Chapter 27

Diode Theory and Application

Diode Models

Ideal model is a switch

Forward-biased ideal model

Short Circuit

V_D = 0

R_D = 0

Diode Models

Reverse-biased ideal model

Open Circuit

V_D = Supply Voltage

R_D = ∞

Diode Models

Characteristic curve shows Current vs Voltage

1^st approximation

Ideal Switch

Diode Models

Circuit model – Ideal Diode

Diode Models

2^nd approximation

Ideal Switch

Barrier potential, V_B

Si ≈ 0.7 volts

Ge ≈ 0.3 volts

Diode Models

Circuit Model

Ideal Diode

V_B

Diode Models

3^rd approximation

Barrier potential, V_B

V_B

Internal Resistance

Diode Models

Circuit Model

Ideal Diode

V_B

R_D

Diode Characteristic Curve

Regions of a real diode curve

Forward Region

Conduction

Knee

High resistance

Diode Characteristic Curve

Reverse Region

High resistance blocking

Diode breakdown

Diode Characteristic Curve

Real Diode I/V curve

Diode Characteristic Curve

Forward region

Conduction region

Dynamic resistance is

Diode Characteristic Curve

Reverse region

Small reverse voltages yield very small currents (uA)

Reverse breakdown voltage, V_R(BR)

Peak Inverse Voltage (PIV), Peak Reverse Voltage (PRV) or V_R(max)

Diode Data Sheets

Manufacturer specifications (specs)

Describe product electrical characteristics

Recommended operating conditions

Maximum ratings (PIV, power = I²R)

ac

dc

Diode Data Sheets

Part number with 1N prefix

Diode Data Sheets

Two sections

Maximum ratings: limits that must not be exceeded

Electrical Characteristics: typical and max values during operation

Forward voltage drop

Reverse voltage

Diode Data Sheets

Always need a safety margin

At least 20% more I or V than your circuit

e.g., if PIV expected in your circuit is 150 V, choose diode with PIV of >180 V

Diode Data Sheets

Many manufacturers

Many diode types (e.g. bridge, high-speed switching, small signal, Varactor, Zener)

Data sheets on Internet

Diode Data Sheets

Example

Diode Data Sheets

Reverse Voltage

dc

Repetitive sinusoidal

Full-wave

Half-wave

Forward Current

Average

Surge

Diode Data Sheets

Maximum Instantaneous Forward Voltage Drop, v_F

Maximum Full-Cycle Average Voltage Drop, V_F(AV)

Temperature Derating

I²R in diode generates heat

Derating curve

Diode Data Sheets

Parameter Shifts

Temperature increase Forward Region

Generates more e^- - hole pairs

Changes Barrier Potential, V_B

V_B decreases ≈ 2.5mV per 1° C increase

Diode Data Sheets

Temperature increase Reverse Region

More minority carriers

Reverse current, I_S ≈ doubles per 10° C increase

Diode Data Sheets

Reverse Recovery Time

Switching time

From On to Off state

t_rr

Nanoseconds for switching diodes

Microseconds for rectifier diodes

The Zener Diode

Special purpose diode

Operates in reverse-bias region

Breakover voltage called Zener Voltage, V_Z

The Zener Diode

V_Z is close to constant

From knee current, I_ZK

To maximum rated current, I_ZM

V_Z is set by amount of doping used

The Zener Diode

Symbol:

Available with ~2.4 V < V_Z < ~200 V

Forward direction – like a standard diode

Reverse direction – sharp breakdown region

The Zener Diode

Characteristic curve

The Zener Diode

Zener Specification Sheet

Zener test current, I_ZT

Nominal Zener Voltage, V_Z (measured at I_ZT)

