2.1 Power Supply

Power supply is a group of circuits that convert the standard ac voltage (120 V, 60 Hz) provided by the wall outlet to constant dc voltage.

The voltage produced is used to power all types of electronic circuits including:

Consumer electronics (ex: radio, television, DVD, etc.)

Computers

Industrial controllers

Most laboratory instrumentation systems and equipment

The dc voltage level required depends on the application, but most applications require relatively low voltage.

There are two basic types of power supplies: a linear power supply and a switching

power supply. These components are described as follows:

1. A linear power supply is one that provides a constant current path between its input and its load.

2. A switching power supply provides an intermittent current path between its input and its output.

Fig. 2-1: Block diagram of a dc power supply with a load and rectifier.

(a) Complete power supply with transformer, rectifier, filter, and regulator

(b) Half-wave rectifier

2.2 Transformer

A transformer is a device that changes ac electric power at one voltage level to ac electric power at another voltage level through the action of a magnetic field.

Simple transformer consist of:

1. Primary winding (input winding)

2. Secondary winding (output winding)

3. Magnetic core

If the secondary has more turns than the primary, the output voltage across the secondary will be higher and the current will be smaller. If the secondary has fewer turns than the primary, the output voltage across the secondary will be lower and the current will be higher.

The core has a function to concentrate the magnetic flux.

Fig.2-2: The general arrangement of a transformer

There are three types of transformers: step-up, step-down, and isolation. These

components are described as follows:

1. The step-up transformer provides a secondary voltage that is greater than the primary voltage. Ex: a step-up transformer may provides a 240 Vac output with a 120 Vac input.

2. The step-down transformer provides a secondary voltage that is less than the primary voltage. Ex: a step-down transformer may provides a 30 Vac output with a 120 Vac input.

3. An isolation transformer provides an output voltage that is equal to the input voltage. This type of transformer is used to isolate the power supply electrically from the ac power line.

+

-

NP

+

-

NS

120 Vac 240 Vac

+

-

NP

+

-

NS

120 Vac 30 Vac

+

-

NP

+

-

NS

120 Vac 120 Vac

1:2 4:1 1:1

Step-up Step-down Isolation

(a) (b) (c)

Fig.2-3

The turns ratio of a transformer is equal to the voltage ratio of the component and since, the voltage ratio is the inverse of the current ratio. By formula:

sec

secsec

I

I

V

V

N

N pri

pripri

where

NSec = the number of turns in the secondary

NPri = the number of turns in the primary

VSec = the secondary voltage

VPri = the primary voltage

ISec = the secondary current

IPri = the primary current

By the equation (2-1) can be stated that:

Step-down transformer secondary current is greater than its primary current

(ISec > IPri).

Step-up transformer secondary current is less than its primary current (IPri > ISec).

(2-1)

2.3 Half-Wave Rectifiers

A rectifier is a diode circuits that converts the ac input voltage to a pulsating dc voltage.

There are three basic types of rectifier circuits:

1. Half-wave rectifier

2. Full-wave rectifier

3. Bridge rectifier

The half-wave rectifier is simply a diode that is placed in series between a transformer (or ac line input) and its load.

When a half-wave rectifier is positioned as shown in Fig.2-4 (a), it eliminates the negative alternation of the input. And when positioned as shown in Fig.2-4 (b), it eliminates the positive alternation of the input.

The direction of the diode determines whether the output from the rectifier is positive or negative:

1. When the diode points toward the load (RL), the output from the rectifier will be positive.

2. When the diode points toward the source, the output from the rectifier will be negative.

Fig.2-4: Half-wave rectifiers.

2.3.1 Load Voltage and Load Current

When we take the value of VF (barrier potential) into account, the peak out voltage (peak load voltage) is found as

Fpoutp VVV (sec))(

Vp(sec) is the peak secondary voltage which is the peak input voltage (Vp(in)) of the transformer and is found as

)(sec

(sec) prippri

p VN

NV

where

Nsec/Npri = the ratio of transformer secondary turns to primary turns

Vp(pri) = the peak primary voltage of a transformer

(2-2)

(2-3)

Fig.2-5: Ideal half-wave rectifier operation.

