ZENER DIODES AND REGULATORS INEL 4201 Electronics I Textbook - - PowerPoint PPT Presentation

ZENER DIODES AND REGULATORS

Textbook section 4.4 INEL 4201 Electronics I

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Oxford University Publishing Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)

4.2. Terminal Characteristics


• f Junction Diodes

▪ Most common implementation of a diode utilizes pn junction. ▪ I-V curve consists of three characteristic regions ▪ forward bias: v > 0 ▪ reverse bias: v < 0 ▪ breakdown: v << 0 discontinuity caused by differences in scale

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Oxford University Publishing Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)

= − S i I

<< − S i I

−

=

/ T v V S

i I e

V = -VZK V = -VT V = 10VT

= −

/

( 1)

T

v V S

i I e

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• vD

+ iD VZ Zener diodes

• reverse-bias operation
• keeps an approx. constant voltage independent of

diode’s current

• Operation in this regime can destroy a regular

diode

4

R

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VPS

R

• VZ : Zener diode’s nominal voltage.
• iZ,min : minimum current through diode required for proper operation.
• iL,max, iL,min : maximum and minimum load’s current.
• VS,max, VS,max : maximum and minimum source voltage levels

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Voltage regulator VS R

Let:

• VZ : Zener diode’s nominal voltage.
• iZ,min : minimum current through diode required for proper operation.
• iL,max, iL,min : maximum and minimum load’s current.
• VS,max, VS,max : maximum and minimum source voltage levels
• Minimum zener current ≥ iZ, min when load current = iL, max
• VZ is specified at a specific current; use piece-wise linear

model to estimate the diode’s voltage at a different current

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=> rZ VZ + − VS R1 D1 RL vL +

• +

− VS R1 D1 RL vL +

• rZ

VZ => Piece-wise linear approx. iD vD iD vD +

• VZ

slope = 1/rZ

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• PROB. 4.61
• Design a regulator with output voltage VL = 7.5V
• VZ = 7.5V at IZ = 12mA
• Iknee (minimum) current is 0.5mA
• incremental resistance rz = 30Ω
• Power Supply: VPS = 10V
• Load: RL = 1.2kΩ

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• PROB. 4.61 DESIGN

IL = 7.5V 1.2kΩ = 6.25mA IR = IL + IZ = 12mA + 6.25mA = 18.25mA R = VP S VL IR = 10V 7.5V 18.25mA 137Ω

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• 10V

139.5 RL 1200 30 7.14V IZ IL IR

What's the output voltage if the supply is 10% high? Extrapolate to find VZ0 VZ0 = 7.5V − 30Ω × 12mA = 7.14V

10V 139.5 RL 1200 10V

• 137Ω

10V + ΔVPS

• 10V

139.5 RL 1200 30 7.14V IZ IL IR

137Ω

ΔVPS ΔvO = ΔVPS 30Ω∥1200Ω 30Ω∥1200Ω + 137Ω = 0.176V

+

• 137Ω

10V 139.5 RL 1200 10V 139.5 RL 1200 30 7.14V IZ IL IR

What’s the output voltage if the supply is 10% high? VZ = IZ 30Ω + VZ0 VZ0 = 7.5V 12mA 30Ω = 7.14V VP S VL R = VL 7.14V 30Ω + VL RL VL = (R30ΩRL) VP S R + 7.14V 30Ω

• For R = 137Ω, VP S = 10V and RL = 1200Ω,

VL = (24.116Ω) 10V 137Ω + 7.14V 30Ω

• = 7.5V

as expected. For VP S = 11V , VL = (24.116Ω) 11V 137Ω + 7.14V 30Ω

• = 7.676V

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137Ω 137Ω

Another way

What's the output voltage when both the supply is 10% high and the load is removed?

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What's the smallest load resistor that can be used while the zener

• perates at a current no lower than the knee current while the

supply is 10% low? What's the load voltage in this case?

Exercise 4.16: A zener diode whose nominal voltage is 10 V at 10 mA has an incremental resistance of 50 Ω. What voltage do you expect if the diode current is halved? Doubled? What is the value of VZ0 in the zener model?

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–

Exercise 4.17

A zener diode exhibit a constant voltage of 5.6V for current greater than five times the knee

• current. IZK is specified to be 1mA. The zener is to be used in the design of a shunt regulator fed

from a 15-V supply. The load current varies over the range of 0mA to 15mA. Find a suitable value for the resistor R. What is the maximum power dissipation of the zener diode?