INDUSTRIAL ELECTRONICS

THYRISTORS

• 4 layer, 3 terminal semiconducting devices

• “Rectifier and Transistor Combined”
• N-P-N-P or P-N-P-N
• General Rule: “All junctions must be forward bias” para masabing ito ay “ON’ or buhay.

3 STATES
1. Reverse Blocking Mode – OFF
-V (Negative Supply)

RB
FB
RB

+V (Positive Supply)

2. Forward Blocking Mode – OFF

+V (Positive Supply)

FB
RB
FB

-V (Negative Supply)
3. Forward Conduction Mode – ON As long as:
+V (Positive Supply)
VAK > VFB – the device is “ON”

FB
RB I(g) Gate Triggering “If Gated”
FB Forward Breakover Voltage

-V (Negative Supply)

I. SHOCKLEY DIODE
• 4-Layer Diode, 2 terminals (Anode and Cathode)
• Used as a trigger switch for SCR
• Also known as SUS (Silicon Unilateral Switch)

II. SILICON-CONTROLLED RECTIFIER

• 4 Layer diode
• 3 terminals (Anode, Cathode, Gate)
• Introduced in 1956 by Bell Telephone Laboratories
• Operates on the principle of Current Conduction
• Operation of diode ≠ Operation of SCR
• Has a control element (gate) that can trigger the device into conduction
• Turn off time is usually from 5μs to 30μs
Terms Normally Encountered:

• Holding Current – Minimum amount of current to keep the SCR “on”

• Triggering Gate Current – Gate current required to make the SCR “conduct”
• Forward Breakover Voltage – Voltage above which the SCR enters the conduction region.
• Reverse Breakdown Voltage – Equivalent to Zener avalanche region
• Forward Conduction Region – Corresponds to the “on” condition of the SCR
• Forward and Reverse Blocking Regions – Corresponds to the “off” condition of the SCR
Remember:
If the Ia > IH, SCR is “ON”
If the Ia < IH, SCR is “OFF”

TURNING ON METHODS: (Gate Triggering and Forward Break-over Voltage)

TURNING OFF METHODS:
Anode Current Interruption Force Commutation

To solve for the force commutation capacitance,

Force Commutation Capacitance Formula:
𝑰𝒕𝒐𝒇𝒇
𝑪 = 𝟏. 𝟒𝟑
𝑽

What value of the capacitor is required to force commutate a thyristor with a turn-off time of 20μs with a 96V battery a full-load
current of 100A?
𝑰𝒕𝒐𝒇𝒇 (𝟏𝟎𝟎)(𝟐𝟎𝐱𝟏𝟎−𝟔 )
𝑪 = 𝟏. 𝟒𝟑 = 𝟏. 𝟒𝟑 = 𝟐𝟗. 𝟕𝟗
𝑽 𝟗𝟔
FORMULAS USED:
Instantaneous Voltage Formula:
𝑽(𝒕) = 𝑽𝒑 𝐬𝐢 𝐧 𝜽𝒇

Gate Triggering Current Formula:

𝑽𝑻 − 𝑽𝑮
𝑰𝒈𝒕 =
𝑹𝑮
Average Load Voltage Formula:
𝑽𝒑
𝑽𝑳𝒂𝒗𝒆 = [𝟏 + 𝒄𝒐𝒔 (𝜽𝒇 )]
𝟐𝝅
Average Load Voltage Formula:
𝑽𝑳𝒂𝒗𝒆 = (𝑰𝑳𝒂𝒗𝒆 )(𝑹𝑳 )
RMS Load Voltage:

𝑽𝒑 𝟐(𝝅 − 𝜽𝒇 ) + 𝒔𝒊𝒏 𝟐𝜽𝒇

𝑽𝑳𝒓𝒎𝒔 = √
𝟐 𝟐𝝅
Half-wave, variable-resistance, phase-control circuit

By adjusting R2 the SCR can be made to trigger at any point on the

positive half-cycle of the ac waveform between 0° and 90°

Sets the trigger level

𝑽
DERIVATION of AVERAGE LOAD VOLTAGE: 𝑽𝑳𝒂𝒗𝒆 = 𝟐𝝅𝒑 [𝟏 + 𝒄𝒐𝒔 (𝜽𝒇 )]
𝝅
∫𝜽 𝑽𝒑 𝒔𝒊𝒏 𝜽𝒇 𝒅𝜽 𝑽𝒑 (−𝒄𝒐𝒔𝜽)𝝅
𝜽
𝒇 𝒇
𝑽𝒂𝒗𝒆 = 𝟐𝝅
𝑽𝒂𝒗𝒆 = 𝟐𝝅