The source voltage are given as rms values as follows:

707.0rms

p

VV (2-4)

2.3.2 Average Voltage and Current

The value of average voltage (VAVG) for an ac (or other) waveform is the value that would be measured with a dc voltmeter. For a half-wave rectifier, VAVG is found as:

p

AVG

VV

where Vp = the peak value of the voltage.

Fig.2-6 : Average value of the half-wave rectified signal.

(2-5)

We can also convert a peak current to an average current. The value of the average

current (IAVG) for an ac waveform is the value that would be measured with a dc

ammeter. The value of IAVG can be calculated in one of two ways:

1. We can determine the value of VAVG and then use Ohm’s law as follows:

L

AVGAVG R

VI

2. We can calculate the value of Ip. Then, we can convert this peak value to average

form using equations similar to those we used to convert Vp to VAVG. The current

forms of these equations are:

p

AVG

II

(2-6)

(2-7)

2.3.3 Peak Inverse Voltage (PIV)

The maximum reverse bias that will be applied to a diode in a given circuit is called the peak inverse voltage (PIV) of the circuit. For the half-wave rectifier, the PIV is found as

(sec)pVPIV

When the diode is reverse biased (Fig.2-5 (b)), no voltage is dropped across the load.

Therefore, all of Vsec is dropped across the diode.

(2-8)

Ex. 2-1:

The fuse shown in Fig. 2-7 is used to limit the current in the primary of the transformer. Assuming that the fuse limits the value of Ipri to 1 A, what is the limit on the value of the secondary current if the transformer has a turns ratio of 1:4? Ans.: 250 mA

Ex. 2-2:

A circuit like the one shown in Fig.2-7 has a turns ratio of 1:5 and a fuse that limits the primary current to 750 mA. Calculate the maximum allowable value of Isec. Ans.: 150 mA

Ex. 2-3:

Determine the peak load voltage for the circuit shown in Fig.2-7. The ratio of secondary turns to primary turns is 5:1. Ans.: 33.3 V

Ex. 2-4:

A half-wave rectifier has values of Npri = 10, Nsec = 1, and Vp(pri) = 180 V. What is the peak load voltage for the circuit? Ans.: 17.3 V

Ex. 2-5:

Using the results of ex. 2-3 and 2-4, what are the peak load current, the value of VAVG and the value of IAVG for the circuit if the load current of the circuit is 120 Ω?

Figure 2-7.

F1

1 A

2.4 Full-Wave Rectifiers

There are two types of full-wave rectifiers: center-tapped and bridge full-wave rectifier.

2.4.1 The Center-Tapped Full-Wave Rectifier

A center-tapped rectifier is a type of rectifier that uses two diodes connected to the center of the secondary winding of a center-tapped transformer, as shown in Fig. 2-8.

The input voltage is coupled through the transformer to the center-tapped secondary.

Half of the total secondary voltage appears between the center tap and each end of the secondary winding as shown.

Fig.2-8: A center-tapped full-wave rectifier.

The number of positive alternations that make up the full-wave rectified voltage is twice that of the half-wave voltage for the same time interval, as illustrated by Fig. 2-9. Therefore, the average load voltage for the full-wave rectifier is found as:

Fig. 2-9: Typical rectifier waveforms.

to

Input signal

Half-wave rectifier output

Full-wave rectifier output

p

AVG

VV

2 (2-9)

2(sec)pV

2(sec)pV

Fig. 2-30: The operation of the center-tapped full-wave rectifier during the one complete cycle of the input signal.

During a positive half-cycle of the input voltage, the polarities of the secondary voltages are shown in Fig. 2-30 (a). This condition causes diode D1 is forward-biased, and diode D2 is reverse-biased. The current path is through D1 and the load resistor RL, as indicated.

During a negative half-cycle of the input voltage, the polarities of the secondary voltages are shown in Fig. 2-30 (b). This condition reverse-biases diode D1, and forward-biases diode D2. The current path is through D2 and RL, as indicated.

Because the current through the load resistor is in the same direction, so the output voltage always has the same polarity.

Assuming the diode is an ideal model, the peak output voltage (value of peak load voltage, Vp(load) ) can be approximated as:

2(sec)

)(p

outp

VV

The load voltage is approximately equal to half the secondary voltage because the transformer is center tapped. The voltage from either end of a center-tapped transformer to the center tap is always half the total secondary voltage.