Maximum Zener current, I_ZM

Knee current, I_ZK

The Zener Diode

Zener impedance, Z_Z @ I_ZT

Dynamic Z =

2 – 45 Ω

Almost constant in operating region

The Zener Diode

Power Rating

Maximum dc power dissipation, P_Dmax

P_Dmax = V_Z * I_ZM watts

.25 W < P_Dmax < 50 W

Power Derating

Factor such as 6.67 mW per °C

Graph

The Zener Diode

Modeling

Ideal

2^nd approximation

Open circuit if I_Z < I_ZT

The Zener Diode

Applications

Use ideal model

Commercial Tolerance, ±5% to 10% for V_Z

Design

Determine limits imposed by I_ZK and I_ZM

Design circuit well within these limits

The Zener Diode

Voltage regulator

The Zener Diode

Current divider between Zener and Load

I_ZK < I_Z < I_ZM

Input regulation

Limits input voltage: E_min < E_in < E_max

Load regulation

Limits load resistance: R_Lmin < R_L < R_Lmax

The Zener Diode

Clippers

Limit amplitude of input ac waveform

Single sided

Dual sided

Clampers

If V_in ≥ V_Z then V_out = V_Z

The Zener Diode

Transient suppression

Greater power capability

Use Back-to-Back Zeners

The Varactor Diode

Also called varicap, epicap, or tuning diode

Voltage variable capacitor

Symbols:

or

The Varactor Diode

Nonlinear V vs C curve

Increase reverse Voltage decrease C

Reverse biased

Increase voltage decreases diode junction

Capacitance inversely proportional to distance between plates

The Varactor Diode

Normal diode operation when forward biased

The Varactor Diode

Specs

Nominal capacitance, C_T (given at a specific voltage)

Reverse breakdown voltage

Temperature coefficient

The Varactor Diode

Specs

Figure of Merit, Q

Capacitance ratio (tuning ratio)

C_R e.g. if 5 pF < C < 30 pF, C_R = 6 (30 pF/5 pF = 6)

Half-Wave and Full-Wave
Rectifier Circuits

Half-Wave rectification

Sine Wave input with no dc component

Single diode

Half-Wave and Full-Wave
Rectifier Circuits

Half-Wave output

Upper ½ of sine wave

Diode in forward direction

Lower ½ of sine wave

Diode in reverse direction

dc value = .318 V_m (not counting V_B)

PIV ≈ 2 * E_m

Half-Wave and Full-Wave
Rectifier Circuits

Full-Wave Bridge

Half-Wave and Full-Wave
Rectifier Circuits

Full-Wave rectification

Sine Wave input with no dc component

Center-tap transformer with two diodes

Full-Wave Bridge with four diodes

Half-Wave and Full-Wave
Rectifier Circuits

Full-Wave output

Upper ½ of sine wave and inverted lower half of sine wave

dc value = .637 V_m (not counting V_B)

PIV ≈ E_m

Bridge rectifier package (4 matched diodes)

Power Supply Filtering

Parallel RC circuit with half-wave rectified input

Capacitor charges during first ¼ cycle

Capacitor holds during rest of cycle

Power Supply Filtering

Output

Less ripple

Closer to steady dc

Larger capacitor yields less ripple

Power Supply Filtering

Parallel RC circuit with full-wave input

T = Period of Sinusoid

R_L = Load resistance

C = Filter capacitance

Power Supply Filtering

Ripple

Expressed in rms volts

Ripple factor

Power Supply Filtering

Diode Forward Current

Repetitive surge currents

Maximum listed on many rectifier data sheets

Unregulated power supplies

Output dc voltage varies with input voltage

Regulated power supplies

Simplest regulator is a Zener diode


	Ideal model is a switch
	Forward-biased ideal model
		Short Circuit
		V_D = 0
		R_D = 0


	Reverse-biased ideal model
		Open Circuit
		V_D = Supply Voltage
		R_D = ∞


	Characteristic curve shows Current vs Voltage
	1^st approximation
		Ideal Switch


	2^nd approximation
		Ideal Switch
		Barrier potential, V_B
		Si ≈ 0.7 volts
		Ge ≈ 0.3 volts


	3^rd approximation
		Barrier potential, V_B
		V_B
		Internal Resistance


	Regions of a real diode curve
	Forward Region
		Conduction
		Knee
		High resistance


