power electronics - college of engineering, trivandrumfiring angle is 90 degrees solution dept. of...

Power Electronics

TRIGGERING CIRCUITS

DEPT. OF ELECTRICAL ENGINEERING, COLLEGE OF ENGINEERING TRIVANDRUM 1

2018 Dr. Francis M. Fernandez

Gate Characteristics


OA and OB represent the spread of characteristics for the thyristor of same rating

Vg

Ig

Vgmax

Vgmin

Igmin IgmaxO

B

A

E

S1

S2

R - Triggering Circuit


1

maxR

VI m

g max

1

g

m

I

VR

R1 is the gate current limiting resistance

R2 is used to vary the gate current and hence firing angle

max

1max

gm

g

VV

RVR

R limits the voltage at Gate terminal

Diode D prevents build-up of negative voltage at Gate terminal

R-Trig Waveforms


Vi

VL

VT

t

α

The phase angle at which the SCR starts conducting is called firing angle, α

Features of R-Trig Circuit


Vi

VL

VT

t

α

Simple circuit

Disadvantages:

Performance depends on temperature and SCR characteristics

Minimum phase angle is typically 2-4 degrees only (not zero degree)

Maximum phase angle is only 90 degrees

Problem

Design an R-triggering circuit for a half wave controlled rectifier circuit for 24 V ac supply. The SCR to be used has the following data.

Igmin = 0.1 mA, Igmax = 12 mA, Vgmin = 0.6V, Vgmax = 1.5 V

6DEPT. OF ELECTRICAL ENGINEERING, COLLEGE OF ENGINEERING TRIVANDRUM

�� = 24 2 1 3max

24 22.8 3.3

12 10m

g

VR k k

I

� ≤��

�� − ��=1.5 × 3.3 × 10�

24 2 − 1.5= 145.8 � ≃ 120 �

��

�� − ��=

�

�� + ��

�� =�� − ��

��− �� =

24 2 − 0.6 × 120

0.6− 3.3 × 10� = 3.37 ��

For finding R2

Select 4.7 k Potentiometer for R2(Drop across diode D is neglected)

Solution:

Problem


In a resistance firing circuit for SCR, the following parameters are applicable

Igt(min) = 0.5 mA,

V gt(min) = 0.7V,

Supply voltage = 48 V, 50Hz

Resistance in gate current path = 70 kΩ.

a) Find the firing angle for the abovecondition

b) What is the resistance for which firing angle is 90 degrees

Solution


V3.36

7.06.01070105.0 33

21

gDgt VVRRIe

1

sin

36.3 2 48sin

36.3sin 0.535

2 48

sin 32.3

t me V t

t

t

t t

Firing angle = 32.3 degrees

a)

Solution


kΩ133105.0

6.66

105.06.66

7.06.0105.0482

3

3

3

21

R

R

R

VVRRIe gDgt

b)

RC Triggering Circuit


Capacitor charges during the negative half cycle through D2

When SCR is turned on, capacitor C is suddenly discharged through D2

D1 protects the SCR during negative half cycle

2

3.1 TRC

1min mins g g Dv R I V V

To ensure minimum gate current

1min

min

s g D

g

v V VR

I

Advantage over R-triggering Circuit:

Controls upto 180 degrees

RC Trig Waveforms


2

3.1 TRC

RC Full wave trigger circuit


2

50 TRC

Initial Capacitor voltage in each half cycle is almost zero

min

min

s g

g

v VR

I

Unijunction Transistor (UJT)


Has a lightly doped n-type silicon layer to which a heavily doped p-type emitter is embedded

The inter-base resistance is in the range of 5 – 10 kΩ

This device cannot ‘amplify’

UJT Equivalent Circuit


BBBB

BB

B

AB VVRR

RV

21

1

1

η is called intrinsic standoff ratio

Value of η varies from 0.5 – 0.8

When Ve is more than V1+VD, then the diode is forward biased and a current flows through RB1

Number of carriers in RB1 increases and the resistance reduces

Ve decreases with increase in Ie and the therefore the device is said to exhibit negative resistance

UJT Characteristics


At peak point, Ve = V1+VD,

At Valley point, RB1 is minimum

UJT parameters


Maximum emitter reverse voltage◦ Maximum reverse bias which the emitter – base2 junction can tolerate without

breakdown. Typ: 30V

Maximum inter-base voltage◦ Maximum voltage possible between base1 and base2. Decided by the power

dissipation. Typ: 35 V

Interbase resistance◦ Typ: 4.7 k – 9.1 k

Intrinsic stand off ratio ◦ Typ: 0.56 – 0.75

Maximum peak emitter current◦ Typ : 2A

Emitter leakage current◦ The emitter current when Ve is less than Vp and the UJT is off.

◦ Typ 12 μA

Typical values are of 2N2646

UJT Oscillator


• R1 and R2 are much less than the inter-base resistance

• The output pulses can be used to trigger an SCR

Design


RC

t

BBC eVV 1

Time required for C to charge from Vv to Vp is obtained as follows

RC

t

BBvDBBp eVVVVV 1

Assuming D vV V

RC

t

e1

1

1ln

1RC

fT

Tt ,case For this

R1 is selected based on voltage level required to trigger the SCR

R2 is selected using the empirical formula:

BBVR

4

2

10

UJT firing circuit for Half Wave Controller


Waveforms for Half Wave Controller


Vi

VP

VO

t

VC

VV

Full wave UJT trigger Circuit


Problem


Design a UJT relaxation oscillator using UJT2646 for triggering an SCR. The UJT has the following parameters

η = 0.63, VBB = 20 V, VP = 13.2 V, IP = 50 μAVV = 2 V, IV = 6 mA, RBB = 7 kΩ, leakage current = 2.5 mA

Also find the minimum and maximum time period of oscillation.

Solution:

Assume C = 0.1 μF

�� =�� − ��

��=20 − 13.2

50 × 10��= 136 ��

�� =�� − ��

��=

20 − 2

6 × 10��= 3 ��


Approximate value of �2 =10�

� ��=

10�

0.63 × 20= 794 Ω

�� =��

�� =

0.7

2.5 × 10��= 280 Ω

�� = ��ln1

1 − �= 136 × 10� × 0.1 × 10�� × ln

1

1 − 0.63= 13.5 ��

�� = 3 × 10� × 0.1 × 10�� × ln1

1 − 0.63= 0.3 ��

Commutation Commutation us the process by which a thyristor is turned off or

current diverted to another path.

There are two types of commutation Natural Commutation

Forced Commutation

24DEPT. OF ELECTRICAL ENGINEERING, COLLEGE OF ENGINEERING TRIVANDRUM

• In natural commutation, the reversing nature of alternating voltages turn off the thyristor

• Suitable for AC circuits only

• Current passes through a zero in every half cycle

• No external circuit is required for natural commutation

• In DC circuits, external circuits are necessary for turn off of thyristors

• Turn off with external circuits is called forced commutation

Pulse Transformer


Used to trigger SCR, TRIAC etc

Provides Electrical isolation between power circuit and control circuit

Optical Isolation


Used to trigger SCR, TRIAC etc

Provides Electrical isolation between power circuit and control circuit

power electronics - college of engineering, trivandrumfiring angle is 90 degrees solution dept. of...

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