chapter 2 diode applications · chapter 2 diode applications basic dc power supply - a dc power...

PHYS 162 - Chapter 2 Diode Applications

Prepared By: Syed Muhammad Asad – Semester 102

Figure 1 DC Power Supply

CHAPTER 2 DIODE APPLICATIONS

BASIC DC POWER SUPPLY - A DC power supply is one of the most important part of an electronic circuit.

- It converts the supply line AC voltage to an appropriate DC voltage which can be used to power

up the electronic circuit inside various electronic appliances.

- The DC power supply consists of the following parts

o Transformer – It is responsible to step down the AC line voltage to a lower AC voltage.

o Rectifier – Converts the AC voltage to a pulsating DC voltage.

o Filter – Filters the rectified voltage close to the DC voltage but with ripples.

o Voltage Regulator – Smoothes out the entire ripple in the filtered output and maintains

the output to a specified DC voltage level.

2-1 HALF – WAVE RECTIFIER - A diode circuit that produces one half cycle output for one complete cycle of input is called a

half-wave rectifier as shown in Figure 2.

- In the positive half cycle, the diode is forward biased and produces a positive cycle at the

output taken across RL.

- In the negative cycle the diode is reverse biased and the output is zero.

Average Value of Half Wave Output Voltage

- The average value of the output would the value measured on a DC multimeter.

- The average output is 31.8% of 𝑽𝒑 and is given by 𝑽𝑨𝑽𝑮 = 𝑽𝒑

𝝅.



Figure 2 Half Wave Rectification (Ideal Diode)

Figure 3 Half Wave Rectification (Practical Diode)

2.1.1 Effect of Barrier Potential on the Half-Wave Rectifier Output

- If the diodes are not

ideal, then the barrier

potential of 0.7 V is

taken into account.

- During the positive half

cycle, the input voltage

must overcome the

barrier potential for the

diode to become

forward bias.

- This results in the half cycle to appear at the output.

- During the negative cycle the diode is reverse biased so there is no output.

- The peak output voltage in half wave rectifier is given by

𝑽𝒑(𝒐𝒖𝒕) = 𝑽𝒑 𝒊𝒏 − 𝟎. 𝟕 𝑽

NOTE – Refer to Example 2-2 Page 49



Figure 4 Half Wave Rectifier with Transformer Coupling

Figure 5 Full Wave Rectification

2.1.2 Peak Inverse Voltage (PIV)

- PIV is the peak value of the input

voltage and is given as 𝐏𝐈𝐕 = 𝑉𝑝 𝑖𝑛

2.1.3 Transformer Coupling

- Transformer is used to either step

down or step up the source

voltage.

- The amount of step down is

determined by the turn ratio.

- Turn Ratio ‘n’ is defined as the ratio of number of secondary turns (NSEC) to the number of

primary turns (NPRI).

- If n < 1, the transformer is step down and if n > 1, the transformer is step up.

- The secondary voltage of a transformer is given by

𝑽𝒔𝒆𝒄 = 𝒏𝑽𝒑𝒓𝒊

- If n > 1, secondary voltage is greater than primary while if n < 1, the secondary voltage is less

than the primary.

- The peak output voltage in transformer coupled half wave rectifier is given by

𝑽𝒑 𝒐𝒖𝒕 = 𝑽𝒑 𝒔𝒆𝒄 − 𝟎. 𝟕 𝑽

- And PIV is given by

𝑷𝑰𝑽 = 𝑽𝒑 𝒔𝒆𝒄


2-2 FULL-WAVE RECTIFIER - For each complete cycle of the input voltage, the full wave rectifier produces two cycles at the

output that are both in the same direction.

- The frequency of the output doubles

compared to the input, i.e. 𝑓𝑜𝑢𝑡 = 2𝑓𝑖𝑛 .

- There are two types of full wave rectifier

circuit.

o Center – tapped Full Wave Rectifier

o Bridge Full Wave Rectifier

Average Value of Full Wave Output Voltage

- The average value of the output would the value measured on a DC multimeter. The average

output is 63.7% of 𝑽𝒑 and is given by

𝑽𝑨𝑽𝑮 = 𝟐𝑽𝒑

𝝅



Figure 6 Center-Tapped Full Wave Rectifier

2.2.1 Center-Tapped Full-Wave Rectifier

- A center-tapped rectifier uses two diode connected to the secondary of the transformer.

- Half of the total secondary voltage appears at the two ends of the winding.

Rectification Mechanism

- The rectification mechanism of the center-tapped rectifier can be described as follows

o In the positive half cycle of the input, diode D1 is forward biased and diode D2 is reverse

biased.

o The flow of current is shown in Figure 7. This results in a positive cycle at the output.

o During the negative cycle of the input, D2 is forward biased while D1 is reverse.

o The flow of current again results in a positive cycle at the output.

Figure 7 Rectification Mechanism



Figure 8 Bridge Rectifier

Effect of Turn Ratio on the Output Voltage and PIV

- If n = 1, then the peak rectified output voltage equals half the peak primary input voltage

minus the diode drop.

