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Lecture 4

Examples of Diode Circuits:

Rectifier Circuits

ELEC 1250 - Diodes andDiode Circuits

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DC Power Supply.

Mains Diode

RectifierFilter

Voltage

Regulatorv

sv

o

N1 N 2

Load

vm

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Half-Wave Rectifier

mains vi

vo RL

vd

vs

vi

vo

RL

vdrs

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Equivalent Circuit using the Constant-Voltage drop and Resistance Model

vs

rs

RL

vi

vo

i e l VDO rd

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Peak Inverse Voltage (PIV)

Two important design parameters:

l Maximum diode current.l Peak Inverse Voltage (PIV). PIV = V (peak)

The PIV is the largest reverse bias voltage the

diode will experience in the circuit without breaking

down.

s

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Half-wave rectifier:only half (max!) waveform used

Time [s]

0.00 10.00m 20.00m 30.00m 40.00m

I

-100.00m

200.00m

Axislab

el

-2.00

2.00

0v

0v

VDO

+ Vb

i

d

vs

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Full-Wave Rectifier:both half-cycles used

vi

vi

vo

RL

D1

D2

Mains-

vo

RL

+-

D1D4

D2 D3

vi

Two types:

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Full-Wave Rectifier

vi

vi

vo

RL

D1

D2

Mai

+

-

+

-

+_

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Operation of Full-Wave Rectifier

vi

vi

vo

RL

D1

D2

Main

vi

vi

vo

RL

D1

D2

Main

v Positivei

v Negativei

+

-

+

-

+

-

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Operation of Full-Wave Rectifier

vi

vi

vo

RL

D1

D2

Main

vi

vi

vo

RL

D1

D2

Main

v Positivei

v Negativei

ON

OFF

ON

OFF

+

-

-

+

+

-

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Operation of Full-Wave Rectifier

vi

vi

vo

RL

D1

D2

Main

vi

vi

vo

RL

D1

D2

Main

v Positivei

v Negativei

ON

OFF

ON

OFF

+

-

-

+

+

-

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Operation of Full-Wave Rectifier

vi

vi

vo

RL

D1

D2

Main

v Positivei ON

OFF

+

-

+

What is PIV (peakinverse voltage) here?

For bottom diode, will occur when sine wave is max positive

at top

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Steady-State Waveforms

Ti [ ]

0.00 10 .00m 20 .00m 30 .00m 40 .00m

Outputvoltage

[V]

-2.00

-1.00

0.00

1.00

2.00

Vi

vL

~VDO

Assuming a constant-voltage-drop model for the diodes.

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The Bridge Rectifier.

vo

RL

+-

D1D4

D2 D3

vi

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Operation in Positive half cycle

vo

RL

+-

D1D4

D2 D3

vi

ON

ON

OFF

OFF

+

-

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PIV for bridge rectifier

vo

RL

+-

D1D4

D2 D3

v i

D2 D3

D

vo

RL

v reverse v v

Thus

PIV v peak V V v peak V

D o DO

i DO DO i DO

3

2

( )

( ) ( )

!

! !

+

-

Assuming constant voltage

drop model

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Operation in Negative Half Cycle

vo

L

+-

D1D4

D2 D3

vi

OFF

OFF

ON

ON

+

-

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Centre-tapped transformer vsbridge rectifier

What are the relative (dis)advantages of each?

CT transfo: more complicated transformer

diodes need higher PIV

Bridge: 4 diodes instead of 2

Which is better?

Are there sustainability / moral issues?

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Transformers

Core made of steel

l need for metal resources

l recyclable but do we do so?

Wire made of copper

l often mined in poorer countries

l working conditions often badl but does earn income & foreign currency

l recyclable as above

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Steady-State Waveforms

Time [s]

0.00 10.00m 20.00m 30.00m 40.00m

O

t

tvoltage[

]

-2.00

-1.00

0.00

1.00

2.00

Vi

Vo

2VDO

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DC Power Supply: Filter

Diode

RectifierFilter

Voltage

RegulatorLoad

DC Power Supply

vs

vo

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Filter Circuit

Filter Capacitor

C RL

v i

vo

CR >> T

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Voltage Waveforms for fullwave rectifier

v

Ti [s]

0.00 10.00 m

0.00m

0.00m

0.00m

O

t

t

.00

-1.00

0.00

1.00

2.00

Output

Ripple

i

vo

Vr

Vp

T

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Capacitance Filter

At the end of the discharge period

Since CR >> T then

Giving

V

where is the frequency

r

V V V e

eCR

VT

CR

V

fCR

f

T

o r o

T CR

T CR

oo

$

$

$ !

!

/

/

2

21

2

2 2

1

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Example

Consider a fullwave rectifier fed by a 50Hz

sinusoid with a peak value output voltage of

100V. If the load resistance is 10Kohms, find

the size of capacitance necessary to give a

peak to peak ripple of 2V.

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Without Filter Capacitor

Time [s]

0.00 10.00m 20.00m 30.00m 40.00m

vout

-200.00m

800.00m

rectifier

curre

t

-20.00u

80.00u

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With filter capacitor

Ti [ ]

0.00 10.00m 20.00m 30.00m 40.00m

vout

0.00

800.00m

r

ctifi

rcurr

nt

-1.00m

2.00m

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Summary - Rectifiers #1

l Rectifier circuits have been shown for a practical dc

power supply.

halfwave rectifier

full wave rectifier

bridge rectifier

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Problem for next time:Battery charger circuit

RL

iD

vs

rs

vD

+

+

-

Vb

Battery

V5Vand100

20,V7.0:modelWL

2),sin(10:upply

b !;!

;!!

;!!

L

dDO

SS

rV

rtv [

Determine the peak and average current through

the diode and its Average Power Dissipation