(2-10)

(2-11)

Effect of the Turns Ratio on the Output Voltage

Using the practical diode model, in the any case, the peak load voltage for a full-wave rectifier is always one-half of the peak secondary voltage less the diode drop (due to the barrier potential):

VV

V ploadp 7.0

2(sec)

)(

If the transformer’s turns ratio is 1, the output voltage equals half the peak primary voltage less the voltage drop, as shown in Fig. 2-31.

Fig.2-31: Center-tapped full-wave rectifier with a transformer turns ratio of 1.

VV

V priploadp 7.0

2)(

)( (2-12)

In order to obtain an output voltage with a peak equal to the input peak, a step-transformer with a turns ratio of n = 2 must be used, as shown in Fig. 2-32. Meaning that the total peak secondary voltage is twice the peak primary voltage (Vp(sec) = 2Vp(pri)),

so the voltage across each half of the secondary is equal to Vp(pri).

Fig.2-32: Center-tapped full-wave rectifier with a transformer turns ratio of 2.

VVV priploadp 7.0)()( (2-13)

Peak Inverse Voltage

When one of the diodes in a full-wave rectifier is reverse-biased as shown in Fig. 2-33 where D2 is assumed to be reverse-biased, the maximum anode voltage of D1 is

2(sec)

)(p

anodem

VV

and the maximum anode voltage of D2 is

2(sec)

)(p

anodem

VV

(2-14)

(2-15)

Fig.2-33: Full-wave rectifier PIV (D1 is forward-biased and D2 is reverse-biased.

Thus, the peak inverse voltage across diode D2 is

VVV

VV

PIV ppp 7.02

7.02 (sec)

(sec)(sec)

Since D1 is assumed to be forward-biased, its cathode is at the same voltage as its anode minus diode drop; this is also the voltage on the cathode of D2. By formula,

VV

V pcathodem 7.0

2(sec)

)(

Because the peak load voltage supplied by the full-wave rectifier is approximately half the secondary voltage. By formula,

)((sec) 2 outpp VV 2(sec)

)()(p

outploadp

VVV or

(2-16)

(2-17)

(2-18)

Then, by substitution, the peak inverse voltage across either diode is

VVPIV outp 7.02 )(

2.4.2 The Bridge Full-Wave Rectifier

A bridge rectifier is a type of rectifier that uses four diodes connected as shown in Fig.

2-34.

The bridge rectifier is the most commonly used full-wave rectifier for several reasons:

1. It does not require the use of a center-tapped transformer and therefore can be coupled directly to the ac power line if desired.

2. When connected to a transformer with the same secondary voltage, it produces nearly twice the peak output voltage of the conventional full-wave rectifier. This results in a higher dc output voltage from the supply.

(2-19)

Fig.2-34: Bridge rectifier operation.

(a) During the positive half-cycle of the input, D1 and D2 are forward-biased and conduct current. D3 and D4 are reverse-biased.

(b) During the negative half-cycle of the input, D3 and D4 are forward-biased and conduct current. D1 and D2 are reverse-biased.

(sec)pV

(sec)pV

If the center-tapped full-wave rectifier produces its output by alternating conduction between two diodes, the bridge full-wave rectifier alternates conduction between two diode pairs.

When the input half-cycle is positive, the transformer has the polarity as shown in Fig. 2-34(a), causing diodes D1 and D2 are forward-biased and conduct the current in the direction shown. A voltage is developed across RL that looks like the positive half of the input cycle. During the time, diodes D3 and D4 are reverse-biased.

When the input half-cycle is negative, the transformer has the polarity as shown in Fig. 2-34(b). Diodes D3 and D4 now are forward-biased and conduct the current in the same direction through RL. During the time, diodes D1 and D2 are reverse-biased. A full-wave rectified output voltage appears across RL as a result of this action.

Bridge Output Voltage

Assuming the diodes in the bridge as shown in Fig. 2-35 to be ideal (in other word, the diode drops are neglected), the rectifier has a peak output voltage of

(sec))( poutp VV (2-20)

Fig. 2-35: Bridge operation during a positive half-cycle of the primary and secondary voltages for a ideal diodes.

