A Quick Reference to the most Common Electronic Components

In this page, we continue our reference to the most common electronic components with a short description of transistors and diodes.

Transistors

By many the transistor is considered as the biggest discovery/innovation of the 20th century. Indeed inside every electronic device nowadays, someone will find transistors working effortlessly and reliably. The two most common types of transistors are Bipolar Junction Transistors (BJTs) which can be broke down in NPN and PNP transistors and Field Effect Transistors (FETs). Similar to BJTs, FETS come in N-channel and P-channel ones. The two major types of FETs are MOSFETS (Metal-Oxide Semiconductor Fets) and JFETS (Junction Fets).


A transistor has three leads, source, gate and drain. To explain its operation we will use a very simple paradigm. Think of a pipe that connects a source of water to a drain. The valve (gate) controls the flow of water either by being fully closed, partially open or fully open. The same goes with a transistor, by applying voltage to the gate we can control the current flowing from source to drain. In NPN transistors the source, gate and drain are called collector, base and emitter respectively. The two main roles of transistors are amplifying small signals or switching.

In PSUs mostly NPN MOSFETs are used, in APFC stage and as main switches. In order to further increase efficiency they are also used in the secondary side, to rectify the DC outputs (synchronous design).

Diodes

A diode can be considered as a one-way valve, which allows current to flow in one direction but not through the other, when we apply voltage on it. The above sometimes is also called rectifying process. The one end of a diode is called anode and the other cathode. Most of the diodes allow current to freely flow from anode to cathode. When current starts to flow from a diode then there is a constant voltage drop on it. For most diodes this voltage drop is approximately 0.7V.

All diodes have a current rating that informs about the maximum forward current that they can withstand. Also the PIV rating tells the maximum reverse voltage that a diode can handle before it breaks down. Now if you want to find out if a diode is working properly then all you have to do is measure it with a multimeter, using the Ohms scale. In one direction the diode should have low resistance (forward-biased) and on the opposite high resistance (reverse-biased).

Diodes have numerous usages. Some of the most common are voltage regulation, AC rectifying (bridge rectifiers), leds, overvoltage protection etc. In many PSUs, besides the common diodes, almost always we find bridge rectifiers (four diodes in bridge arrangement that provide full-wave rectification to incoming AC signal) and Schottky Barrier Diodes (SBRs). The latter are used in the APFC section (boost diode) and sometimes for the rectifying process of the DC outputs in the secondary side. Schottky diodes are special diodes with lower forward voltage drop than the common diodes. However in high efficiency PSUs they are fully replaced by MOSFETs that dissipate less energy.