𝑽𝒑 [−𝒄𝒐𝒔 𝝅 − (−𝒄𝒐𝒔 𝜽𝒇 )
𝑽𝒂𝒗𝒆 =
𝟐𝝅
𝑽𝒑 [−(−𝟏) − (−𝒄𝒐𝒔 𝜽𝒇 )
𝑽𝒂𝒗𝒆 =
𝟐𝝅
Sample Problems:
Single Phase Half-Wave Controlled Rectifier
A thyristor half-wave-controlled converter has a supply voltage of 240V at 50Hz and a load resistance of 100Ω. What are the
average values of load voltage and current when the firing delay angle is 30 degrees?
Solution:
Average Load Voltage: Average Load Current:
𝑽 𝑽𝑳𝒂𝒗𝒆
𝑽𝑳𝒂𝒗𝒆 = 𝟐𝝅𝒑 [𝟏 + 𝐜𝐨𝐬 (𝜽𝒇 )] 𝑽𝑳𝒂𝒗𝒆 = (𝑰𝑳𝒂𝒗𝒆 )(𝑹𝑳 ) → 𝑰𝑳𝒂𝒗𝒆 = 𝑹𝑳

240√2 𝑉𝐿𝑎𝑣𝑒 100.8

𝑉𝐿𝑎𝑣𝑒 = [1 + 𝑐𝑜𝑠 (30)] = 𝟏𝟎𝟎. 𝟖𝐕 𝐼𝐿𝑎𝑣𝑒 = = = 𝟏𝐀
2𝜋 𝑅𝐿 100

Full-Wave Half-Controlled Bridge with Resistive Load

A full-wave half-controlled bridge has a supply voltage of 220V at 50Hz. The firing delay angle delay 90 degrees. Determine
the values of average rms currents load power for a resistive load of 100Ω.
Solution:
Average Load Voltage:
𝑽𝒑
𝑽𝑳𝒂𝒗𝒆 = 𝝅
[𝟏 + 𝐜𝐨𝐬 (𝜽𝒇 )]

220√2
𝑉𝐿𝑎𝑣𝑒 = [1 + 𝑐𝑜𝑠 (90)] = 𝟗𝟗𝐕
𝜋

RMS Load Voltage: RMS Load Current:

𝑽𝒑 𝟐(𝝅−𝜽𝒇 )+𝒔𝒊𝒏 𝟐𝜽𝒇

𝑽𝑳𝒓𝒎𝒔 = √ 𝑽𝑳𝒓𝒎𝒔 = (𝑰𝑳𝒓𝒎𝒔 )(𝑹𝑳 )
𝟐 𝟐𝝅

𝛼 𝑠𝑖𝑛 2𝛼 𝑉𝐿𝑟𝑚𝑠
𝑉𝐿𝑟𝑚𝑠 = 𝑉𝑠𝑢𝑝𝑝𝑙𝑦 √[1 − ( ) + ] 𝐼𝐿𝑟𝑚𝑠 =
𝜋 2𝜋 𝑅𝐿

155.56
In converting to radiant, shift + mode + 4 (Rad) 𝐼𝐿𝑟𝑚𝑠 =
100

𝜋⁄ si n 2(𝜋⁄2)
2
𝑉𝐿𝑟𝑚𝑠 = 220√[1 − ( )+ ] 𝐼𝐿𝑟𝑚𝑠 = 𝟏. 𝟓𝟓𝟓𝟔𝐀
𝜋 2𝜋

𝑽𝑳𝒓𝒎𝒔 = 𝟏𝟓𝟓. 𝟓𝟔𝐕

Power Factor:

𝑷 𝑰𝟐 𝑹 𝑻𝑹𝑼𝑬
𝑷𝒇 = 𝒄𝒐𝒔 𝜽 = = =
𝑽𝒔 𝑰𝒓𝒎𝒔 𝑽𝒔 𝑰𝒓𝒎𝒔 𝑨𝑷𝑷𝑨𝑹𝑬𝑵𝑻
𝐼2 𝑅 1.55562 (100)
𝑷𝒇 = = = 𝟎. 𝟕𝟏
𝑉𝑠 𝐼𝑟𝑚𝑠 220(1.5556)
 III. SILICON-CONTROLLED SWITCH (SCS)
• 4 Layer Diode
• 4 terminals (Anode, Cathode, Gate A and Gate K)
• Construction is the same as SCR but with an addition of anode gate
• Also called “tetrode”
• The addition of anode gate is used to turn off the SCS by applying a positive input/voltage.
• Has faster turn on/off times than SCR
• Increased control and triggering sensitivity
• Limited to low power, current and voltage ratings

Turning ON and Turning OFF

IV. LIGHT ACTIVATED SCR (LASCR)
• A thyristor that operates like an SCR except that it can be light triggered except that it can be light triggered
• Most sensitive to light when the gate is open, a resistor from gate to cathode is used to control the sensitivity.

Schematic Symbol

V. GATE TURN-OFF SWITCH (GTO)

• Characteristics are similar to SCR
• Like SCR, it has 3 terminals (Gate terminal, anode and cathode)
• Used in high-speed applications
• GTOs, as opposed to normal thyristors, are fully controllable switches which can be turned on and off by their
third lead, the gate lead.

