Rectifier and SMPS

Unit -V

Rectifiers and Switched Mode Power Supply

Power Supplies

• Power supplies provide the necessary power, voltage and current requirements for electronic devices.

• They usually change ac to dc voltage.– For example, 120 volts ac is changed to 13.8 volts

dc.

Power SuppliesConsist of:

1. Transformer – steps ac voltage up or down.2. Rectifier Diodes – change ac to “bumpy” dc.3. Filter Network – includes capacitors and

inductors, smooths out the bumps.4. Voltage Regulator – keeps the voltage constant

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Power Supply Overview

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• Transformer – sets the appropriate voltage level• Rectifier – rectifies AC input voltage to pulsating DC

voltage (can be half wave or full wave rectified • Filter – eliminates fluctuations in the rectified voltage

and produces a relatively smooth AC voltage (this function was performed by the capacitor)

• Regulator – maintains a constant voltage despite variations in the input line voltage or the load

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Power Supply Circuits

• To achieve its purpose a power supply must:– Step down the voltage supplied;– Convert ac to dc by rectifying the ac.

• A transformer is used to step down the magnitude of the voltages from the wall receptacle.

Transformer

• A transformer consists of two coils of wire on a common iron core. The voltages on these two coils are related by the turns ratio, which is the ratio of the number of turns of wire in the secondary coil to that in the primary coil.

RMS Values

• Note that the 110-120 volts and 220-240 volts are RMS values.

• The actual amplitude of that sinusoidal signal is a factor of √2 larger.

Rectification

• Converting ac to dc is accomplished by the process of rectification.

• Two processes are used:– Half-wave rectification;– Full-wave rectification.

IMPORTANCE & REQUIREMENTS

• For reasons associated with economics of generation and transmission, the electric power available is usually an a.c. supply.

• The voltage varies sinusoidal and has a frequency of 50Hz. It is used for lighting, heating and electric motors.

• But there are many applications (e.g. electronic circuits) where d.c. supply is needed. When such a d.c. supply is required, the mains a.c. supply is rectified by using crystal diodes.

TYPES OF RECTIFIERS

Centre-tape full-wave rec.

Full-waveBridge rec.

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Half Wave Rectifier

Effect of the Barrier Potential

Vp(out) = Vp(in) – 0.7 Volts

Note:

Vin must overcome the barrier potential (0.7V) before the diode becomes forward biased.

Half Wave Rectifier – Barrier Potential

Vp(in) – 0.7V. = Vp(out)

• Approx. 0.7V is “dropped” across the forward-biased diode junction. • This voltage is removed at the broad base of the waveform.

1207Chap2Fig2_5].ms8See Ex. 2-2 Pg.53

Half Wave Rectifier - Peak Inverse Voltage

Peak inverse voltage is the maximum voltage across the diode when it is in reverse bias. (blocking mode)

The diode must be capable of withstanding this amount of voltage. (-Vp(in)).

Peak Inverse Voltage (PIV)

• Diode must be able to withstand PIV (Peak Inverse) voltage

PIV

PIV calculations

PIV = ((Vpsec/2) – 0.7V) – (-Vpsec/2) =

((Vpsec/2) + (Vpsec/2)) – 0.7V =

Vpsec – 0.7V

1207Chap2Ex2_5.ms8

Half Wave Rectifier• We initially consider the diode to be ideal, such

that VC =0 and Rf =0

• The (ideal) diode conducts for vi >0 and since Rf =0 v0 vi

• For vi < 0 the (ideal) diode is an open circuit (it doesn’t conduct) andv0 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

0

0sin21 dVV MAV

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 ripple, i.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 (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)


• 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

TVΔV M


• We can define a ripple factor as

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

The lower the ripple factor the better

d.cVΔVfactor 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

Vvif

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)

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Average Voltage Value• the average voltage is a measure of the efficiency of

the rectifier circuit• the “straight line” dc equivalent of the pulsating dc

created by half wave rectification• the value you would measure on a dc voltmeter

• V p(out)

• V AVG = ----------• π

Transformer-Coupled Half-Wave Rectifier

• Transformers are often used for voltage change and isolation.

• The turns ratio determines the output voltage.

• Islolation between the primary and secondary windings.prevents shock hazards in the secondary circuit

Vsec = N Vpri

Vp(out) = Vp(sec) – 0.7V

See Ex.2-3 pg.55

Full-Wave Rectifiers

A full-wave rectifier allows current to flow during both the positive and negative half cycles or the full 360º. Note that the output frequency is twice the input frequency.

