Power Semi-Conductors

Learning Outcomes At the end of the lesson, students should be able to : Explain the characteristic and operation of Silicon Controlled Rectifier (SCR) Explain the characteristic and operation of Diac Explain the characteristic and operation of Triac

Thyristors … is a family of semiconductor devices, 4 and 5 layers (P & N), in two groups:- –Controllers and –Triggers Ref Lowe p108, Fig 16.1

SCR An SCR is a 4 layer, 3 terminal, Uni- directional Thyristor “Controller” device and can handle high currents (up to 6kA) at high voltages (up to 8.5kV). It may be turned-on (made to conduct) by:- – applying a positive current pulse to the gate (normal) or –By increasing the anode potential to the “forward breakover voltage” point

+ ve at anode - ve at cathode Neither PN junction is forward biased No conduction (anode to cathode)

+ ve at gate - ve at cathode Only lower PN junction is forward biased No conduction (anode to cathode)

By applying a small voltage between gate and cathode, the lower transistor will be forced on by the resulting base current, which will cause the upper transistor to conduct, which then supplies the lower transistor's base with current so that it no longer needs to be activated by a gate voltage.

+ ve at anode - ve at cathode + ve at gate both PN junctions are forward biased Conduction (anode to cathode)

After conduction initiated, + ve at anode - ve at cathode + ve removed from gate both PN junctions are still forward biased Conduction continues

The gate current pulse required to cause the SCR to conduct is much lower than the current through the SCR, 35mA from 2 to 5V for 10 to 50 micro-secs SCR’s are rated up to 6kA at 8.5kV

Using the gate to activate the SCR’s conduction is called triggering, and it is by far the most common way that SCRs are latched in actual practice.

In practice, SCRs are usually chosen so that their break over voltage is far beyond the greatest voltage expected to be experienced from the power source, so that it can be turned on only by an intentional voltage pulse applied to the gate.

Turn-off or commutation is accomplished by reducing current below the “holding current” value (this causes one or both of the internal transistors fall into cut-off) There are 3 ways to achieve natural- commutation and 6 ways to achieve forced commutation ref Lowe p111

PB 1 to trigger on the SCR PB 2 to turn off the SCR

Conduction

Applications of SCRs SCR’s are often used in power electronics applications for the control of AC Voltage. They are also used frequently in motor controllers. Usually an AC voltage controller circuit for an SCR will comprise of a switching method that will switch the SCR on partway through the cycle of an AC waveform, only delivering part of the voltage.

Application of S C R

Circuit above only shows the gate connections for two out of the four SCRs. Pulse transformers and triggering sources for SCR 1 and SCR 3, as well as the details of the pulse sources themselves, have been omitted for the sake of simplicity.

Application of S C R Controlled bridge rectifiers are not limited to single-phase designs. In most industrial control systems, AC power is available in three-phase form for maximum efficiency, and solid-state control circuits are built to take advantage of that. A three-phase controlled rectifier circuit built with SCRs, without pulse transformers or triggering circuitry shown, would look like this-

Application of S C R

DIAC A Diac is a 4 layer, two terminal, Bi- directional, Thyristor “Controller” device It will conduct in each direction when breakover voltage is reached

Diac

AC repeatedly reverses direction, DIACs will not stay latched longer than one-half cycle. If a DIAC becomes latched, it will continue to conduct current only as long as there is voltage available to push enough current in that direction. When the AC polarity reverses, the DIAC will drop out due to insufficient current, necessitating another breakover before it conducts again.

Diac Operating Characteristics V BR+ V BR- Unstable region V+ I+

Diac

Symbol of Diac

+50V 0V R1 R2

Diac Operating Characteristics V BR+ V BR- Unstable region V+ I+

With the DIAC, the breakover voltage is fixed therefore the conduction period is fixed. With the SCR, we have control over when conduction occurs and therefore control of the conduction period. By connecting a suitable control circuit to the gate of an SCR, we can delay the turn on point over almost the full half cycle of the AC supply

Testing Diacs Diacs are thyristors without any gate terminal. They depend on the leakage current to switch them on once the voltage across the device exceeds their specified ratings. With an ohmmeter, they can be tested only for shorts. Resistance should be infinite in both directions.

TRIAC A Triac is a 5 layer, 3 terminal, bi- directional Thyristor “Trigger” device They are like two SCRs joined in back-to- back parallel configuration

Triac

Uses TRIACs are usually seen in simple, low- power applications like household dimmer switches.

Triac A simple lamp dimmer circuit

Triac TRIACs are notorious for not firing symmetrically. One way to make the TRIAC's current waveform more symmetrical is to use a device external to the TRIAC to time the triggering pulse. A DIAC placed in series with the gate does a fair job of this:

Triac DIAC breakover voltages tend to be much more symmetrical (the same in one polarity as the other) than TRIAC triggering voltage thresholds. The DIAC prevents any gate current until the triggering voltage has reached a certain, repeatable level in either direction, the firing point of the TRIAC from one half-cycle to the next tends to be more consistent, The waveform more symmetrical above and below its centerline.

Triac Practically all the characteristics and ratings of SCRs apply equally to TRIACs, Except that TRIACs of course are bidirectional (can handle current in both directions). Not much more needs to be said about this device except for an important caveat concerning its terminal designations.

Triac Main terminals 1 and 2 on a TRIAC are not interchangeable. To successfully trigger a TRIAC, gate current must come from the main terminal 2 (MT 2 ) side of the circuit! Identification of the MT 1 and MT 2 terminals must be done via the TRIAC's part number with reference to a data sheet or book.

Triac

Triac, RC phase control using a Diac Ref Lowe p121