Introduction

A buck converter or step-down switch mode power supply can also be called a switch mode regulator. Popularity of a switch mode regulator is due to its fairly high efficiency and compact size and a switch mode regulator is used in place of a linear voltage regulator at relatively high output. Linear voltage regulators tend to be inefficient. Since the power devices used in linear regulators have to dissipate a fairly large amount of power, they have to be adequately cooled, by mounting them on heatsinks and the heat is transferred from the heatsinks to the surrounding air either by natural convection or by forced-air cooling. Heatsinks and provision for cooling makes the regulator bulky and large. In applications where size and efficiency are critical, linear voltage regulators cannot be used.

A switch mode regulator overcomes the drawbacks of linear regulators. Switched power supplies are more efficient and they tend to have an efficiency of 80% or more. They can be packaged in a fraction of the size of linear regulators. Unlike linear regulators, switched power supplies can step up or step down the input voltage.

The buck converter is introduced using the evolutionary approach. Let us consider the circuit in Fig. 1, containing a single pole double-throw switch.

For the circuit in Fig. 1, the output voltage equals the input voltage when the switch is in position A and it is zero when the switch is in position B. By varying the duration for which the switch is in position A and B, it can be seen that the average output voltage can be varied, but the output voltage is not pure dc. The output voltage contains an average voltage with a square-voltage superimposed on it, as shown in Fig. 2.. Usually the desired outcome is a dc voltage without any noticeable ripple content and the circuit in Fig. 1 is to be modified.

The circuit in Fig. 1 can be modified as shown in Fig. 3 by adding an inductor in series with the load resistor. An inductor reduces ripple in current passing through it and the output voltage would contain less ripple content since the current through the load resistor is the same as that of the inductor. When the switch is in position A, the current through the inductor increases and the energy stored in the inductor increases. When the switch is in position B, the inductor acts as a source and maintains the current through the load resistor. During this period, the energy stored in the inductor decreases and its current falls. It is important to note that there is continuous conduction through the load for this circuit. If the time constant due to the inductor and load resistor is relatively large compared with the period for which the switch is in position A or B, then the rise and fall of current through inductor is more or less linear, as shown in Fig. 3.

The next step in evolutionary development of the buck converter is to add a capacitor across the load resistor and this circuit is shown in Fig. 4. A capacitor reduces the ripple content in voltage across it, whereas an inductor smoothes the current passing through it. The combined action of LC filter reduces the ripple in output to a very low level.

The circuit in Fig. 4 contains a single-pole double-throw switch. It is a difficult configuration to realize using power semiconductor devices. On the other hand, an understanding of the circuit in Fig. 4 leads to a realizable and simple configuration. When the switch is in position A, the current through the inductor and it decreases when the switch is in position B. It is possible to have a power semiconductor switch to correspond to the switch in position A. When switch is in position B, the inductor current free-wheels through it and hence a diode can be used for free-wheeling operation. Then only the power semiconductor switch needs to be controlled, and in practice, a pulse-width modulating IC is used. The circuit that results is shown in Fig. 5.

Generally any basic switched power supply consists of five standard components:

1. a pulse-width modulating controller,
2. a transistor switch,
3. an inductor ,
4. a capacitor and
5. a diode.

Control by pulse-width modulation, usually effected by an IC, is necessary for regulating the output. The transistor switch is the heart of the switched supply and it controls the power supplied to the load. Power MOSFETs are more suited than BJTs at power outputs of the order of 50 W. Transistors chosen for use in switching power supplies must have fast switching times and be able to withstand the voltage spikes produced by the inductor.

An inductor is used in a filter to reduce the ripple in current. This reduction occurs because current through the inductor cannot change suddenly. When the current through an inductor tends to fall, the inductor tends to maintain the current by acting as a source. Inductors used in switched supplies are usually wound on toroidal cores, often made of ferrite or powdered iron core with distributed air-gap to minimize core losses at high frequencies.

A capacitor is used in a filter to reduce ripple in voltage. Since switched power regulators are usually used in high current, high-performance power supplies, the capacitor should be chosen for minimum loss. Loss in a capacitor occurs because of its internal series resistance and inductance. Capacitors for switched regulators are chosen on the basis of effective series resistance (ESR). Solid tantalum capacitors are the best in this respect. For very high performance power supplies, sometimes it is necessary to parallel capacitors to get a low enough effective series resistance.

The diode used in a switched regulator is usually referred to as free-wheeling diode or sometimes as a catch diode. The purpose of this diode is not to rectify, but to direct current flow in the circuit and to ensure that there is always a path for the current to flow into the inductor. It is also necessary that this diode should be able to turn off relatively fast. Diodes known as the fast recovery diodes are used in these applications.

Most of the switched supplies needs a minimum load, in order to ensure that the inductor carries current always. If the current flow through the inductor is not continuous, regulation may become poorer.

The buck converter or SMPS can be controlled in two ways, known as :

1. Constant-frequency operation or pulse-width modulation control
2. Variable-frequency operation or control by frequency modulation

With pulse-width modulation control, the regulation of output voltage is achieved by varying the duty cycle of the switch, keeping the frequency of operation constant. Duty cycle refers to the ratio of the period for which the power semiconductor is kept ON to the cycle period. Usually control by pulse width modulation is the preferred method since constant frequency operation leads to optimization of LC filter and the ripple content in output voltage can be controlled within the set limits. On the other hand, if the load on the converter is below a certain level, voltage regulation of output becomes a problem and in such a case, control by frequency modulation is to be preferred.

When control by frequency modulation is to be achieved, the ON period of the power semiconductor switch is kept constant and the frequency of operation is varied to effect voltage regulation. Design of LC filter is not easy in such a case.

If a micro-controller is used instead of a specific PWM IC, it is possible to switch from one mode of control to the other depending on the load conditions.

 
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