Chapter 2 Diodes

Chapter 2 Diodes

2.1 Introduction to PN-junction diode 2.2 The ideal diode

2.3 The practical diode model

2.1 Introduction to the PN-Junction Diode

Diode :

A two-terminal device that acts as a one-way conductor When forward biased, diode conduct.

when reverse biased, diode conduction drops to nearly zero PN-junction diode schematic symbol

forward

reverse

The diode conducts fully when the forward voltage (V_F) across the component is approximately 0.7 V for a Si diode and 0.3 V for a Ge diode ~ due to the potential barrier.

Forward biased diode Reverse biased diode Forward & reverse biased diode:

2.1.1 Diode Models

Ideal model: the diode as a simple switch that is either closed (conducting) or open (non-conducting).

When reverse biased (open switch) -The diode has infinite resistance.

-There is no current flow across the diode.

-The voltage drop occurs through the diode.

When forward biased (closed switch) -The diode has no resistance.

-Infinite current flows through the diode.

-There is no voltage drop across the

diode. Characteristics of the ideal diode.

EXAMPLE 2.1

Determine the values of V_D1, I_T, and V_R1 for the circuit shown in Figure 2.5.

The arrow in the schematic symbol is pointing toward the positive terminal of the source, so we know that the diode is reverse biased. Therefore:

a. Total voltage drops across D1 ~ V_D1 = V_S = 5 V a. There is no current flow. Therefore, I_T = 0 A

b. Since there is no current through R₁, there is no voltage drop across the resistor ~ V_R1 = 0 V

EXAMPLE 2.2

Determine the values of V_D1, V_R1, and I_T for the circuit shown in Fig. 2.6.

The arrow in the schematic symbol is pointing toward the negative terminal of the source, so we know that the diode is forward biased. Therefore:

a. V_D1 = 0 V, leaving the total applied voltage across R₁ b. V_R1 = V_S = 5 V

c. I_T is determined by the source voltage and R₁. By formula.

1

5 5

1

R T

V m

I A

R k

V  

 

Summary of Ideal Diode

When Do We Use Ideal Diode Model?

Normally, the ideal model of the diode is used in the initial stages of circuit troubleshooting.

When troubleshooting most diode circuits, your initial concern is only whether a given diode is acting as a one-way conductor or not. If it is, the component is assumed to be good. If it is not, the component is faulty and must be replaced.

2.3 The Practical Diode Model

Knee voltage (V_K) ~ the voltage at which the device current suddenly increases under the forward bias voltage (V_F)

• Diode current remains at zero until the knee voltage is reached

• Once the applied voltage reaches the value of V_K, the diode is turned on, and forward current flows

• When the diode is conducting, the value of V_F is approximately equal to V_K

The Effect of V_F on Circuit Analysis

1

1

0.7 5 0.7 4.3

0.7 5 0.7

1 4.3

R S

S T

V V V V V V

V V V V

I mA

R k

    

 

  



Ex. Predict the voltmeter reading in Fig. 2.10.

Ex. Determine the total current in the circuit

Determine the value of I_T for the circuit shown in Fig. 2.11

1

0.7 5 0.7

3.4 1.26

S T

V V V V

I mA

R k

 

  



Determine the value of I_T for circuit shown in Fig. 2.12.

With two diodes in the circuit, the total value of V_F is 1.4V.

Using this value instead of 0.7V, we can determine I_T as follows.

1

1.4 4 1.4

5.1 0.51

S T

V V V V

I mA

R k

 

  



Determine the value of I_T for the circuit shown in Fig.

2.13 using the ideal diode model. Then recalculate the value using the practical diode model. What is the

percentage of error introduced by using the ideal diode model?

EXAMPLE 2.7

Since the ideal diode model assumes that V_F = 0 V, total applied voltage is dropped across the two resistors. Thus I_T is found as

10 3.03

3.3

S T

T

V V

I mA

R k

  



The practical diode model assumes that V_F = 0.7 V for each diode. Therefore, the value of I_T is found to be

1.4 10 1.4 3.3 2.61

S T

T

V V V V

I mA

R k

 

  



The percentage of error between the two calculations is given by

One Final Note

It isn’t always necessary to include the diode forward voltage or knee voltage for the circuit calculations.

For example, if a forward-biased diode and a resistor are in series with a 100V source, ignoring the 0.7V across the diode causes an error of less than 1% in the circuit calculations (which is well within acceptable limits).

While this argument is valid, we are always interested in getting the most accurate results possible (within reason), so we always use the practical diode model in our circuit calculations.

2.4 Other practical considerations peak reverse voltage

average forward current forward power dissipation 2.5 The complete diode model 2.6 Diode specification sheets

Chap. 2 Diodes

2.4 Other practical considerations

When a diode in the circuit is faulty, you may troubleshoot to replace it.

Resistance check

Diode test

Common types of diodes

Q. Which electrode is anode (or cathode) ?

2.4 Other practical considerations

Several diode characteristics must be considered when determining whether a specific diode can be used in a given circuit or not.

