L5 Zener Effect,Voltage Regulator,Half Wave Rectifier

8/11/2019 L5 Zener Effect,Voltage Regulator,Half Wave Rectifier

1/20

DC Analysis Representation of diode into three models

Ideal case model 1 with V= 0

Piecewise linear model 2 with Vhas a given value

Piecewise linear model 3 with Vand forward resistance, rf

Recall-Lecture 4


2/20

Diode AC equivalent model

During analysis, must perform DC analysis first tocalculate ID in order to obtain rd


3/20

DC ANALYSIS

DIODE = MODEL 1 ,2

OR 3

CALCULATE DC

CURRENT, ID

CALCULATE

rd

AC ANALYSIS

DIODE = RESISTOR,

rd

CALCULATE AC

CURRENT, id


4/20

Electronics ECE 1231

BREAKDOWN VOLTAGE

The breakdown voltage is a function of the doping concentrations

in the n- and p-region of the pn junction.

Large doping concentrations result in smaller break-down voltage.

Reverse biased voltage ET

The electric field may become large enough for the covalent bond

to break, causing electron-hole pairs to be created.

So, electrons from p-type are swept to n-region by the electric

field and holes from the n-type are swept to the p-region

The movement will create reverse biased current known as the

Zener Effect .


5/20


Zener Effect and Zener Diode

The applied reverse biased voltage cannot increase without limit since at

some point breakdown occurs causing current to increase rapidly.

The voltage at that point is known as the breakdown voltage, VZ

Diodes are fabricated with a specifically design breakdown voltage and are

designed to operate in the breakdown region are called Zener diodes. Circuit

symbol of the Zener diode:

Such a diode can be used as a constant-voltage

reference in a circuit.

The large current that may exist at breakdowncan cause heating effects and catastrophic failure

of the diode due to the large power dissipated in

the device.

Diodes can be operated in the breakdown region

by limiting the current to a value within the

capacities of the device.

NOTE:When a Zener diode is reverse-

biased, it acts at the breakdown region,

when it is forward biased, it acts like a

normal PN junction diode


6/20


Avalanche Effect

While these carriers crossing the space-charge

region, they also gain enough kinetic energy.

Hence, during collision with other atoms, covalent

bond is broken and more electron-holes pairs are

created, and they contribute to the collision process

as well. Refer to figure below

e

he

atom

e

h

atom

Electron with

high kineticenergy

e

h

atom


7/20

Chapter 3

Diode Circuits


8/20

VoltageRegulator


9/20

A voltage regulator supplies constant voltage to a load.

Voltage Regulator - Zener Diode


10/20

The breakdown voltage of a Zener

diode is nearly constant over a wide

range of reverse-bias currents.

This make the Zener diode useful ina voltage regulator, or a constant-

voltage reference circuit.

1. The zener diode holds

the voltage constant

regardless of the current

2. The load

resistor

sees a

constantvoltage

regardless

of the

current

3. The remainder

of VPSdrops

across Ri


11/20

A Zener diode is connected in a voltage regulator circuit. It is given that VPS

= 20V, the Zener

voltage, VZ= 10V, R

i= 222 and P

Z(max)= 400 mW.

a. Determine the values of IL, I

Zand IIif RL= 380 .

b. Determine the value of RLthat will establish PZ(max)= 400 mW in the diode.

Example


12/20

For proper function the circuit the following conditions must be satisfied the following

conditions.

1. The power dissipation in the Zener diode is less than the rated value

2. When the power supply is a minimum, VPS(min), there must be minimum current in

the zener diode IZ(min), hence the load current is a maximum, IL(max),

3. When the power supply is a maximum, VPS(max), the current in the diode is a

maximum, IZ(max), hence the load current is a minimum, IL(min)

AND Or, we can write


13/20

For general thumb of rule for design this circuit is, so from the last Equation

Maximum power dispassion in the Zener diode is

EXAMPLE 1 (Example 3.3 from textbook)

Design a Zener diode voltage regulator circuit as shown

in Figure . Consider voltage regulator is used to powerthe cell phone at 2.5 V from the lithium ion battery,

which voltage may vary between 3 and 3.6 V. The

current in the phone will vary 0 (off) to 100 mA(when

talking).

simple Zener diode voltage

regulator circuit


14/20

Solution:

The stabilized voltage VL= 2.5 V, so the Zener diode voltage must be VZ= 2.5 V. The

maximum Zener diode current is

The maximum power dispassion in the Zener diode is

The value of the current limiting resistance is


15/20

Example 2

Range of VPS: 10V14V

RL= 20100

VZ= 5.6VFind value of Riand calculate the maximum power rating of the diode


16/20

Rectifier


17/20

Rectifier Circuits

A dc power supply is required to bias all electronic circuits.

A diode rectifier forms the first stage of a dc power supply.

Diagram of an Electronic Power Supply

Rectificationis the process of converting an alternating(ac)

voltage into one that is limited to one polarity.

Rectification is classified as half-waveor full-waverectifier.


18/20

Rectifier Parameters

Relationship between the number of turns of a

step-down transformer and the input/output

voltages

The peak inverse voltage (PIV) of the diode is the peak value of the voltage

that a diode can withstand when it is reversed biased

Duty Cycle: The fraction of the wave cycleover which the diode is conducting.


19/20

i

vD

V

V

Equation of VOand current when diode is conducting

Vs< V, diode off, open circuit, no

current flow,Vo = 0V

Vs> V, diode conducts, current flows,

Vo = Vs

V

Vp


20/20

Vs< V, diode off, open circuit, no current flow,Vo = 0V

Vs> V, diode conducts,current flows and Vo = Vs V

Vs = Vpsin t

Notice that the

peak voltage of Vo

is lower

Vs >V

V

Vp

Documents

L5 Zener Effect,Voltage Regulator,Half Wave Rectifier