If the voltage drops across the two conducting diodes as shown in Fig. 2-36 are taken into account, the output voltage is

The 1.4 V in the equation represents the sum of the diode voltage drops. Note that in the bridge rectifier, two diodes are always in series with the load resistor during both the positive and negative half-cycles.

VVV poutp 4.1(sec))(

Fig. 2-36: Bridge operation during a positive half-cycle of the primary and secondary voltages for a practical diodes.

(2-21)

Peak Inverse Voltage

When Vsec has the polarity as shown in Fig. 2-37(a), D1 and D2 are forward-biased that the current flows through D1 and D2 as shorts (ideal model). As shown, D3 and D4 are connected across the transformer secondary and thus, D3 and D4 have a peak inverse voltage equal to the peak secondary voltage. Since the output voltage is ideally equal to the secondary voltage,

)(outpVPIV

If the voltage drops of the forward-biased diodes are included as shown in Fig. 2-36(b), the peak inverse voltage across each reverse-biased diode in terms of Vp(out) is

VVPIV outp 7.0)(

(2-22)

(2-23)

Fig.2-37: Peak inverse voltages across diode D3 and D4 in a bridge rectifier during the positive half-cycle of the secondary voltage.

Fig.2-37: Peak inverse voltages across diode D3 and D4 in a bridge rectifier during the positive half-cycle of the secondary voltage.

2.5 Filters and Regulators

Filter is a circuit implemented with capacitor that follows the rectifier in a power supply.

Filters are used to reduce the fluctuation in the rectified output voltage and produces a constant dc output voltage as shown in Fig. 2-38.

The small amount of fluctuation in the filter output voltage is called ripple.

Fig.2-38: The effects of filtering on the output of a half-wave rectifier.

The amount of ripple in the output voltage from a given filter depends on rectifier used,

filter component values, and load resistance.

Too much ripple in the output can have different adverse effects, depending on the

application of the power supply.

For example:

In an audio amplifier, excessive ripple can produce an “annoying hum” at 60 or

120 Hz, depending on the type rectifier used.

In video circuits, excessive ripple can produce “video hum bars” in the picture.

In the digital circuits, excessive ripple can result in “erroneous outputs” from logic

gates.

Therefore, filtering is necessary to provide power and biasing for proper operation of

electronic circuits.

2.5.1 Capacitive Filter

Capacitive filter is simply a capacitor connected in parallel with the load resistance or connected from the rectifier output to ground, as shown in Fig.2-39.

During the positive first quarter-cycle of the input, the diode is forward-biased, allowing the capacitor charges rapidly, as illustrated in Fig.2-39(a).

When the input begins to go negative, the diode is reverse-biased, and the capacitor slowly discharges through the load resistance (Fig.2-39(b)). As the output from the rectifier drops below the charged voltage of the capacitor, the capacitor acts as the voltage source for the load.

During first quarter of the next cycle, as illustrated in part (c), the diode will again become forward-biased when the input voltage exceeds the capacitor voltage.

Capacitor

Fig.2-39: Basic capacitive filter. Current indicates charging or discharging of the capacitor

Ripple Voltage

Ripple voltage is the fluctuation in the capacitor voltage due to the difference between the charge and discharge times.

The difference between the charge and discharge times is caused by two distinct RC time constant in the circuit. One time constant is found as:

RC (2-24)

where R and C are the total circuit resistance and capacitance, respectively.

Since it takes five time constants for a capacitor to charge or discharge fully, this time period (T) can be found as:

55 RCT (2-25)

The discharge path for the capacitor is through the resistor as shown in Fig. 2-40(b). For this circuit, the time constant is found as:

msFk 100)100)(1(

msmsT 500)100)(5(

and the total capacitor discharge time is found as:

For example, refer to Fig. 2-40(a), the capacitor charges through the diode. Assuming that diode has a forward resistance of 5 Ω, so the time constant for the circuit is found as:

sF 500)100)(5(

and the total capacitor charge time is found as:

mssT 5.2)500)(5(

(a) Charge circuit

(b) Discharge circuit

Fig.2-40: The basic capacitive filter.

Fig.2-41: Comparison of ripple voltages for half-wave and full-wave rectified voltages with the same filter capacitor and load and derived from the same sinusoidal input voltage.

