Diodes

CENT-112 Fundamentals of Electricity and Electronics1

Impurity Atoms:•Trivalent: Boron (B), Aluminum (Al), Gallium (Ga), Indium (ln). Has three (3) valence electrons.

–Known as an “Acceptor Impurity.”•Pentavalent: Phosphorous (P), Arsenic (As), Antimony (Sb), and Bismuth (Bi). Has five (5) valence electrons.

–Known as a “Donor Impurity.”

Diodes


–“N - Type” Material:•Pure base material doped with a Donor Impurity.•Majority Current Carrier: Electrons•Minority Current Carrier: Holes

–“P - Type” Material:•Pure base material doped with an Acceptor Impurity.•Majority Current Carrier: Holes•Minority Current Carrier: Electrons

PN Material


–Old Method: Grown Crystals.

–Newer Methods:

•Alloy Fused: N & P material made using heat / pressure.

•Diffused: N & P gas and heat.

–Both methods are used to produce a “PN” Junction.

Construction


Questions

Q) What is meant by a donor impurity?

A) 5 valiant electrons in outer shell.

Q) What are 4 examples of a donor impurity?

A) Phosphorous, Arsenic, Antimony and Bismuth.


• Potential Hill (Junction Barrier) : Electrostatic field set up across a PN junction which prevents further combination of majority current carriers.

• The value of the voltage of the potential hill depends on the type of base material used during diode construction.

1. Silicon (.5 - .8V)

2. Germanium (.2V)• Rated for up to 1500A / 3000V.• Used primarily in Rectifiers.

Diode Definitions


Operations & Definitions

•Forward Bias: External voltage applied which opposes the potential hill, effectively reducing the width and resistance of the depletion region. => Majority Current Carriers flow through the PN junction.

•Reverse Bias: External voltage applied which aids the potential hill, effectively increasing the width and resistance of the depletion region. => No Majority Current Carriers flow through the PN junction.


Rectifier Diode Block Diagram

+ + +

+ P +

+ + +

Anode Cathode

Potential Hill (Junction Barrier)

Depletion Region

- - -

- N -

- - -

- -

- -

- -

- -

+ +

+ +

+ +

+ +


Rectifier Diode Schematic Diagram

Anode Cathode


Diode Forward Bias

+ + +

+ P +

+ + +

Anode Cathode


Depletion Region

- - -

- N -

- - -

-

-

-

-

+

+

+

+ + -


Diode Reverse Bias

+ + +

+ P +

+ + +

Anode Cathode


Depletion Region

- - -

- N -

- - -

- - -

- - -

- - -

- - -

+ + +

+ + +

+ + +

+ + + +-


Characteristic Curve

+I (mA)

Forward Bias

Reverse Bias

-I (uA)Avalanche Breakdown

+V a -c-V a -c


Zener Diode

–The Zener diode is a heavily doped diode which, as a result of doping, has a very narrow depletion region. This allows the diode to be operated in the reverse biased region of the characteristic curve without damaging the PN junction.

–“Zener Effect”: The area of Zener diode operation (<5V) where the Diode maintains a constant voltage output while operating reverse biased.

–“Avalanche Effect”: >5V applied to the diode while reverse biased which tends to cause the diode to eventually breakdown due to heat generation within the lattice structure of the crystal.


Zener Diode Schematic Symbol

Anode Cathode


Characteristic Curve

Operating Region

Reverse Bias

Forward Bias

+ V a - c- V a - c

I (mA)

I (uA)


Zener Operation

•Ratings: .25V to 1500V

•Used in SSMG / SSTG AC voltage regulator for the reference circuit.

When a higher constant voltage is desired, the zener diodes will be “Stacked” together in series and their voltages will add together to make the higher desired voltage.

This is the case in the SSMG / SSTG AC voltage regulators where four (4) 6v zener diodes are stacked to provide a 24V reference to the comparison circuit.


Zener Diode Voltage Regulator

R1

CR1

Vin Vout


Signal Diode

•Same construction as the Rectifier Diode except that it is designed to operate with a very short “reverse recovery time” to allow it to rectify high frequency AC inputs.


