Power Electronics

These converters are used to obtain a variable ac output voltage Vo from a fixed ac source and a single-phase converter with a TRIAC. A TRIAC allows a current flow in both directions. It can be turned on by applying gate voltages at ωt = α for a current flow in the positive direction, and also at ωt = π + α for a current flow in the negative direction. The output voltage is controlled by varying the conduction time of a TRIAC or firing delay angle, α. These types of converters are also known as ac voltage controllers.

A single-phase converter with two natural commutated thyristors

- is also known as an inverter. A singlephase transistor inverter is shown in the figure.

When MOSFETs M1 and M2 are turned on by applying gate voltages, the dc supply voltage Vs appears across the load
and the instantaneous output voltage is Vo = +Vs. Similarly, when MOSFETs M3 and M4 are turned on by applying gate voltages, the dc supply voltage VS appears across the load in the opposite direction. That is, the instantaneous output voltage is Vo = −Vs. If transistors M1 and M2 conduct for one half of a period and M3 and M4 conduct for the other half, the output voltage is of the alternating form. The rms value of the output voltage becomes Vo(rms) = VS. However, the output voltage contains harmonics which could be filtered out before supplying to the load.

also known as a chopper, or switching regulator, and a transistor chopper

When transistor Q1 is turned on by applying a gate voltage VGE, the dc supply is connected to the load and the instantaneous output voltage is Vo = +Vs. When transistor Q1 is turned off by removing the gate voltage VGE, the dc supply is disconnected from the load and the instantaneous output voltage is Vo = 0. The average output voltage becomes Vo(AVG) = t1Vs/T = δ Vs. Therefore, the average output voltage can be varied by controlling the duty cycle. The
average output voltage Vo is controlled by varying the conduction time t, of transistor Q1. If T is the chopping period, then t1 = δT. δ is known as the duty cycle of the chopper.

- circuit converts ac voltage into a fixed dc voltage

• diode conducts when its anode voltage is higher than the cathode voltage, and it offers a very small voltage drop, ideally zero voltage, but typically 0.7 V. A diode behaves as an open circuit when its cathode voltage is higher than the anode voltage, and it offers a very high resistance, ideally infinite resistance, but typically 10 kΩ. The output voltage is a pulsating dc, but it is distorted

free electrons enable current flow in the semiconductor

Semiconductors are materials consisting of elements from group IV of the periodic table and having electrical properties falling somewhere between those of conducting and of insulating materials.

Aconducting material is characterized by a large number of conduction-band electrons, which have a very weak bond with the basic structure of the material.

Because the power devices can be operated as static switches or contactors, the supply to these switches could be either ac or dc and the switches are known as ac static switches or dc switches.