The shorter time between peaks of the rectifier output voltage causes the capacitor reduces the more fluctuation. Therefore, the full-wave rectified voltage has a smaller ripple than the half-wave rectified voltage for the same load resistance and capacitor values (Fig. 2-41).

Ripple Factor

The ripple factor (r) is an indication of the effectiveness of the filter and defined as:

DC

ppr

V

Vr )(

where, Vr(pp) is the peak-to-peak ripple voltage and VDC is the dc (average) value of the filter’s output voltage (Fig.2-42).

Fig.2-42: Vr and VDC determine the ripple factor.

(2-26)

Fig. 2-43.

The lower the ripple factor, the better the filter.

The ripple factor (or the amplitude of the ripple voltage) at the output of a filter can be lowered by increasing the value of filter capacitance or increasing the load resistance, as illustrated in Fig. 2-43:

For a full-wave rectifier with a capacitive filter, approximations for the peak-to-peak

ripple voltage, Vr(pp), and dc value of the filter output voltage, VDC, are given in the

following equations:

)()(

1rectp

Lppr V

CfRV

)(2

11 rectp

LDC V

CfRV

Vp(rect) = the unfiltered peak rectified voltage.

(2-27)

(2-28)

2.5.2 Surge Current

Before the switch in Fig.2-44 is closed, the filter capacitor is uncharged. At the instant

the switch is closed, voltage is connected to the bridge and the uncharged capacitor

acts as a short circuit. As a result, the current is initially limited only by the resistance of

the transformer secondary and the bulk resistance of the diode. Since these

resistances are usually very low, the initial current tends to be extremely high. This high

initial current is referred to as surge current.

Fig.2-44: Surge current in a capacitive filter.

The value of surge current can be calculated as follows:

BW

psurge RR

VI

(sec)

where, Vp(sec) = the peak secondary voltage

RW = the resistance of the secondary windings

RB = the diode bulk resistance

(2-29)

2.5.3 Voltage Regulators

A voltage regulator is connected to the output of a filtered rectifier and maintains a constant output voltage (or current) despite changes in the input, the load current, or the temperature.

The regulator reduces the ripple to negligible amount.

Most regulators are integrated circuits and have three terminals: an input terminal, an output terminal, and a reference (or adjust) terminal.

Three-terminal regulators designed for fixed output voltages require only external capacitors to complete the regulation portion of the power supply, as shown in Fig.2-45.

Fig.2-45: A voltage regulator with input and output capacitors.

Fig.2-46: A basic +5 V regulated power supply

2.5.4 Percent Regulation

The regulation can be stated in a percentage in terms of input (line) regulation or load

regulation.

Line regulation specifies how much change occurs in the output voltage for a given

change in the input voltage. It is mathematically defined as a ratio of a change in output

voltage for a corresponding change in the input voltage expressed as a percentage.

%100xV

VregulationLine

IN

OUT

Load regulation specifies how much change occurs in the output voltage over a certain range of load current values, usually from minimum current (no load, NL) to maximum current (full load, FL). It can be mathematically determined with the following formula:

%100xV

VVregulationLoad

FL

FLNL

(2-30)

(2-31)

2.6 Clippers

Clipper is a diode circuit that is used to limit or clip the positive part of the input voltage.

Clipper is often referred to as a limiter.

2.6.1 Series Clippers

Each series clipper contains a diode that is positioned in series with a load resistor, as shown in Fig.2-47 (a) and (b).

Fig.2-47: Two basic clipper configurations.

+

-

+ -

RL

+

-

+-

RL

I = 0

(a) Negative series clipper (b) Positive series clipper

The operating principles of the series clipper are as follows:

1. When the diode in a negative series clipper is forward biased by the input signal, it

conducts, and the load voltage is found as

VVV inL 7.0

2. When the diode in the negative series clipper is reverse biased by the input signal, it does not conduct. Therefore,

indiode VV

A negative series clipper and its associated waveforms are shown in Fig.2-48(a).

and

VVL 0

3. The positive series clipper operates in the same fashion. The only differences are:

a. The output voltage polarities are reversed.

b. The current directions through the circuit are reversed.

A positive series clipper and its associated waveforms are shown in Fig.2-48(b).

(2-32)

(2-33)

(2-34)

Comparing Fig.2-4 and 2-49, we can see that the operation of the series clipper is identical to that of the half-wave rectifier.