Power Supplies

•Components and their function–Transformer - Receives the AC input from the distribution system and either steps up or down the voltage.–Rectifier - Converts the AC input voltage from the transformer to a pulsating DC voltage.–Filter - Smoothes out the DC pulsations or ripple received from the rectifier.–Regulator - Receives a smoothed DC voltage from the Filter Stage and produces a steady DC voltage to be used by electronic circuitry.


Half - Wave Rectifier

VOUTVIN

1 : 1

T1

CR1

R1


• Positive half-cycle the diode is Forward Bias (FB), negative half-cycle the diode is Reverse Bias (RB).

Half - Wave Rectifier Operation

VDC = VPK X .318Where: VDC = Average DC voltage

VPK = Peak input voltage

.318 = Constant


Full - Wave Rectifier

VOUTVIN

1 : 1

T1

CR1

R1

CR2


• Positive half-cycle, 1 diode is FB, negative half-cycle the other diode is FB.

Full - Wave Rectifier Operation



.637 = Constant


Full – Wave Bridge Rectifier

VOUTVIN

1 : 1

T1

CR1

R1

CR2

CR3CR4


• Positive half-cycle, 1 diode is FB, negative half-cycle the other diode is FB.

Full - Wave Bridge Rectifier Operation



.637 = Constant


Filters

•A filter uses the characteristics of Inductors and Capacitors to smooth the pulsating DC waveform supplied by the Rectifier.

–Types•High Pass - A series RC filter whose output is taken from the resistor.

•Series / Parallel - A filter configuration which uses combinations of capacitors and inductors to smooth the voltage and current pulsations from the rectifier output.


• Rapid charge time constant for filter capacitors and inductors.

• Slow discharge time constant for filter capacitors and inductors.

Ideal filter characteristics


Capacitor Filter Configuration

RB

C1

VIN VOUT

•Capacitor Input Filter Schematic Diagram


Capacitor Filter Operation

•Charge RC time constant is developed from the internal resistance of the rectifier diodes and the capacitance of the filter capacitor. The net result is that the low resistance of the rectifier diodes develop a rapid charge RC time constant.

•Discharge RC time constant is developed from the filter capacitor and the load resistance. Since the load resistance is rather large, the discharge RC time constant is somewhat long.

•RB is called the “Bleeder Resistor” because it provides a path for the filter capacitor(s) to discharge when power is removed from the circuit. RB has a very large resistance and usually draws <10% of normal operating current.


LC Choke Filter Configuration

•LC Choke Filter Schematic Diagram

RB

C1

VIN VOUT

L1


LC Choke Filter Operation

•Charge RC time constant is developed from the internal resistance of the rectifier diodes, the Low DC resistance of the inductor (L1), and the capacitance of the filter capacitor. The net result is that the low resistance of the rectifier diodes and inductor (L1) develop a rapid charge RC time constant.

•Discharge RC time constant is developed from the filter capacitor and the load resistance. Since the load resistance is rather large, the discharge RC time constant is somewhat long.

•The Inductor acts to smooth out the current pulsations produced by the rectifier and / or transformer stage of the power supply.


RC PI Filter Configuration

•RC PI Filter Schematic Diagram

RB

C2

VIN VOUTC1

R1

VOUT(C1)

VOUT (C2)

Charge Path

Discharge Path


RC PI Filter Operation

•First Capacitor provides most of the filtering action.•Second Capacitor Provides additional voltage filtering.•Resistor limits current flow to the desired value and establishes the RC time constants for both filter capacitors.


LC PI Filter Configuration

•LC PI Filter Schematic Diagram

RB

C2

VINC1

L1

VOUT(C1)

VOUT (C2)

Charge Path

Discharge Path


LC PI Filter Operation

•First Capacitor provides most of the filtering action.•Second Capacitor Provides additional voltage filtering.

•Inductor opposes changes in current flow to reduce current spikes and establishes the RC time constants for both filter capacitors.


Voltage Regulators

R1

CR1

Vin Vout

–Series Regulator•Acts as a variable resistor in series with the load.

–Zener Diode Voltage Regulator•Schematic


Voltage Regulator Operation

R1CR1

Vin Vout

VIN

VOUT


Transistor Voltage Regulators

Vin Vout


OPAMP Voltage Regulators

Vin Vout

-+

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