POWER ELECTRONICS
Instructor: Eng. Jalal Al Roumy
The Islamic University of GazaFaculty of EngineeringElectrical Engineering Department
الرحمن الله بسمالرحيم
EELE 5450 — Summer 2012
Lecture 7
Chapter Three
Triacs and Diacs
A power device with four layers conducts in one direction only.Bidirectional device may be obtained by connecting two of these back-to-back.
The five layers, n1,p1,n2,p2,n3 can be combined into a single structure to form a new device.When T1 is positive with respect to T2 by voltage greater than VBO (p2,n2,p1,n1) thyristor will be on.If Reverse polarity (p1,n2,p2,n3) will be on.
Chapter Three
A five-layer device without agate can be designed for various breakdown voltages and current ratings. A diac is a five-layer gateless device.
A diac is a device used to trigger other semiconductor power switches. Current and voltage ratings are determined by the type of device.
Chapter Three
A five-layer device with gate is called triac.( four stages)
Chapter Three
Unijunction Transistor (UJT)Complementary unijunction transistor (CUJT)
Programmable unijunction transistor (PUT)
The UJT has one pn junction and is used mainly as a triggering device in thyristor circuits and can also be used in oscillator circuits. The symbol is similar to a JFET. Note the angle of the emitter. The other terminals are called base 1 and base 2. The characteristics are quite different than any other transistor.
Chapter Three
The resistive equivalent circuit of a UJT shown makes it easier to understand its operation. The emitter current controls the value of rB1
inversely. The total resistance or interbase resistance (rBB) equals the sum of rB1 and rB2. The standoff ratio () is the ratio rB1/ rBB.
Chapter Three
UJT Operation:
Unijunction transistor can trigger larger thyristors with a pulse at base B1.With the emitter disconnected, the total resistance RBB, a datasheet item, is the sum of RB1 and RB2 . RBBO ranges from 4-12kΩ for different device types. The intrinsic standoff ratio η is the ratio of RB1 to RBBO. It varies from 0.4 to 0.8 for different devices.
Chapter Three
As VE increases, current IE increases up IP at the peak point. Beyond the peak point, current increases as voltage decreases in the negative resistance region. The voltage reaches a minimum at the valley point. The resistance of RB1, the saturation resistance is lowest at the valley point.
Chapter Three
IP and IV, are datasheet parameters; For a 2n2647, IP and IV are 2µA and 4mA, respectively. VP is the voltage drop across RB1 plus a 0.7V diode drop; VV is estimated to be approximately 10% of VBB. Peak votage of UJT VpVp=ηVbb +Vd
Chapter Three
Complementary unijunction Transistor (CUJT)
Like standard UJT except that the currents and voltages applied to it are of opposite polarity. Ideal for stable oscillators, timers, frequency dividers.
Chapter Three
Programmable unijunction Transistor (CUJT)
Although it has the same name as a UJT the programmable unijunction transistor’s structure is not the same. It is actually more similar to an SCR.
Chapter Three
Programmable unijunction Transistor (CUJT)
The PUT can be “programmed” to turn on at a certain voltage by an external voltage divider. This yields a curve similar to a UJT.
Chapter Three
Programmable unijunction Transistor (PUT)
External PUT resistors R1 and R2 replace unijunction transistor internal resistors RB1 and RB2, respectively. These resistors allow the calculation of the intrinsic standoff ratio η.
Chapter Three
Programmable unijunction Transistor (PUT)
VR is voltage divider (R1 and R2 can be specified)Vc capacitor voltageWhen Vc > VR the PUT will conduct
Chapter Three
Programmable unijunction Transistor (PUT)
Chapter Three
unijunction Transistor trigger circuits
Chapter Three
LASCR (Light Activated SCR)
End of Lecture
