size and weight of

the components needed for its design is comparatively small.

In this chapter, our aim is to obtain a dc output voltage, which may be higher or

lower, from a fixed dc input voltage. A very simple scheme that illustrates the principle is

shown in Figure 1. In this case, the dc voltage applied to the resistor is controlled via a

switch, which is usually a power semiconductor device such as an SCR, a BJT, a

MOSFET, an IGBT, etc.


Figure 1: A simple dc-to-dc converter

Let us assume that the time period of a switching frequency f is T such that

f/1T = . The switch is closed for a fraction of the time period T and is kept open for the

remainder period. Let us say that the switch is turned on at t = 0 and remains on

for onTt = , where onT is called the on time which is some fraction of T such as TDTon =

and 1D0 ≤≤ . D is usually referred to as the duty cycle. The output voltage obtained by

opening and closing of the switch is shown in Figure 2.

Figure 2: Output voltage as a function of time

The time during which the switch remains closed is customarily referred to as the

off time (period). We can express the off time in terms of the duty cycle as

T)D1(TTT onoff −=−=

The average output voltage may be computed as

son

T

0

so VT

TdtV

T

1V

on

== ∫

Substituting, ,TDTon = the output voltage in terms of the duty cycle is

so VDV =

In this case, the output voltage is directly proportional to the duty cycle. It is

therefore evident that the output voltage is less than the input voltage. For an ideal

switch, the efficiency of the dc-to-dc converter is 100%.


This simple circuit can be designed to meet the dc output-voltage requirements.

However, it has one major drawback. Its percent voltage ripple is 100%. The output

voltage with such a high ripple content may be satisfactory for electric heaters, light-

dimming circuits, etc., it is certainly not suitable for the operation of amplifiers and other

circuits requiring almost constant dc voltage.

The high voltage ripple can be controlled by placing a capacitor across the load.

The capacitor is large enough so that its voltage does not have any noticeable change

during the time the switch is off. Somewhat better circuit can be developed by including

an inductor, which is in series with the switch when the switch is on (closed), to limit the

current in rush. However, this creates another problem. Since the current in the inductor

cannot change suddenly, we have to provide at least one more switch, such a

freewheeling diode, to provide a path for the inductor current when the switch is off

(open).

In summary, a good dc-to-dc converter may have, an inductor, a capacitor, and a

freewheeling diode, and an electronic switch. The placement of these elements in a

circuit dictates the performance of the circuit. The three configurations that utilize these

circuit elements are (a) Buck Converter (lowering the output voltage, step-down

application), (b) Boost Converter (raising the output voltage, step-up application), and

(c) Buck-Boost Converter (lowering or raising the output voltage, step-down or step up

application).

dc to dc converters

Documents