Lecture 7: Diode...

ECE 335: Electronic Engineering

Lecture 7:

Diode Applications

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Diode Applications

• Half Wave Rectifier

• Full Wave Rectifier

• Clipping Circuits

• Clamping Circuits

• Regulator

• Regulated Power Supply

Diode Applications

• Half wave rectifier and equivalent circuit with

piece-wise linear model

vi

v i = VM sin (ωt)

Half Wave Rectifier

• We initially consider the diode to be ideal,

such that Vφ =0

Half Wave Rectifier

• The (ideal) diode conducts for vi >0 , thus

v0 ≈ vi

• For vi < 0, the (ideal) diode is an open circuit

(it doesn’t conduct) and

v0 ≈ 0.

Half Wave Rectifier

• In this simplified (ideal diode) case the

input and output waveforms are as shown

The diode must withstand a peak inverse voltage

of VM

Half Wave Rectifier

• The average d.c. value of this half-wave-

rectified sine wave is

+= ∫

π

θθπ

0

0sin2

1dVV MAV

[ ]π

ππ

MM VV=−−= 0coscos

2

Half Wave Rectifier

• So far this rectifier is not very useful.

• Even though the output does not change polarity it has a lot of ripplei.e. variations in output voltage about a steady value.

• To generate an output voltage that more closely resembles a true d.c. voltage we can use a reservoir or smoothing capacitor in parallel with the output (load) resistance.

Smoothed Half Wave Rectifier

Circuit with reservoir

capacitor

Output voltage

The capacitor charges over the period t1 to t2 when the diode is on

and discharges from t2 to t3 when the diode is off.


• When the supply voltage exceeds the output voltage the (ideal) diode conducts. During the charging period (t1 < t< t2)

vo = VM sin (ωt)


• When the supply voltage falls below the output voltage the diode switches off and the capacitor discharges through the load.

• During the discharge period (t2 < t< t3 ) and

vo = VM exp {- t’ /RC}

where t’= t- t2

• At time t3 the supply voltage once again exceeds the load voltage and the cycle repeats


• The resistance in the discharge phase is the

load resistance R.

• RC can be made large compared to the wave

period.

• The change in output voltage (or ripple) can

then be estimated using a linear

approximation to the exponential discharge.


• vo = VM exp {- t’ /RC} ≈ VM [ 1- (t’ /RC)]

• The change in voltage ∆V is therefore

approximately given by VM t’ /RC

• For a the half wave rectifier this discharge

occurs for a time (t3 - t2 ) close to the period T

= 1/f, with f= frequency.

• Giving the required result:

RC

TV∆V

M≈


• We can define a ripple factor as

where Vd.c. = (VM - ∆V/2)

The lower the ripple factor the better

d.cV

∆Vfactor Ripple =

Non-Ideal Half Wave Rectifier

VM

Vφ

Vφ

Full-Wave (Bridge) Rectifier

• We initially consider the diodes to be ideal, such

that VC =0 and Rf =0

• The four-diode bridge can be bought as a package

vi


• During positive half cycles vi is positive.

• Current is conducted through diodes D1, resistor R and diode D2

• Meanwhile diodes D3 and D4 are reverse biased.

vi


• During negative half cycles vi is negative.

• Current is conducted through diodes D3, resistor R and diode D4

• Meanwhile diodes D1 and D2 are reverse biased.

vi


• Current always flows the same way through the load R.

• Show for yourself that the average d.c. value of this full-wave-rectified sine wave is VAV = 2VM/π(i.e. twice the half-wave value)


• Two diodes are in the conduction path.

• Thus in the case of non-ideal diodes vo will be

lower than vi by 2VC.

• As for the half-wave rectifier a reservoir

capacitor can be used. In the full wave case

the discharge time is T/2 and

2RC

TV∆V

M≈

Diode Clipper Circuits

• These circuits clip off portions of signal

voltages above or below certain limits, i.e. the

circuits limit the range of the output signal.

• Such a circuit may be used to protect the

input of a CMOS logic gate against static.


• When the diode is off the output of these

circuits resembles a voltage divider

i

SL

L

o vRR

Rv

=

+


• If RS << RL

• The level at which the signal is clipped can be

adjusted by adding a d.c. bias voltage in series

with the diode.

v0 ≈≈≈≈ vi

For instance


• Let’s look at a few other examples of clipper

circuits.

Clipper circuits using zeners

Figure 3.24 A voltage regulator supplies constant voltage to a load.

Voltage Regulator

Designing a power supply

Diode Clamper Circuits

• The following circuit acts as a d.c. restorer.

Diode Clamper Circuits

• A bias voltage can be added to pin the output

to a level other than zero.

Lecture 7: Diode...

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