- If we want the output rectified voltage to be equal to the peak input voltage then the turn ratio

should be equal to 2, i.e., n = 2.

- In general, the output voltage of a center-tapped rectifier is given by

𝑽𝒐𝒖𝒕 =𝑽𝒔𝒆𝒄

𝟐− 𝟎. 𝟕𝑽

- The PIV voltage is given by 𝑃𝐼𝑉 = 2𝑉𝑝 𝑜𝑢𝑡 + 0.7𝑉.


22.2 Bridge Full-Wave Rectifier

- First note that the transformer is not center-tapped therefore the secondary voltage will not be

divided in half.

Rectification Mechanism

- The rectification mechanism of the bridge rectifier can be described as follows

o In the positive input cycle, D1 and D2 are forward biased and conduct current in the

direction shown in Figure 8(a).

o This produces a positive output at the load resistance RL.

o During the negative input cycle, D3 and D4 are forward biased and conduct current in

the direction shown in Figure 8(b).

o As this direction is also the same as in the positive cycle, it produces positive output at RL.



Figure 9 Filter Operation

Bridge Output Voltage and PIV

- For the case of ideal diodes the

output voltage is given by 𝑽𝒐𝒖𝒕 =

𝑽𝒑 𝒔𝒆𝒄 .

- For practical diodes, the output

rectified voltage is given by

𝑽𝒐𝒖𝒕 = 𝑽𝒑 𝒔𝒆𝒄 − 𝟏. 𝟒𝑽

- The PIV for is ideal diodes and

practical diodes is given respectively

as

𝑃𝐼𝑉 = 𝑉𝑝 𝑜𝑢𝑡

And

𝑃𝐼𝑉 = 𝑉𝑝 𝑜𝑢𝑡 + 0.7𝑉


2-3 POWER SUPPLY FILTERS AND REGULATORS - A power supply filter is used to remove the fluctuations in the output of a rectifier to produce a

constant DC level as shown in Figure 9.

- Filters are made from capacitors.

2.3.1 Capacitor-Input Filter

- This filter uses a capacitor connected

to the rectifier output to the ground.

Filter Mechanism

- The basic filter mechanism can be

given by an example of half wave

rectifier (Figure 10).

- During the positive quarter cycle of

the input (Figure 10a), the diode is

forward biased and charges the

capacitor to 𝑽𝒑 − 𝟎. 𝟕𝑽.

- As the input begins to decrease from

its peak (Figure 10b), the diode

becomes reverse biased because the

capacitor remains charged.

- This means that voltage at the anode

of the diode is lower than the voltage

at the cathode.

- During the rest of the cycle, the

capacitor discharges through RL.

- The rate of discharge is determined by Figure 10 Filter Mechanism



Figure 11 Effect of capacitor on ripples

Figure 12 Voltage Regulator

the RLC time constant. Bigger constant means that the capacitor will discharge slowly.

- In rectifier filter, the constant is kept long as compared to the period of the input.

- When the next quarter positive cycle comes (Figure 10c), the diode will again become forward

bias and recharges the capacitor.

Ripple Voltage

- As the capacitor is continuously charging and discharging, this variation of voltage appears as

ripple at the output of the filter and is called ripple voltage, 𝑽𝒓 𝒑𝒑 .

- Big capacitors result in lower ripples (good filter) and small capacitor give higher ripples (bad

filter).

Ripple Factor

- The ripple factor is an

indication of how good a filter

is working.

- Small ripple factor means the

filter is good. It can be made as

small as possible by putting

large value of capacitor or load

resistance in the circuit.

- The ripple factor is given by

𝒓 =𝑽𝒓 𝒑𝒑

𝑽𝑫𝑪

- The 𝑉𝑟 𝑝𝑝 and 𝑉𝐷𝐶 in the filtered rectifier circuit are given by

𝑽𝒓 𝒑𝒑 = 𝟏

𝒇𝑹𝑳𝑪 𝑽𝒑 𝒓𝒆𝒄𝒕

𝑽𝑫𝑪 = 𝟏 −𝟏

𝟐𝒇𝑹𝑳𝑪 𝑽𝒑 𝒓𝒆𝒄𝒕


2.3.2 Voltage Regulator

- Voltage regulators are used to remove

the ripple completely and give smooth

DC output voltage.

- It is connected to the output of the filtered rectifier.

- The ripple factor of less than 10% is ideal for the regulator to smooth.

- Voltage regulators usually come in three-terminal IC’s. One is input; one is output and third is

ground.

- C1 acts as a rectifier filter and C2 act as smoothing filter as shown in Figure 12.



Figure 13 Diode Limiter

2-4 DIODE LIMITING AND CLAMPING CIRCUITS

2.4.1 Diode Limiters or Clippers

- Diode limiters or clippers are used to clip some portion of the signal voltage above or below a

voltage level.

Circuit Operation

- For the circuit in Figure 13(a), during the positive cycle of the input, diode is forward biased and

the output is limited to 0.7V (positive limiter).

- During negative cycle, the diode is reverse biased and the output will look like the input.