These characteristics are :

- Peak reverse voltage ^{(피크 역전압)} (V_RRM)

- Average forward current (순방향 직류 전류) (I₀) - Forward power dissipation (순방향 소비 전력)

When a diode in the circuit is faulty, you may troubleshoot to replace it.

2.4.1 Peak Reverse Voltage (V_RRM)

Peak Reverse Voltage (V_RRM): the maximum reverse voltage before the breakdown of the pn diode ~ repetitive reverse maximum voltage

When V_RRM is exceeded, the depletion layer may break down and allow the diode to conduct in the reverse direction.

When pn-junction diode is forced to conduct in the reverse direction, the device is destroyed.

Avalanche current: ^{사태 전류}

The current that flows when V_RRM is exceeded.

One electron bumps other electron to make it free from its covalent bond

-> Another electrons are bumped to be free from the covalent bond successively -> Avalanche current generates sufficient heat to destroy a pn junction diode

V_RRM as a parameter of diode

The component’s “V_RRM rating” should be greater than the maximum reverse voltage in the circuit ~ V_RRM 정격

V_RRM rating for D1 must be greater than 50 V to ensure that the component isn’t damaged by the voltage source

In practice, the diode should have a V_RRM that is at least 20% greater than the maximum voltage in the circuit ~ safety margin of 20%

( )

1.2 1.2 50 60 at minimum

RRM R pk pk

V  V   V  V

2.4.2 Average Forward Current (I₀)

Average forward current (I₀): the maximum allowable value of DC forward current for a diode.

If dc current exceed I₀, the diode may be destroyed by excessive heat. I₀ should be at least 20% greater than the value of forward current.

Ex. 2.9 Determine the minimum average forward current rating that would be required for the diode in the figure.

0

0.7 50 0.7

246.5 200

Considering 20% safety margin, 1.2 246.5 295.8

S F

L

V V V V

I mA

R

I mA mA

 

  



  

2.4.3 Forward Power Dissipation (P_D(max))

Forward Power Dissipation (P_D(max)) : the maximum power dissipation of the diode in forward biased circuit. (^{순방향 소비전력})

Power dissipated by a component : P = I V I: current , V: voltage across the device

Ex. 2.10 Calculate the minimum forward power dissipation rating.

(max)

Total circuit current, 10 0.7

100 93

Corresponding power dissipation, (93 )(0.7 ) 65.1

Considering a safety margin of 20%, 1.2 (1.2)(65.1 ) 78.1

F

F

F

F F F

D F

I

V V

I mA

P

P I V mA V mW

P P mW mW

  



  

  

How to get I₀ from P_D(max)

Average forward current(I₀) from Forward power dissipation (P_D(max))

(max) 0

D F

I P

 V

Ex. 2.11 A diode has a forward power dissipation rating of 500mW.

What is the maximum allowed value of forward current for the device?

(max) 0

0

(max) 0

500 714.29 ~ rating value 0.7

Considering a safety margin, is normally restricted to 80% of . 0.8 (0.8)(714.29 ) 571.43

D F

F F

P mW

I mA

V V

I I

I I mA mA

  

  

2.4.4 Summary

When you are trying to replace one diode with another…..

1. Whether the peak reverse voltage (V_RRM) rating of the replacement diode is at least 20% larger than the maximum reverse voltage?

2. Whether the average forward current (I₀) rating of the replacement diode is at least 20% bigger than the average value of I_F?

3. Whether the forward power dissipation (P_D(max)) rating of the replacement diode is at least 20% greater than the value of P_F in the circuit?

2.5 The complete diode model

Inclusion of two parameters such as the bulk resistance and reverse current enables us to describe most accurately a real operating characteristics of the diode

2.5.1 Bulk resistance (R_B)

The natural resistance of the p- or n-type bulk materials in a p-n diode

~ the effect of R_B on diode operation can be seen in the forward bias region.

Equivalent circuit of diode considering R_B

V_B: the voltage required to overcome the barrier potential of the component

R_B: the bulk resistance of the component Diode current (I_F) develops a voltage drop of I_FR_B

V_F varies linearly with I_F for V_F > V_K

F B F B K F B

V  V  I R  V  I R

Current flow due to a reduction of V_B

Linear slope due to R_B 2.5.1 Bulk resistance (R_B)

p

n

Reverse bias

2.5.2 Effect of bulk resistance on circuit measurement

Low-current circuit: little voltage drop on bulk resistance V_F ~ 0.7 V

High-current circuit: large voltage drop on bulk resistance V_F ~ 1.1 V > 0.7 V

2.5.3 Reverse Current (I_R)

When a diode is reverse biased, the depletion layer reaches its maximum width, and the conduction through the diode is prohibited.

However, there exists a very small amount of current flowing through the reverse-biased diode.

R S SL

I  I  I

I_S : the reverse saturation current I_SL : the surface-leakage current

I_S is generated by thermal activation, increasing with temperature, but independent of the reverse voltage.

I_SL is generated along the surface of the diode, varying directly with the reverse voltage. Much smaller than I_S.

2.5.5 Diode capacitance

An insulator placed between two closely spaced conductor forms a capacitor.