	Reverse region
		Small reverse voltages yield very small currents (uA)
		Reverse breakdown voltage, V_R(BR)
		Peak Inverse Voltage (PIV), Peak Reverse Voltage (PRV) or V_R(max)


	Manufacturer specifications (specs)
	Describe product electrical characteristics
		Recommended operating conditions
		Maximum ratings (PIV, power = I²R)
		ac
		dc


Two sections
	Maximum ratings: limits that must not be exceeded
	Electrical Characteristics: typical and max values during operation
		Forward voltage drop
		Reverse voltage


	Always need a safety margin
		At least 20% more I or V than your circuit
		e.g., if PIV expected in your circuit is 150 V, choose diode with PIV of >180 V


	Many manufacturers
	Many diode types (e.g. bridge, high-speed switching, small signal, Varactor, Zener)
	Data sheets on Internet


Reverse Voltage
	dc
	Repetitive sinusoidal
		Full-wave
		Half-wave
Forward Current
	Average
	Surge


	Maximum Instantaneous Forward Voltage Drop, v_F
	Maximum Full-Cycle Average Voltage Drop, V_F(AV)
	Temperature Derating
		I²R in diode generates heat
		Derating curve


	Parameter Shifts
	Temperature increase Forward Region
		Generates more e^- - hole pairs
		Changes Barrier Potential, V_B
		V_B decreases ≈ 2.5mV per 1° C increase


	Temperature increase Reverse Region
		More minority carriers
		Reverse current, I_S ≈ doubles per 10° C increase


	Reverse Recovery Time
		Switching time
		From On to Off state
		t_rr
		Nanoseconds for switching diodes
		Microseconds for rectifier diodes


	Special purpose diode
	Operates in reverse-bias region
	Breakover voltage called Zener Voltage, V_Z


	V_Z is close to constant
		From knee current, I_ZK
		To maximum rated current, I_ZM
	V_Z is set by amount of doping used


	Symbol:


	Available with ~2.4 V < V_Z < ~200 V
	Forward direction – like a standard diode
	Reverse direction – sharp breakdown region


	Zener Specification Sheet
		Zener test current, I_ZT
		Nominal Zener Voltage, V_Z (measured at I_ZT)
		Maximum Zener current, I_ZM
		Knee current, I_ZK


	Zener impedance, Z_Z @ I_ZT
		Dynamic Z =
		2 – 45 Ω
		Almost constant in operating region


	Power Rating
		Maximum dc power dissipation, P_Dmax
		P_Dmax = V_Z * I_ZM watts
		.25 W < P_Dmax < 50 W
	Power Derating
		Factor such as 6.67 mW per °C
		Graph


	Applications
		Use ideal model
		Commercial Tolerance, ±5% to 10% for V_Z
	Design
		Determine limits imposed by I_ZK and I_ZM
		Design circuit well within these limits


	Current divider between Zener and Load
	I_ZK < I_Z < I_ZM
	Input regulation
		Limits input voltage: E_min < E_in < E_max
	Load regulation
		Limits load resistance: R_Lmin < R_L < R_Lmax


Clippers
	Limit amplitude of input ac waveform
		Single sided
		Dual sided
Clampers
	If V_in ≥ V_Z then V_out = V_Z


	Transient suppression
		Greater power capability
		Use Back-to-Back Zeners


	Also called varicap, epicap, or tuning diode
	Voltage variable capacitor
	Symbols:
	or


	Nonlinear V vs C curve
	Increase reverse Voltage decrease C
		Reverse biased
		Increase voltage decreases diode junction
		Capacitance inversely proportional to distance between plates


	Specs
		Nominal capacitance, C_T (given at a specific voltage)
		Reverse breakdown voltage
		Temperature coefficient


	Specs
		Figure of Merit, Q
		Capacitance ratio (tuning ratio)
		C_R e.g. if 5 pF < C < 30 pF, C_R = 6 (30 pF/5 pF = 6)