- The output voltage is given by the voltage divider as

𝑽𝒐𝒖𝒕 = 𝑹𝑳

𝑹𝟏 + 𝑹𝑳 𝑽𝒊𝒏

- For the circuit in Figure 13(b), the output will be reverse as now diode will be reverse biased in

positive cycle and forward biased in the negative cycle (negative limiter).




Figure 14 Biased Limiters

Biased Limiters

- By adding a bias voltage, the level of AC voltage limit can be adjusted.

Circuit Operation

First Circuit (Figure 14)

- The voltage at point A must be

more than 𝑽𝑩𝑰𝑨𝑺 + 𝟎. 𝟕𝑽 for the

diode to become forward

biased.

- Once the input is greater than

𝑉𝐵𝐼𝐴𝑆 + 0.7𝑉, all the voltage will

clip and the output will remain

𝑽𝑩𝑰𝑨𝑺 + 𝟎. 𝟕𝑽 (positive limiter).

- For the negative cycle, the

output will follow the input

according to the voltage

divider.

Second Circuit

- The input a point A must be

below −𝑽𝑩𝑰𝑨𝑺 − 𝟎. 𝟕𝑽 for the

diode to become forward

biased.

- The output in this circuit will

be opposite of the first

(negative limiter).

Third Circuit

- If we turn around the diode in

the first circuit in Figure 14,

we get the third circuit.

- Until the input voltage is less

than 𝑽𝑩𝑰𝑨𝑺 − 𝟎. 𝟕𝑽, the diode

will remain forward biased

and the output will be

𝑽𝑩𝑰𝑨𝑺 − 𝟎. 𝟕𝑽.

- When the voltage at Point A is

greater than 𝑽𝑩𝑰𝑨𝑺 − 𝟎. 𝟕𝑽,

the diode will become reverse biased and the output will follow the input voltage based on the

voltage divider.

Fourth Circuit

- If we turn around the diode in the second circuit in Figure 14, we get the fourth circuit.

- Until the input voltage is more than −𝑽𝑩𝑰𝑨𝑺 + 𝟎. 𝟕𝑽, the diode will remain forward biased and

the output will be −𝑽𝑩𝑰𝑨𝑺 + 𝟎. 𝟕𝑽.



Figure 15 Diode Clampers

- When the voltage at Point A is less than −𝑉𝐵𝐼𝐴𝑆 + 0.7𝑉, the diode will become reverse biased

and the output will follow the input voltage based on the voltage divider.


2.4.2 Diode Clampers

- Diode clampers are used to add

a DC level to an AC voltage.

- The circuit in Figure 15(a & b)

add a positive DC level to the

AC Voltage while the circuit in

Figure 15(c) adds a negative

level.

Circuit Operation

- Taking the negative quarter of

the input signal, diode is

forward biased and charges the

capacitor to 𝑽𝒑(𝒊𝒏) − 𝟎. 𝟕𝑽.

- After that the diode is reverse

biased because the cathode of

the diode is at higher potential

than the signal voltage.

- This is because the capacitor

keeps its charge and

discharges very slowly through

RL.

- The slow discharge is due to

the large RC time constant

required for the circuit to work

properly.

- In this way the capacitor acts like a battery in series with the input voltage. This results in

adding the capacitor voltage to the input voltage and appears as DC level at the output.

Condition of Operation

- If the RC time constant is 100 times the period of the signal, the clamping is excellent.

- If the time constant is 10 times the period then the clamping is good.



Figure 16 Voltage Doubler

Figure 17 Voltage Tripler

2-5 VOLTAGE MULTIPLIERS - Voltage multiplier use clamping

action to increase the peak

rectified voltage.

2.5.1 Voltage Doubler

- Voltage doubler is a circuit with

multiplication factor of 2.

Circuit Operation Figure (16)

- During the positive half cycle of

the secondary voltage, diode D1

is forward and D2 is reverse

biased.

- Capacitor C1 is charged to

𝑽𝒑 − 𝟎. 𝟕𝑽.

- During the negative cycle, diode

D2 is forward and D1 is reverse

biased.

- As C1 cannot discharge, it acts

as a battery and adds to the secondary voltage to charge C2 to approximately 𝟐𝑽𝒑.

- In this way, the input voltage is doubled and output across C2 will give us twice the input voltage.

2.5.2 Voltage Tripler

- Adding another diode and

capacitor to the doubler creates a

voltage tripler.

Circuit Operation Figure (17)

- During the positive half cycle of

the secondary voltage, C1 charges

to 𝑽𝒑 through D1.

- During the negative cycle, C2

charges to 𝟐𝑽𝒑 through D2 as

described in the doubler.

- During the next positive cycle, C3

charges to 𝟐𝑽𝒑 through D3 as D1

and D2 are reverse biased.

- The tripler output is taken across C1 and C3.



Figure 18 Voltage Quadrupler

2.5.3 Voltage Quadrupler

- Adding another diode and

capacitor to the tripler produces

a voltage quadrupler.

Circuit Operation

- C4 is charged to 𝟐𝑽𝒑 through D4

on a negative cycle.

- The 4𝑉𝑝 output is taken across

C2 and C4.

chapter 2 diode applications · chapter 2 diode applications basic dc power supply - a dc power...

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