Chapter 6

Diode Applications

Rectifier Circuits

The basic application of junction diodes is to convert ac signal to dc signal (ac-to-dc converter) or Rectifier

Half-wave rectifier: The input is ac signal and the output is a half-wave dc signal

The diode conducts in the +ve half cycle of the input signal (forward bias) and is off (reverse bias) in the –ve half cycle of the input signal

Ideal and second diode approximations can be used

Half-wave Rectification

Uin Uout

t0

+5V

t

Uin

0

+5V

-5V

Uout

S ON

S ON

S OFF

Half-wave dc voltage

T

Half-wave Rectification

Lm

Ldcdc

dcave

dcave

T

T

TT

dcave

RVRVP

VmVV

VmTTVmVV

dtdttVmdtUinVV

22

2

2

00

cos0cos22

2cos0cos2

.0.)sin(

Half-wave Rectifier

Out put voltage and Average voltage for Half-wave rectifier.

The output voltage may be viewed as a DC voltage plus a ripple voltage.

Half-wave Rectifier with Capacitor Filter

Capacitor is the most commonly used as a filter circuit. A capacitor filter is connected in parallel with the load.

Positive half cycle

Negative half cycle

Load Voltage

Half-wave Rectifier with Capacitor Filter

Output voltage waveform of half-wave rectifier with capacitorThe value of capacitor filter will depends on charge removed from capacitor during discharging cycle (t2 and t3), Load current IL and Peak-to-peak ripple voltage Vp-p

CVQTIQ

pp

L

Time between t2 and t3 is

approximate to T

Full-wave Rectification

Full wave rectifier consists of two diodes and Transformer with center tapped in secondary side.

During positive half cycle

During negative half cycle

Full-wave Rectification

Load voltage when full wave rectifier is used;

m

dcaveVVV 2

Full-wave Rectifier with Capacitor Filter

Full wave rectifier with filter

Load voltage

The Capacitance required for output filter of Full wave rectifier is only half the value of the one used in Half wave rectifier.

pp

L

VTIC

2

Full-wave Bridge Rectifier

Uin

Uout

D1

D2

D3

D4

t0

+5V

t

Uin

0

+5V

-5V

Uout

D3, D4

D1, D2

T

Full-wave dc voltage

Bridge Rectifier

Voltage Regulator

Varying voltage

System requires regulate supply

+

-VZ

10 – 15V

9V

9V

Us

Zener diode are most frequently used in regulator networks or as reference voltage. If voltage across Zener diode is greater than required to turn the Zener diode “on” the voltage across load will be maintained at Vz volts.

Clipper Circuits

A clipper circuit has the ability to cutoff /CLIP a portion of the input signal without distorting the remaining part

A half-wave rectifier is the simplest form of a diode clipper

There are two general categories of clippers Series: Diode is in series with the load Parallel: The diode is a branch in parallel

with the load

Series Clipper Circuits

Uin Uout

t

Uin

t

Uout

tUin

t

Uout

Parallel Clipper Circuits

Uin Uout

t

Uin

t

Uout

tUin

t

Uout

Clamper Circuits

Clamper circuit is the one that will “clamp” a signal to a different DC level. The circuit must have a capacitor, diode and resistor.

Vo clamped at 0 from t=0 up to T/2

Vo clamped at 0 from t=T/2 up to T

Voltage Multiplier circuits

There are several requirements in electronics for voltage levels which are greater than the normally available levels

Voltage multipliers can be used to produce higher voltages and eliminate the need for a transformer

Multiplication factors of two, three, and four are commonly used

Voltage Doubler

D2C1

U D1 C2

t

U

0

+Vp

-VpT

Voltage Doubler

D2C1

UD1 C2

During +ve half cycle

Forward Biased

Reverse Biased

+ -

C1 is charged to peak voltage Vp

Vp +

-

Voltage Doubler

D2C1

UD1 C2

During -ve half cycle

ReverseBiased

Forward Biased

+ -

-Vp

Vp

+

-

The peak voltage on C1 adds to Vp of the source to charge C2

C2 is therefore charged to twice peak voltage (Vp + Vp)

2Vp+-