Switching Power Supply: Design and Schematic

solar windDesign of a Switching Power Supply

Design of a Switching Power Supply

Welcome to this informative page. With the aim of saving energy and a cleaner and better future we could not avoid telling switching power supplies. In this page we will explain what is a switching power supply, how does it work, what is its wiring diagram, what are the differences between the old stabilized linear power supply and these new power supplies and the differences between switching step up power supplies (boost ) And those of type step down (buck) and finally how to make a proper design.

Due to their small size and the ability to obtain a wide range of voltages, switching power supplies have now replaced traditional linear stabilized power supplies in many applications. Many electronics enthusiasts, who would be in a position to build a linear stabilized power supply, encounter no less difficulties when deciding to make a switching power supply. If it is not difficult to understand in great detail the principle of operation of this latest generation of power supplies, in fact, designing one and running it as it should be, it's just another matter. Switching power supplies have a much more complex electrical scheme compared to a classical power supply, without the need to implement some constructive design even during the implementation phase.

Difference between linear power supply and switching power supply

Born from aerospace technology, in which the use of lightweight and small-scale equipment with high efficiency is of paramount importance, switching power supplies have for some years now become bullied in common use, finding widespread diffusion in most of electronic equipment.

It is thanks to their very small size that it has been possible to produce more and more miniaturized and efficient devices such as personal laptops, DVD players, universal chargers and many other everyday devices. Reduced size and lightness are, however, not the only prerogative, because switching offers the electronic designer other possibilities, which make them almost irreplaceable in some applications. To sum up, one can get higher output voltage than the input applied, the so-called step-up (or boost) function, which can not be assured by the classic linear power supply.

In fact, the latter uses an adjustment element, generally constituted by a power transistor, on which a voltage drop is produced that allows to adjust the output voltage. In this case, the transistor works as a variable resistance in series to the load. As a result, the output voltage is always lower than the input voltage. It is a regulation system that works fine but has the disadvantage of a rather low efficiency, generally between 30% and 60%, since a negligible part of the input power is dissipated and therefore lost on the element Of regulation. The latter must be mounted on a suitable heat sink so that it will work at not too high temperatures that, incidentally, this power supply is also called "dissipative".

This does not happen with the switching power supply, which works in a completely different way. With this type of power supply, not only is it possible to produce output voltages higher than input voltages but above all achieve a much higher performance, in the order of 80% -90%, which can significantly reduce both the Its dimensions, those of the cooling fins and the power supply transformer and to extend operating times on battery-powered equipment. It soon becomes clear how this type of power supply can be useful even in small wind and photovoltaic systems where it is important to maximize the production and management of energy.

On the other hand, the switching power supply has some disadvantages, such as a ripple superimposed on the rather large output voltage and high frequency noise, which make it unobvious in some sensitive applications such as laboratory stabilized power supplies or hi-power amplifiers -fi, in which the traditional power supply is still beneficial. To overcome the difficulty of switching design, numerous integrated circuits have long been available on the market, which give the hobbyist the opportunity to realize the type of power supply he needs from time to time. One of these is the integrated MC34063A (see datasheet), which allows you to create a wide range of switching power supplies.

Linear and Switching: Table of Differences

Difference Linear Power Supply Switching Power Supply
Dimension Big Small
Weight Heavy Light
Efficiency 30 to 40% 70 to 95%
Complexity Simple Complex
Electromagnetic Interference (EMI) Low noise Filtration is required
Cost High (due to the materials used) Low

Switching power supplies: step-down and step-up

Below we will show you the two main types of power supply, namely step-down, with which the output voltage is lower than the input applied and the step-up, which allows to obtain a continuous output voltage Higher than the incoming one. We will also explain what are the differences between these two configurations and how to calculate the different components needed for their realization. You will find out that with this integrated, the design of a switching does not present any particular difficulty and really becomes within reach of everyone. Once you've got confidence in this subject, you'll be able to enjoy more complex switching power supplies and more sophisticated performance.


Schematic of a step-down switching power supply

The operating principle of a step-down switching power supply is shown in the figure above. Inputs are applied to the DC voltage coming from the rectifier and voltage leveler, or from a battery. In this case, the switching power supply can also be seen as a DC-DC converter, ie continuous voltage - continuous voltage. A switch (S1) is located on the input line, behind which there is an inductance L1 which is in series with the load, schemated by the RC resistor. The capacitor C1 is parallel to the load. At one end of the inductance, the DS1 diode cathode is connected.

To understand how the power supply works, it is necessary to observe what is happening in the closing phase and in the switch opening time. We call Ton the time the switch remains closed, Toff the time it stays open, and T the sum of the two Ton + Toff sums. When the switch is closed, a current flows through it, which partially crosses the inductance and the load in series, and partially charges the condenser C1. The value of the current flowing in the inductance grows progressively during Ton's time, because this component has the characteristic of opposing the change of current flowing through it.

If Ton time has elapsed, the switch will open, the inductor will tend to circulate in the circuit the same current value that was reached at that time. The head of the inductance produces a voltage, with the polarity indicated in the figure, which also tends to circulate for the time Toff the current on the load through the diode DS1, which is now polarized directly. In this way the load voltage is also present in the Toff time, that is, with the open switch. When the current passing through the inductance decreases, the condenser takes charge of the load, maintaining the constant voltage. Attaching and disconnecting the switch periodically, a voltage value is obtained whose amplitude depends on the relationship between the time Tone and the T period. This ratio is called a duty cycle.


Schematic of a step-up switching power supply

With this type of converter, it is possible to extract a higher voltage from the input applied to the input, provided that it can only be obtained with a switching power supply. In the figure below we have reported the basic scheme of such a configuration. The inductance L1 with respect to the step-down configuration is set in series, via the DS1 diode, to the input voltage and to the output voltage.

During the Ton phase, when the transistor is saturated, the inductance accumulates energy to transfer it to the output during the Toff phase, thus adding an additional voltage to the input voltage, which outputs a higher voltage than the input voltage . The DS1 diode avoids that the output voltage is shorted out by the transistor during the Ton phase.

Did you find this information useful?
Let's keep in touch through the Eco-Newsletter or via Facebook!