Fig.2-48: Series clipper operation

2.6.2 Shunt Clippers

Each shunt clipper contains a diode that is positioned in parallel with a load resistor, as shown in Fig.2-49 (a) and (c).

The operating principles of the shunt clipper are as follows:

As the input signal goes positive (Fig.2-49(a)), the diode is forward biased and conducts current. With the diode conducting, the voltage across the diode equals the diode forward voltage (VF). Since the diode and load are in parallel, the output voltage also equals the diode forward voltage. By formula:

Fig.2-49: Two basic shunt clipper configurations.

(a) Positive shunt clipper. The diode is forward-biased during the positive alternation (above 0.7 V).

(b) Positive shunt clipper. The diode is reverse-biased

during the negative alternation (below 0.7 V).

VVV Fout 7.0

When the input signal goes back below 0.7 V (Fig.2-49(b)), the diode becomes reverse biased and acts as an open circuit. The output voltage looks like the negative part of the input voltage, but with a magnitude determined by the voltage divider formed by R1 and RL, as follows:

+

-

(2-35)

If the diode is turned around, as in Fig. 2-50, the negative part of the input signal is clipped off. When the diode is forward-biased during the negative part of the input signal, the voltage across the diode is held to - 0.7 V. When the input signal goes above -0.7 V, the diode is no longer forward-biased; and the output voltage is proportional to the input voltage.

inL

Lout V

RR

RV

1

If R1 is small compared to RL, then

inout VV

(2-36)

(2-37)

Fig.2-50: Negative shunt clipper. The diode is forward-biased during the negative alternation and reverse-biased during the positive alternation.

2.6.3 Biased Clippers/Limiters

A biased clipper uses a dc bias

source, VBias, in series with the diode

to limit an output voltage to certain

level, as shown in Fig. 2-51.

The positive-biased clipper (Fig.2-

51(a)) clips the input signal at the

values of VBias + 0.7 V before the

diode will become forward-biased

and conduct. Once the diode starts to

conduct, the voltage at point A is

limited to VBias + 0.7 V so that all input

signal above this level is clipped off.

The negative-biased clipper (Fig.2-

51(b)) works in the same fashion, but

it clips the input signal at the values

of -VBias - 0.7 V.

(a) A positive clipper or limiter

(b) A negative clipper or limiter

Fig.2-51: Biased shunt clipper or limiter

By turning the diode around, the positive clipper can be modified to limit the output voltage to the portion of the input voltage waveform above VBias - 0.7 V, as

shown in Fig.2-52(a).

Similarly, the negative clipper can be modified to limit the output voltage to the portion of the input voltage waveform below –VBias +

0.7 V, as shown in part (b).

Fig. 2-52.

2.6.4 Voltage-Divider Bias

The bias voltage sources can be replaced by a resistive voltage divider that derives the

desired bias voltage from the dc supply voltage, as illustrated in Fig.2-53.

The bias voltage is calculated by using the voltage-divider formula as follows:

Fig. 2–53: Diode clippers implemented with voltage–divider bias.

SupplyBias VRR

RV

32

3

(a) A positive clipper (b) A negative clipper (b) A variable positive clipper

(2-38)

Fig.2-54: The input/output characteristics of the positive clamper circuit

2.7 Clampers (DC Restorers)

Clamper is a diode circuit designed to shift a waveform either above or below a given

reference voltage without distorting the waveform.

There are two types of clampers: the positive clamper and the negative clamper.

1. A positive clamper shifts its input waveform so that the negative peak of the

waveform is equal to the clamper dc reference voltage.

For example: Fig. 2-54 shows what happens when a 20 Vpp sin wave is applied to a

positive clamper with a dc reference of 0 V. The input and output waveforms have the

value of 20 Vpp. However, the clamper output waveform has the positive peak of +20 V and

the negative peak of 0 V. The positive clamper has shifted the entire waveform so that

its negative peak is equal to the circuit’s dc reference voltage.

2. A negative clamper shifts its input waveform so that the positive peak of the waveform

is equal to the clamper dc reference voltage.