Most power supplies use full-wave rectifiers. Half-wave rectifiers see lesser applications like lo-cost power supplies.

The average VDC or VAVG = 2Vp/.

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Center Tapped Full Wave Rectifier

Full-Wave RectifierCenter-Tapped

This method of rectification employs two diodes connected to a center-tapped transformer.

The peak output is only half of the transformer’s peak secondary voltage.

Center-tap

Transformer Turns Ratio

Non-center-tapped transformer:• For a turns ratio (output/input) = 1, Output Vp = Input

Vp.• For a turns ratio = 2, Output Vp = Input Vp/2

Center-tapped transformer:• For a turns ratio (output/input) = 1, Output Vp = Input

Vp/2.• For a turns ratio = 2, Output Vp = Input Vp

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Bridge Full Wave Rectifier

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Average Voltage Value• twice that of half wave rectified output• 2V p(out)

• V AVG = ----------

• π

• Since 2/ π = 0.637, you can calculate • V AVG = 0.637 V p(out)

• The full wave rectifier is twice as efficient as the half wave rectifier

COMPARISON OF RECTIFIERS

Half-wave Centre-tap Bridge type

No. of diode 1 2 4

Transformer necessary

No Yes No

Maximum efficiency

40.6% 81.2% 81.2%

Summary of Rectifier Circuits

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Vm = peak of the AC voltage.

In the center tapped transformer rectifier circuit, the peak AC voltage is the transformer secondary voltage to the tap.

Rectifier Ideal VDC Realistic VDC

Half Wave Rectifier VDC = 0.318Vm VDC = 0.318Vm – 0.7

Bridge Rectifier VDC = 0.636Vm VDC = 0.636Vm – 2(0.7 V)

Center-Tapped Transformer Rectifier VDC = 0.636Vm VDC = 0.636Vm – 0.7 V

Important Concepts• Half Wave Rectifier:

– Simplest.– Hard to filter well.

• Full Wave Rectifier:– Easier to filter.– Requires transformer with center-tap.– Transformer secondary must be twice intended voltage.

• Bridge Rectifier:– Easier to filter (just like full wave).– Center-tap transformer not required.– Transformer secondary same as intended voltage.– Higher parts count.

Power Supply Filters And RegulatorsAs we have seen, the output of a rectifier is a pulsating DC. With filtration and regulation this pulsating voltage can be smoothed out and kept to a steady value.

Power Supply Filters And Regulators

A capacitor-input filter will charge and discharge such that it fills in the “gaps” between each peak. This reduces variations of voltage. The remaining voltage variation is called ripple voltage.


The advantage of a full-wave rectifier over a half-wave is quite clear. The capacitor can more effectively reduce the ripple when the time between peaks is shorter.

Ripple is approx. ½ with full-wave rectification.

Ripple Voltage Calculations

Vr(pp) = (1/fRLC)Vprect

VDC = (1 – 1/2fRLC)Vp(rect)


Being that the capacitor appears as a short during the initial charging, the current through the diodes can momentarily be quite high. To reduce risk of damaging the diodes, a surge current limiting resistor is placed in series with the filter and load.


Regulation is the last step in eliminating the remaining ripple and maintaining the output voltage to a specific value. Typically this regulation is performed by an integrated circuit regulator. There are many different types used based on the voltage and current requirements.


How well the regulation is performed by a regulator is measured by it’s regulation percentage.

There are two types of regulation, line and load.

Line and load regulation percentage is simply a ratio of change in voltage (line) or current (load) stated as a percentage.

Line Regulation = (VOUT/VIN)100%

Load Regulation = (VNL – VFL)/VFL)100%

Filter NetworkCapacitors and inductors are used in power supply filter networks. The capacitors and inductors smooth out the “bumpy” ac to dc.

Electrolytic Capacitor

• Electrolytic capacitors are used to filter rectified dc.

• They change “bumpy” dc to “smooth” dc.

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The Basic Capacitor

Capacitors are one of the fundamental passive components. In its most basic form, it is composed of two conductive plates separated by an insulating dielectric.

The ability to store charge is the definition of capacitance.

Dielectric

Conductors

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Die le c tric

Pla te sLe a d s

Ele c tro ns

BA

+

+

+

+

+

+

+

+

Initially uncharged

+ BA

+

+

+

+

+

+

+Charging+

BA

V S+

+

+++++++++

Fully charged BA

VS+

+

+++++++++

Source removed

The charging process…

A capacitor with stored charge can act as a temporary battery.

