Electronics 1 Lecture 7

Diode types and application

Ahsan Khawajaahsan_khawaja@comsats.edu.pkLecturerRoom 102Department of Electrical Engineering

Outline

• Working of Full wave rectifier.• Ripple effect• Clippers• Clampers• Voltage multiplier (Doubler Circuit)• Refrences

Full Wave Rectifier(FWR)

FWR Working

Ripple Effect (RE)...

cont...

• So far this rectifier is not very useful.• Even though the output does not change polarity

it has a lot of ripple. • “The variation in the capacitor voltage due to

charging and discharging is called the ripple voltage”.

• 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 (prev slide fig b).

Ripple Effect...

Ripple Effect...

• For a given input frequency, the output frequency of a full wave rectifier is twice that of a half wave rectifier.

• As a result, a full wave rectifier is easier to filter because of the shorter time between peaks.

Compraison of HWR & FWR

Ripple Voltage Calculations

Vr(pp) = (1/fRLC)Vp(rect)

VDC = (1 – 1/2fRLC)Vp(rect)

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.

Smoothed Half Wave Rectifier

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

vo = VM sin (t)(The resistance in the charging circuit is strictly

Rf which we have assumed to be zero. Even for a practical diode RfC will be very small)

Smoothed Half Wave Rectifier

• 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

Smoothed Half Wave Rectifier

• 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.

Smoothed Half Wave Rectifier

• 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

TVΔV

M

Smoothed Half Wave Rectifier

• We can define a ripple factor as

where Vd.c. = (VM - V/2)

The lower the ripple factor the better

d.cV

ΔVfactor Ripple

Half Wave Rectifier

• If we don’t consider the diode to be ideal then from the equivalent circuit we obtain, for vi >Vc:

vi – Vc – i Rf - iR =0

i.e.

• Giving)( RR

Vvi

f

ci

cicif

o VvVvRR

RiRv

)(

)(

Non-Ideal Half Wave Rectifier

VM

Non-Ideal Half Wave Rectifier

• A plot of v0 against vi is known as the transfer characteristic.

VC vi

R/(R + Rf)

Non-Ideal Half Wave Rectifier

• We usually have R>> Rf so that Rf can be neglected in comparison to R.

• Often VM >> Vc so Vc can also be neglected.

The transfer characteristic then reduces tov0 vi

SURGE CURRENT IN THE CAPACITORINPUT FILTER

• When the power is first applied to a power supply, the filter capacitor is uncharged..

• At the instant the switch is closed, voltage is connected to the rectifier and the uncharged capacitor appears as a short.

• An initial “surge” of current is produced through the forward-biased diodes.

SURGE CURRENT IN THE CAPACITORINPUT FILTER

• It is possible that the surge current could destroy the diodes, for this reason a surge limiting resistor Rsurge, is sometimes connected.

• The value of this resistor must be small to avoid a significant voltage drop across it.

• The diode must have a forward current rating that can handle the momentary surge of current.

Diode Clipper Circuits

• Clipper circuits have the ability to ‘clip’ off a portion of the input signal without distorting the remaining part of the alternating waveform.

• Such a circuit may be used to protect the input of a CMOS logic gate against static error-prone states.

• There are two types of clipper circuit, namely series and parallel.

Series Clipper Circuits

Parallel Clipper Circuits

Diode Clipper Circuits

• When the diode is off the output of these circuits resembles a voltage divider

• 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.

iSL

Lo v

RR

Rv

v0 vi

Diode Clampers (DC Restorers)A diode clamper adds a DC level to an AC voltage. The capacitor charges to the peak of the supply minus the diode drop. Once charged, the capacitor acts like a battery in series with the input voltage. The AC voltage will “ride” along with the DC voltage. The polarity arrangement of the diode determines whether the DC voltage is negative or positive.

0V.

Diode Clampers (DC Restorers)Applications:

Amplifiers of all types use capacitive coupling between stages. Why?

To simplify the DC biasing; allows stage by stage independent biasing.

This capacitive coupling “loses” the DC component, stage to stage. To “restore” DC, the Diode Clamper can be used.

Here is a –DC Restorer circuit

Voltage MultipliersClamping action can be used to increase peak rectified voltage. Once C1 and C2 charges to the peak voltage they act like two batteries in series, effectively doubling the voltage output. The current capacity for voltage multipliers is low.

Voltage Doubler

Voltage MultipliersThe full-wave voltage doubler arrangement of diodes and capacitors takes advantage of both positive and negative peaks to charge the capacitors giving it more current capacity. Voltage triplers and quadruplers utilize three and four diode-capacitor arrangements respectively.

Voltage Multipliers - TriplersThe voltage tripler arrangement adds another diode/capacitor set.

+ half-cycle: C1 charges to Vp through D1, - half-cycle: C2 charges to 2Vp through C2,

Next + half-cycle: C3 charges to 2Vp through C3. Output is across C1 & C3.

Voltage Multipliers - QuadruplersThe voltage tripler arrangement adds another diode/capacitor set.

+ half-cycle: C1 charges to Vp through D1, - half-cycle: C2 charges to 2Vp through C2,

Next + half-cycle: C3 charges to 2Vp through C3. Next - half-cycle: C4 charges to 2Vp through C4 Quadruple Output is across C2 & C4.