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

ZENER DIODES AND REGULATORS INEL 4201 Electronics I Textbook section 4.4 1

discontinuity caused by 4.2. Terminal Characteristics 
 differences in scale 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 � 2 Oxford University Publishing Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)

v V / i I e ( 1) = T − S i I i I v / T V i I e << − S = − S − = S V = -V ZK V = -V T 10 V T V = � 3 Oxford University Publishing Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)

V Z - i D v D + 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 R V PS • V Z : Zener diode’s nominal voltage. • i Z,min : minimum current through diode required for proper operation. • i L,max , i L,min : maximum and minimum load’s current. • V S,max , V S,max : maximum and minimum source voltage levels 5

6

Voltage regulator R V S Let: • V Z : Zener diode’s nominal voltage. • i Z,min : minimum current through diode required for proper operation. • i L,max , i L,min : maximum and minimum load’s current. • V S,max , V S,max : maximum and minimum source voltage levels • Minimum zener current ≥ i Z, min when load current = i L, max • V Z is specified at a specific current; use piece-wise linear model to estimate the diode’s voltage at a different current 7

Piece-wise linear approx. i D V Z v D + r Z i D v D => - slope = 1/r Z V Z R 1 R 1 r Z + + + R L v L + v L => V S D 1 − V S D 1 R L − - - V Z 8

9

PROB. 4.61 • Design a regulator with output voltage V L = 7.5V • V Z = 7.5V at I Z = 12mA • I knee (minimum) current is 0.5mA • incremental resistance r z = 30 Ω • Power Supply: V PS = 10V • Load: R L = 1.2k Ω 10

PROB. 4.61 DESIGN I L = 7 . 5 V 1 . 2 k Ω = 6 . 25 mA I R = I L + I Z = 12 mA + 6 . 25 mA = 18 . 25 mA R = V P S � V L = 10 V � 7 . 5 V � 137 Ω I R 18 . 25 mA 11

What's the output voltage if the supply is 10% high? Extrapolate to find V Z0 V Z 0 = 7.5 V − 30 Ω × 12 mA = 7.14 V 137 Ω 139.5 � � 137 Ω 139.5 � � I L 10 V + Δ V PS I R I Z � R L 30 � R L 10V 10V 1200 � 10V � 1200 � � 7.14V 137 Ω 139.5 � � + I L I R I Z 30 Ω∥ 1200 Ω 30 � Δ V PS R L Δ v O = Δ V PS 30 Ω∥ 1200 Ω + 137 Ω = 0.176 V 10V 1200 � � - 7.14V

Another way 137 Ω 137 Ω 139.5 � 139.5 � I L I R I Z 30 � R L R L 10V 10V 1200 � 1200 � 7.14V What’s the output voltage if the supply is 10% high? = I Z � 30 Ω + V Z 0 � V Z 0 = 7 . 5 V � 12 mA � 30 Ω = 7 . 14 V V Z V P S � V L V L � 7 . 14 V + V L = 30 Ω R R L � V P S + 7 . 14 V � = ( R � 30 Ω � R L ) V L 30 Ω R For R = 137 Ω , V P S = 10 V and R L = 1200 Ω , � 10 V 137 Ω + 7 . 14 V � V L = (24 . 116 Ω ) = 7 . 5 V 30 Ω as expected. For V P S = 11 V , � 11 V � 137 Ω + 7 . 14 V V L = (24 . 116 Ω ) = 7 . 676 V 30 Ω 13

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

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? 15

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 V Z0 in the zener model? 16

– Exercise 4.17 A zener diode exhibit a constant voltage of 5.6V for current greater than five times the knee current. I ZK 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?