For a reverse biased case, a diode contains a depletion layer (insulator)

between two semiconductor materials, which are conducting in comparison.

Usually the diode capacitance is negligible.

In a high-frequency circuit, however, the diode capacitance becomes important.

2.5.6 Temperature effects on diode

When temperature increases,

I_F increases at a fixed value of V_F and V_F decrease at a fixed value of I_F.

~ due to an increase of thermal activation and an effective decrease of V_B Magnitude of I_R (or I_S) also increases due to the same reason.

2.5.7 Summary

1. When the forward bias voltage reaches the barrier potential (V_K), the diode starts to conduct. The value of V_F is approximated to be V_K.

2. As V_F decreases below V_K, the diode is turned off and acts like an open switch. When V_R > V_RRM, the diode may be broken.

3. The forward current through a diode is limited by the applied voltage and the resistance connected in series.

Chap. 2 Diodes

2.7 Zener Diodes

2.8 Zener Diode Specification Sheets 2.9 Light Emitting Diode (LED)

2.7 Zener diode

Zener diode is designed to work in the reverse breakdown region When the reverse breakdown voltage is reached

1. the current increases drastically

2. the reverse voltage across the diode (V_R) remains constant relatively

2.7 Zener diode as a voltage regulator ^{(전압 조정기)}

Relatively low impedance of the diode keeps the voltage across the diode to be V_Z , while resistor R is used to limit current through the circuit.

.

source out

diode out source diode

V V

I V V I R const

R

     

The breakdown voltage of diode D is stable over a wide current range and holds V_OUT relatively constant even though the input voltage, V_source, may fluctuate over a fairly wide range.

Voltage regulator : a circuit designed to maintain a constant voltage without

respect to the load current or input voltage

2.7 Zener diode as a voltage regulator ^{(전압 조정기)} Voltage regulator :

a circuit designed to maintain a constant voltage without respect to the load current or input voltage

Zener voltage (V_Z) :

the approximate voltage across a zener diode when it is operated near the

reverse breakdown

Zener breakdown :

a type of reverse breakdown that occurs at relatively low reverse voltage

Note that V_Z is much lower than the

avalanche breakdown voltage, because of the very thin depletion layer caused by highly doped p- and n-type materials

I_ZK : the minimum current required to maintain constant voltage

I_ZT : the value of Zener current at which the nominal values of the component are measured I_ZM : the maximum amount of current the diode can handle without being damaged

2.7.2 Zener operating characteristics

Zener impedance (Z_Z) :

Zener diode’s opposition to a change in current

Example : 2mA variation in I_Z and 56mV variation in V_Z

2.7.3 Zener equivalent circuit

(cf) Diode equivalent circuit in forward bias

2.8 Zener Diode Specification Sheets (1)

Maximum steady state power dissipation rating (P_D(max)) :

The maximum allowable average power dissipation for a Zener diode that is operating in reverse breakdown (최대 정적 상태 소비전력)

Example 2.13

A 1N754A zener diode has a dc power dissipation rating of 500 mW and a nominal zener voltage of 6.8 V. What is the value of I_ZM for the device?

Remember, the value of I_ZM is important because it determines the maximum current where the diode can tolerate. If the average current through this

diode exceeds 73.5 mA, you’ll end up replacing the diode.

2.8 Zener Diode Specification Sheets (2) Power derating factor :

The rate at which the maximum power rating (P_D(max)) decreases per 1 ^oC rise above a specified temperature (T)

Example 2.14

The power derating curve in Figure 2.33 shows that the 1N4370A has a maximum power dissipation rating of 400 mW at T = 100 ^oC. Find out the component’s derating factor and P_D rating using the curve.

(max) 500

PD  mW

LED is a diode that emit light in a forward biased circuit 2.9 Light-emitting diode (LED)

The color emitted by a LED depends on the band gap energy of the semiconductor material consisting of it

Blue LED

Anode (+)

Cathode (-) 2.9 Light-emitting diode (LED)

Charge carriers of electrons and

holes flow into the pn junction from electrodes with different voltages.

When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon ~ “radiative recombination”

The wavelength of the light emitted, and thus its color depends on the band gap energy of the materials forming the p-n junction.

In silicon or germanium diodes, the electrons and holes recombine by a non-

radiative transition, which produces no optical emission, because these are indirect band gap materials.

The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light.

2.9 Light-emitting diode (LED)

2.9.2 Current limiting resistor

V_out(pk) = the maximum output voltage

V_F = minimum forward bias voltage of LED I_F = forward operation current of LED

Example 2.17

When the driving circuit has V_out(pk) = 8 V and the LED has the maximum

forward current of I = 16 mA with V_F ranging from 1.8 V to 2.0 V. Find out the current limiting resistor.

( )

0.8 16 12.8

8 1.8 12.8 484

F

out pk F

S

F

I mA mA

V V V V

R I mA

  

 

   

2.9.3 Multicolor LED

Only one LED is emitting the light at a time

Oscillating voltage in a driving

circuit generates a new color of light as a combination of two different LED’s colors, such as yellow light as a combination of red and green