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All About power Electronics - LPS

A linear power supply was the first type of electronic power supply ever invented. It was generally used to convert high voltage AC(around 110-250v) from our homes to a low voltage stable DC(around 24-5v) to power household equipment like audio amplifiers, led lights, DC fans, televisions, dvd players etc. Any power supply that converts high voltage AC to a low voltage DC is called an AC adapter or a  can also be called an AC-DC converter.

 A linear power supply can either be internal (inbuilt in an equipment) or external( a separate equipment) and it also good to note that it's no more a commonly used power supply as a much more efficient type of power supply called the smps (which we will learn later in this tutorial) has taken over because of it high efficiency and size. Below is a photo of a typical LPS circuit.

 It contains few components, including a transformer, diodes, capacitors and a regulation IC. To understand the whole working of the circuit, we have to break it into different stages. Each of the stages of conversion is highlighted below in the block diagram.



The first stage shows the input voltage source, followed by the transformer then to the rectifiers and to the filters before reaching the regulation stage. Each of these stages will be explained in full in this tutorial.

Input voltage

The input voltage is an alternating current ranging from 110 to 250v at 50-60hz frequency, depending on the country. Like in the USA, it's 120v at 60hz, and in places like India, Nigeria and the UK, it around 220v at 50hz frequency. It usually gotten from the wall outlet in our homes and can also be called "line voltage" or "mains voltage". In many power supplies, the input parameters are always written on it body as shown below. 

<image>

The transformer stage

This stage is only made up of a step-down transformer. The purpose of the transformer here is to step down the high voltage AC(120-250v) into a low voltage AC(24-9v). But how does it accomplish this? To understand this, let have a brief explanation on the working of a transformer. 

A transformer is an electrical device used for stepping-up or stepping-down an alternating or a fluctuating signal. It has a primary and a secondary winding and also a main core which is made up of iron or ferrite material.

The working of a transformer is based on the action of electromagnetic induction. Whenever there is a changing magnetic field or magnetic flux linking a coil of wires, an electric current is produced and this current is called an induced current. Also whenever there is a changing electric field surrounding a coil, a magnetic field is also produced. These phenomenon is called electromagnetic induction.

In the case of transformers, when the primary winding experience a changing or varying electric field, a magnetic flux will be generated around it and this magnetic flux will be linked to the secondary winding and then induce voltage on it. The magnitude of the induced voltage is dependent on the turns ratio of the primary and secondary winding, for example, if the primary winding has the same number of turns as the secondary, the induced voltage will have the same magnitude. But if the number of turns in the secondary winding is lesser or greater than the primary winding, then the magnitude of the induced voltage or current will turn to be greater or smaller. 

<image>

From the above image description we can see that in other for our transformer to step down the voltage, the number of turns of primary winding must be greater than that of the secondary winding and vise-versa.

One important point to note here is that for voltage to be induced to the secondary winding, the magnitude of current in the primary winding must be changing or fluctuating(or we can say varying) as respect to Faraday's law. That is why transformers work with only AC because the current is alternating, that is, it reverses it polarity 50-60 times per second. The output of this transformer is a low voltage AC.

The rectifier stage

 This stage can consist of just a single diode or multiple diodes. It job is to convert the low voltage AC which was previously stepped downed by the transformer into a pulsating direct current. Any component or circuit that convert AC to pulsating DC is called a rectifier and the process is called rectification.

Generally, diodes are always used as the rectifiers because of their ability of allowing electric current to flow only in one direction but blocks it if is coming in the other direction. So Since alternating current flows in both directions, from the positive half cycle to the negative half cycle periodically, a diode can be employed to force the current to flow in only one direction making the AC waveform appear to have only one cycle as shown below.

<image>

We have 2 types of rectifiers namely; The half wave and the full wave rectifier. Each of them is used to obtain different waveform and different level of rectifications.

Even if a rectifier converts AC to DC, the output of any rectifier is not a pure direct current but rather a pulsating one. It pulsating because the current is not linear or constant but it contains ripples. Ripple voltage or current simply means that it continuously varying and it needs to be smoothened or the ripples needs to be filtered out.

The filter stage

An electrolytic capacitor is used to filter out the unwanted harmonics present in the rectified DC voltage and tend to make the voltage linear. It also called a power factor correction(PTF) circuit as the filtering process also serves to increase the overall power factor of the circuit. A capacitor  or an inductor or the combination of both is usually employed in this stage to form passive filters.

We have up to 5 configurations of passive filters consisting of only capacitors, inductors and resistors, arranged in different topologies to achieve different filtering actions. Examples are :

  1. Series capacitor filters
  2. Series inductor filters
  3. Choke input filter or LC filters
  4. Capacitor input filters
  5. RC filters

Most of these filters are no longer in use today, so we not no waste time studying them we will just focus on the basics of filters and then move forward.

Working of series capacitor filers







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