For example: Fig. 2-55 shows what happens when a 20 Vpp sin wave is applied to

a negative clamper with a dc reference of 0 V. In this case, The clamper output

waveform has the positive peak of 0 V and the negative peak of –20 V. The

negative clamper has shifted the entire waveform so that its negative peak is

equal to the circuit’s dc reference voltage.

Fig.2-55: The input/output characteristics of the negative clamper circuit

Clamper Operation

The clamper is similar to a shunt clipper; the difference is added capacitor in the clamper, as illustrated in Fig. 2-56.

For the circuit in Fig. 2-56(a), the diode is forward biased and it charges the capacitor. Thus, the charging time constant is found as: (a) Capacitor charge circuit

Reverse-biased

+

−

(b) Capacitor discharge circuit

Fig.2-56: Clamper charge and discharge

CRD

where RD is bulk resistance of the diode and C is capacitance of the capacitor. The total charge time is:

CRT DeCh 5arg

(2-39)

(2-40)

When the diode is reverse biased, the capacitor starts to discharge through the resistor, as shown in Fig. 2-56(b). Therefore, the discharge time constant is found as:

CRL

CRT LeDisch 5arg

and the total discharge time is found as:

(2-41)

(2-42)

The effect of the clamping action is shown in Fig. 2-56. The capacitor retains a charge approximately equal to the input peak less the diode drop so that it acts as a battery.

Fig.2-57: Positive clamper operation

If the diode is turned around, a negative dc voltage is added to the input voltage to produce the output voltage as shown in Fig. 2-57:

Fig.2-57: Negative clamper operation

Ex. 2-6:

Assume the circuit in Fig.2-40 has an input of 170 Vpk, a turns ratio of 2:1, and values of RW = 0.5 Ω and RB = 8 Ω. What is the initial surge current for the circuit?

Ex. 2-7:

A negative shunt clipper has a values of RL = 510 Ω and R1 = 100 Ω. If the input voltage is +15 Vpk, what is the peak load voltage?

2.8 Voltage Multipliers

A voltage multiplier is a circuit providing a dc output voltage that is a multiple of its

peak input voltage.

When a voltage multiplier increases a peak input voltage by a given factor, the peak

input current is decreased by approximately the same factor.

The typical application of a voltage multiplier is to supply the high-voltage, low-current

input required to operate the cathode-ray tube in a television and to operate the

particle accelerators.

There are several types of the voltage multiplier:

Voltage doublers : half-wave voltage doubler and full-wave voltage doubler.

Voltage tripler

Voltage quadrupler

Fig.2–58: Half-wave voltage doubler operation. Vp is the peak secondary voltage.

2.8.1 Voltage Doubler

Voltage doubler is a voltage multiplier with a multiplication factor of two.

Half-Wave Voltage Doubler

Half-wave voltage doubler consist of two diodes and two capacitors, as shown in Fig.2-58.

During the positive half-cycle of the secondary voltage (in Fig.2-58(a)), diode D1 is forward-biased and D2 is reverse-biased. Capacitor C1 charges until its plate-to-plate voltage equals the peak value of the secondary voltage less the diode drop.

During the negative half-cycle, D2 is forward-biased and D1 is reverse-biased (in part b). AC voltage source and C1 charge C2 until its plate-to-plate voltage is approximately 2 Vp.

Applying Kirchhoff’s law around the loop as shown in part (b), the voltage across C2 is

021 pCC VVV

12 CpC VVV

Neglecting the diode drop of D2, VC1 = Vp. Therefore,

pppC VVVV 22

Input and output waveforms for a half-wave voltage doubler are shown in Fig. 2-59.

Fig. 2-59.

Full-Wave Voltage Doubler

The full-wave voltage doubler closly resembles the half-wave voltage doubler, as shown in Fig.2-60.

During the positive half-cycle of the secondary voltage, D1 is forward-biased and C1 charges until its plate-to-plate voltage is approximately equal to Vp.

During the negative half-cycle, D2 is forward-biased and C2 charges until its plate-to-plate voltage is approximately equal to Vp. Since C1 and C2 are in series, the total output voltage across the two capacitors is 2 Vp.

Fig.2–60: Full-wave voltage doubler operation.

2.8.2 Voltage Tripler

Voltage tripler is variation on the half-wave voltage doubler. It is created by the addition of another diode-capacitor section to the half-wave voltage doubler, as shown in Fig.2-61.

