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RECTIFIERS

RECTIFIERS

and

and

VOLTAGE REGULATION

VOLTAGE REGULATION

(1)

(1)

Analog

Analog

Electronics

Electronics

Pujianto

Pujianto

Department of Physics

Department of Physics

Edu

Edu

.

.

State University of

We have studied that a PN Junction conducts easily when forward biased and practically does not

conduct when reverse biased (very small current flow).

This characteristic of PN Junction is very similar to that of vacuum diode and we may also say that it acts like a switch which conducts current in one

RECTIFIERS

A rectifier may be defined as a device which

converts AC voltage/current into DC voltage/current.

Half Wave Rectifier

Whenever an AC signal e = E_m sin wt is applied

across a circuit consisting of a PN diode junction with some resistance RL called load, during the positive

The output voltage across load appears during positive half of input cycle.

The current flowing through diode during positive half cycle

circuit

the

of

ce

resis

Total

voltage

Applied

i

_b

_

_

_

tan

_

_

=

L f s m b

R

R

R

t

E

i

i

+

+

=

=

sin

w

_{When 0 < wt <p}

ib = 0 When p < wt < 2p

Rs = the Resistance of secondary coil

Rf = the Forward resistance of diode

RL = the Resistance of the load

If Rs and Rf are very small in comparison with the

load RL than

L m

b

R

t

E

i

i

=

=

sin

w

t

I

i

i

=

_b

=

_m

sin

w

When 0 < wt <p

or

where

L m m

R

E

A diode is a frequency converter in which an input frequency “w” is changed to a large number of

output frequencies.

In rectification we desire the DC or zero frequency component and it is

p

m

E

A Fourier analysis of the half sinusoidal voltage pulse appearing across the load yield

t

E

t

E

E

e

m m m

w

p

w

p

3

cos

2

2

,

0

sin

2

The DC or average current Idc is given by

( )

ò

=

p

w

p

₀

2

1

t

id

I

_dc

The second term in the bracket is zero since there is no conduction during p to 2p interval of input.

So

( )

( )

ú

û

ù

ê

ë

é

+

=

ò

ò

p

p p

w

w

p

2

0

2

1

t

id

t

id

( ) ( )

[

]

ò

=

-=

p

p

w

p

w

w

p

₀ sin 2 cos

2 1

o L

m

L m

dc t

R E t

td R

E I

p

p

m_L m

dc

I

R

E

I

=

=

By substituting for i

Or

We can conclude that for a half rectifier the DC output voltage is

p

m

dc

I

I

=

L dc

dc

I

R

V

=

(

s f

)

dc m

dc

I

R

R

V

V

=

-

+

p

and

so

2

m rms

Efficiency of Rectifier

L m L dc dc

R

I

R

I

P

2 2

÷

ø

ö

ç

è

æ

=

=

p

( )

(

)

m

(

_{f L}

)

L f

rms

ac

r

R

I

R

r

I

P

÷

+

Ripple Factor

Ripple factor r _{is defined as the ratio of two current}

(or voltage) components.

( )

( )

dc r

dc r

V

rms

V

I

rms

I

Voltage Regulation

The degree to which a power supply varies in output voltage under conditions of load variations is

measured by the voltage regulation which is usually expressed as percentage

%

100

%

x

V

V

V

V

fullload

fullload noload

r

ú

ú

û

ù

ê

ê

ë

é

Ratio of Rectification

It is used as measure of merit to compare rectifiers

ondary r

transforme from

power input

ac

load the

to delivered

power dc

RoF

sec _

_ _

_ _

_ _

_ _

_

=

ac dc

P

P

Transformer Utilization Factor (TUF)

2

2

2

_ m m

L m

rated ac

dc

I

x

V

R

I

P

P

TUF

÷

ø

ö

ç

è

æ

=

=

p

(

f

L

)

m

m

I

R

R

A half-wave rectifier consists of a diode having a

dynamic resistance of 20 ohm and an output voltage is 12 volt.

It has a secondary resistance of 5 ohm and a load resistance of 100 ohm. Determine:

a. An open-circuit secondary transformer voltage

b. DC output current

c. % Voltage regulation

d. Decreasing voltage because of transformer and diode