ZENER DIODES AND REGULATORS

Textbook section 4.4

INEL 4201 Electronics I

1

Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith

(0195323033)

4.2. Terminal Characteristics

of 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

!2

Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith

(0195323033)

= − Si I<< − Si I −= / Tv VSi I e

V =

-VZK

V =

-VT

V =

10V T

= −/( 1)Tv VSi I e

!3

-

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

5

VPS

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

For proper operation, select R so that

R VS,min � VZ

iZ,min + iL,max

Maximum power dissipated in the resistor:

PR,max =(VS,max � VZ)2

R

Maximum power dissipated in the diode:

PD,max = VZ ⇥✓VS,max � VZ

R� iL,min

◆

D and R must have a power rating greater than PD,max and PR,max, respec-tively.

6

Voltage regulatorVS 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

For proper operation, select R so that

R VS,min � VZ

iZ,min + iL,max

Maximum power dissipated in the resistor:

PR,max =(VS,max � VZ)2

R

Maximum power dissipated in the diode:

PD,max = VZ ⇥✓VS,max � VZ

R� iL,min

◆

D and R must have a power rating greater than PD,max and PR,max, respec-tively.

• 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

7

8

=>

rZ

VZ

+−VS

R1

D1 RLvL

+

-

+−VS

R1

D1RL vL

+

-

rZ

VZ

=>

Piece-wise linear approx.iD

vD

iD vD

+

-

VZ

slope = 1/rZ

9

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

10

PROB. 4.61 DESIGN

IL =7.5V

1.2k�= 6.25mA

IR = IL + IZ = 12mA + 6.25mA = 18.25mA

R =VPS � VL

IR=

10V � 7.5V

18.25mA� 137�

11

10V

139.51

RL12001 10V

139.51

RL12001

301

7.14V

IZIL

IR

What's the output voltage if the supply is 10% high?

Extrapolate to find VZ0

VZ0 = 7.5V − 30Ω × 12mA = 7.14V

10V

139.51

RL12001 10V

139.51

RL12001

301

7.14V

IZIL

IR

137Ω

10V + ΔVPS

10V

139.51

RL12001 10V

139.51

RL12001

301

7.14V

IZIL

IR

137Ω

ΔVPS ΔvO = ΔVPS30Ω∥1200Ω

30Ω∥1200Ω + 137Ω= 0.176V

+

-

137Ω

10V

139.51

RL12001 10V

139.51

RL12001

301

7.14V

IZIL

IR

What’s the output voltage if the supply is 10% high?

VZ = IZ � 30� + VZ0 � VZ0 = 7.5V � 12mA � 30� = 7.14VVPS � VL

R=

VL � 7.14V

30�+

VL

RL

VL = (R�30��RL)

�VPS

R+

7.14V

30�

�

For R = 137�, VPS = 10V and RL = 1200�,

VL = (24.116�)

�10V

137�+

7.14V

30�

�= 7.5V

as expected. For VPS = 11V ,

VL = (24.116�)

�11V

137�+

7.14V

30�

�= 7.676V

13

137Ω137ΩAnother way

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

15

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

16

INEL 4201 – Diodes 9/14/2015

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?