The Basic Capacitor

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Charging

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Discharging

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• How fast does a capacitor charge or discharge?

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The RC time constant

When a capacitor is charged through a series resistor and dc source, the charging curve is exponential.

C

R Iin itia l

t0(b ) C ha rg ing c urre nt

Vfina l

t0(a ) C a p a c ito r c ha rg ing vo lta g e

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Capacitor Charging Voltage Curve

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Tau (T)• The voltage across a capacitor cannot change

instantaneously because a finite time is required to move charge from one plate to another

• The rate at which the capacitor charges or discharges is determined by the RC time constant of the circuit

• The time constant of a series RC circuit is a time interval that equals the product of the resistance and capacitance

• T = RC

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When a capacitor is discharged through a resistor, the discharge curve is also an exponential. (Note that the current is negative.)

t

t

Iin itia l

0( b ) D is c h a rg in g c u rre n t

V in itia l

0( a ) C a p a c i t o r d is c h a r g in g v o l t a g e

C

R

The RC time constant

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Capacitor Discharging Voltage Curve

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Universal exponential curves

Specific values for current and voltage can be read from a universal curve. For an RC circuit, the time constant is

τ RC

100%

80%

60%

40%

20%

00 1t 2t 3t 4t 5t

99%98%95%

86%

63%

37%

14%

5% 2% 1%

Number of time constants

Perc

ent o

f fin

al v

alue

Rising exponential

Falling exponential

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Half Wave Rectifier with Capacitor Filter

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First Quarter Cycle

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Remainder of Cycle

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Second Cycle

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Ripple Voltage

• the variation in the output voltage• much improved when you add filtering• the smaller the ripple, the better the filtering

and the better quality dc output

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Half Wave & Full Wave Ripple Comparison

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Ripple Voltage

• Make RC > 10T• The ripple factor (r) is an indication of the

effectiveness of the filter and is defined as the ratio of the ripple voltage (Vr) to the dc (average) value of the filter output voltage (VDC)

• r = (Vr / VDC) x 100%

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Measures

• Efficiency – Average Voltage Value (VAVG)

• Filter Quality – Ripple Voltage (Vr) & Ripple Factor (r)

• Regulator Quality – Line & Load Regulation

Power Supply Safety

• Filter capacitors hold a charge a long time.• They charge to the output voltage of the

power supply.• Bleeder resistors across the filter capacitors

“bleed off” charge when supply is turned off.

Bleeder Resistors• A bleeder resistor is installed across the filter

capacitors as a safety feature to make sure that the capacitors are discharged.

Voltage Regulation

• This characteristic of the zener diode is very useful for voltage regulation circuits. The zener diode provides an effective way to clamp or limit the voltage at a relatively constant value thus creating a voltage regulation capability.

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Regulators• IC regulators• Line regulation• Load regulation

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IC Regulators

• Filters reduce ripple from a power supply to a relatively low level (<10%)

• Integrated circuit regulators connect to the output of a filtered regulator and reduce the ripple to a negligible level

• Regulators maintain a constant output voltage despite changes in the input voltage, load current or temperature

• Available in a variety of voltages

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A Basic Regulated Power Supply

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Percent Regulation• Regulation as a percentage is a figure of merit used

to measure performance of a voltage regulator• Line Regulation• How much change occurs in the output voltage

for a given change in the input voltage• Line Regulation = (ΔVOUT / ΔVIN)100%• Load Regulation• How much change occurs in the output voltage

from no load to full load• Load Regulation = (VNL – VFL / VFL)100%

Switching-Mode Power Supplies

SMPSWhat is SMPS?

SMPS means Switch Mode Power Supply. This is used for D.C to D.C conversion. This works on the principle of switching regulation. The SMPS system is highly reliable, efficient, noiseless and compact because the switching is done at very high rate in the order of several KHz to MHz

Switching Power Supplies

Switching Power Supplies

• The advantage of a switching-mode power supply is that the relatively high frequency oscillator allows the use of small, lightweight and low-cost transformers.

• This makes them considerably smaller and lighter than linear power supplies. Almost all modern powers supplies, including those in PCs, are switching mode power supplies. Their disadvantages are complexity and RF egress (interference).

Switching Power SuppliesIn a switched mode power supply, the first step in converting 120 V ac to a 12 V dc output is to rectify and filter the 120 V.

Rectifier and SMPS

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