Fig. 2–61: Voltage tripler.

On the positive half-cycle of the secondary voltage, C1 charges through D1 until its plate-to-plate voltage is approximately equal to Vp.

During the negative half-cycle, C2 charges through D2 until its plate-to-plate voltage is approximately equal to 2Vp.

During next positive half-cycle, C3 charges through D3 until its plate-to-plate voltage is approximately equal to 2Vp. The voltage across C1 and C3 add up to 3Vp.

2.8.3 Voltage Quadrupler

Voltage quadrupler is also variation on the half-wave voltage doubler. It is produced by the addition of still another diode-capacitor section, as shown in Fig.2-62.

Fig. 2–62: Voltage quadrupler.

C4 charges through D4 until its plate-to-plate voltage is approximately equal to 2Vp

on a negative half-cycle. The voltage across C1 and C3 now add up to 4Vp.

Summary:

Half-wave voltage doubler provides a dc output voltage that is approximately twice its

peak input voltage.

Full-wave voltage doubler provides a dc output voltage that is approximately twice its

peak input voltage.

Voltage tripler produces a dc output voltage that is approximately three times its peak

input voltage.

Voltage quadrupler produces a dc output voltage that is approximately four times its

peak input voltage.

2.9 The Diode Datasheet

Datasheet gives detailed information on a device as maximum ratings, electrical

characteristics, mechanical data, and graphs of various parameter. Thus, it can be

used properly in a given application.

Diode datasheet is commonly divided into two categories:

1. Data given in table form: (a) absolute maximum ratings, (b) thermal characteristics

and (c) electrical characteristics.

2. Characteristics shown in graphical form: (a) forward current derating curve, (b)

forward characteristics curve, (c) nonrepetitive surge current and (d) reverse

characteristics.

Fig. 2–63: A selection of rectifier diodes based on maximum ratings of IO, IFSM, and VRRM.

Fig. 2–63: A selection of rectifier diodes based on maximum ratings of IO, IFSM, and VRRM.

2.10 Troubleshooting

Power supply faults may occur in the transformer, rectifier, filter, or regulator.

2.10.1 Transformer Faults

The transformer in a power supply develop one of several possible faults:

1. A shorted primary or secondary winding.

2. An open primary or secondary winding.

3. A short between the primary and secondary winding and the transformer frame.

In most cases, a shorted primary or secondary winding will cause the fuse to blow.

2.10.2 Rectifier Faults

Half-wave rectifier is the easiest rectifier to troubleshoot. If the diode in the rectifier

shorts, the output will be a sine wave. If the diode opens, the resulting symptom is an

output of 0 V, as illustrated in Fig. 2-64.

Fig. 2–64: The effect of an open diode in a half-wave rectifier is an output of 0 V.

In full-wave rectifier, a shorted diode will cause the power supply fuse to blow. Full-wave center-tapped rectifier is shown in Fig.2-65. If either of two diodes is open, the output voltage will have twice the normal ripple voltage and the ripple frequency will go from 120 to 60 Hz.

Fig. 2–66: Effect of an open diode in a bridge rectifier.

The symptoms for open diode in the bridge rectifier are the same as those for the center-tapped full-wave rectifiers, as shown in Fig.2-66. In the case, we have more diodes that must be tested.

Fig. 2–66: Effect of an open diode in a bridge rectifier.

2.10.3 Filter Faults

Capacitors store an electrical charge, and they can retain that charge even after the

power switch has been turned off or the ac plug has been disconnected. This is a

danger to the technician and also, the test equipment can be damaged.

As a safety precaution, capacitors should be discharged by shorting the terminals

with a shorting tool before taking any measurements.

Three types of defects of a filter capacitor are illustrated in Fig.2-67:

If the filter capacitor for a full-wave rectifier opens, the output is a full-wave

rectified voltage.

If the filter capacitor shorts, the fuse will blow and the output is 0 V. When the

filter capacitor shorts, this effectively shorts out the transformer secondary,

causing excessive secondary current.

If the filter capacitor leaks, the leakage resistance will reduce the time constant

and allow the capacitor to discharge more rapidly than the normal. This results in

an increase in the ripple voltage on the output. This fault is rare.

Fig. 2–67: Effects of a faulty filter capacitor.