Schematic Symbol
 VI. DIAC
• Diode for AC (bidirectional)
• Constructed like a TRIAC but without a gate terminal
• Used as a trigger for TRIAC circuits
• Symmetrical trigger diode because its break-over voltage is close ±32 volts

Basic Construction Schematic Symbol Characteristic Curve

VII. TRIAC
• Triode for AC
• A three-terminal device used to control the average current flow to a load.
• Can conduct current in either direction when it is turned on so it is called a bidirectional triode thyristor.
• Acts like two SCR’s connected in inverse parallel so that each SCR conducts alternately for every half cycle of
an AC signal.
• Gated DIAC
ADVANTAGES:
1. Single Gate Controls conduction in both directions.
2. TRIAC with high voltage current ratings is available.
3. TRIAC is a bidirectional device; it conducts in both direction
DISADVANTAGES:
1. Gate has no control over conduction once TRIAC is turned on as in the case of SCR
2. TRIAC is not suitable for DC power applications.
 Schematic Symbol

Basic Structure Characteristic Curve

 SUMMARY OF CHARACTERISTICS OF THYRISTORS
O GATE 1 GATE 2 GATES

UNIDIRECTIONAL SHOCKLEY SCR SCS

BIDIRECTIONAL DIAC TRIAC

TURNING-ON Breakover Gate Triggering and Gate Triggering and

Voltage Breakover Voltage Breakover Voltage

VIII. UNIJUNCTION TRANSISTOR (UJT)

• Break-over type switching device
• Double-based diode
• Semiconductor device consisting of thin silicon bar on which a PN junction acting as emitter is formed near one
end
• Operates in the negative resistance region

Basic Construction Schematic Symbol Equivalent Circuit

Characteristic Curve
Peak Point Voltage
Value of emitter voltage that causes the PN junction to become forward biased

Peak Point Voltage Formula:

𝑽𝒑 = 𝜼𝑽𝑩𝑩 + 𝑽𝒑𝒏

where:
Vp = peak point voltage

η = intrinsic standoff ration (usually, the value is 0.4 to 0.6)

VBB = base to base voltage
Vpn = peak point voltage of diode

Application:
Relaxation Oscillator
Formula for the Frequency Oscillator:
DC Thyristor / SCR Circuit
There are many applications where an SCR circuit is required to control the
operation of a DC load. This may be used for DC motors, lamps or any other load
requiring switching.
The basic SCR circuit given below is able to control the power to the load using a
small switch to initiate the application of power to the load.
Initially with S1 closed and S2 open, no current will flow. Only when S2 is closed
and it triggers the gate by causing gate current to flow, will the SCR circuit turn on
and current flow in the load.
Current will continue to flow until the anode circuit is interrupted. This can be done
using S1. An alternative method is to place the switch S1 across the SCR and by
momentarily closing it, the voltage across the SCR will disappear and the SCR will
stop conducting.
As a result of their functions in this SCR circuit S1 may be called the Off switch and S2 the ON switch. In this configuration S1
needs to be able to carry the full load current, while S2 only needs to be able to carry the gate current. Once the SCR is on,
the switch can be released and remain open as the action of the SCR sustains the current flow through the device and hence
the load.
The resistor R1 connects the gate to the supply via the switch. When the switch S2 is closed, current flows through the
resistor, enters the gate and turns SCR on. The resistor R1 has to be calculated to provide sufficient gate current to turn the
SCR circuit on.
R2 is included to reduce the sensitivity of the SCR so that it does not fire on any noise that may be picked up.

Basic AC thyristor / SCR circuit

When AC is used with a thyristor circuit, a few changes need to be made
as seen below.
The reason for this arises because the AC power reverses polarity over
the course of the cycle. This means that the SCR will become be reverse-
biased, effectively reducing the anode voltage to zero causing it to turn
OFF during one half of each cycle. As a result, there is no need to have
an off switch as this is achieved as part of the use of an AC supply.

The operation of the circuit is slightly different to that of the DC SCR

circuit. When the switch is turned on, the circuit will need to wait until
there is sufficient anode voltage available as the AC waveform progresses along its course. Also, the SCR circuit will need to
wait until the voltage within the gate section of the circuit can provide sufficient current to trigger the SCR. For this the switch
has to be on its closed position.
Once triggered the SCR will remain in its conducting state over the positive half of the cycle. As the voltage falls, there will
come a point where the anode cathode voltage is insufficient to support conduction. At this point the SCR will stop conducting.
Then over the negative half of the cycle, the SCR will not conduct. Only when the next positive half of the cycle returns will the
process repeat.
As a result, this circuit will only conduct when the gate switch is in its closed position.
One of the issues with using an SCR circuit of this nature is that it cannot supply more than 50% power to the load, because it
does not conduct during the negative half of the AC cycle because the SCR is reverse biased.