Lecture 9 - Zener diode

Zener Diode Lecture 9

Introduction

• General purpose diodes have light doping resulting in high breakdown voltage.

• Heavily doped P and N regions reduce breakdown voltage.

• Zener Diode: A reverse biased heavily doped PN junction diode that operates in the breakdown region, designed to operate at voltages from a few to several thousand volts.

Operational Characteristics

• Small-signal and rectifier diodes cannot be operated in breakdown region due to potential damage.

• Zener diode can operate in three regions: forward, leakage, and breakdown.

• Doping levels allow Zener diodes to have breakdown voltages ranging from ~2V to over 1000V.

Avalanche Breakdown

• Occurs with a high reverse voltage applied across the diode.

• Increased reverse voltage enhances electric field strength across the junction.

• High electric fields free electrons from covalent bonds, causing them to collide with other atoms, leading to more free electrons and a rapid increase in reverse current.

Zener Breakdown

• With heavy doping, the depletion layer is narrow, creating an intense electric field.

• Increased electric field energy allows carriers to tunnel between regions at lower voltages.

• Energetic charge carriers produce electron-hole pairs without permanent damage due to the narrow depletion region.

Temperature Effect

• Ambient temperature changes slightly affect Zener voltage:

  • Negative temperature coefficient for breakdown voltages < 4V (Zener effect).

  • Positive temperature coefficient for voltages > 6V (Avalanche effect).

  • Temperature coefficient equals zero between 4V and 6V.

Characteristics of Zener Diode

• Forward Region: Conducts around 0.7V like an ordinary silicon diode.

• Leakage Region: Small reverse current between zero and breakdown.

• Breakdown Region: Sharp knee with a vertical increase in current, with voltage remaining almost constant (approximately equal to V_z).

  • V_z: Zener voltage (reverse breakdown voltage).

  • I_ZM: Maximum current the diode can handle without failure.

  • I_ZT: Current level at which V_z rating is measured.

Zener Resistance

• Reverse current must remain below I_ZM to avoid destruction of the diode.

• A current-limiting resistor is required to prevent excessive reverse current.

• In breakdown operation, the reverse voltage across a diode comprises the breakdown voltage plus additional voltage from bulk resistance, termed Zener resistance.

Zener Parameters

• Maximum Power: Dissipated power equals the product of voltage and current.

• Maximum Current (I_ZM) indicates the peak handling capacity without exceeding power rating.

Zener Regulator

• Reverse biased Zener diode acts as a voltage regulator, maintaining constant output voltage amid current changes.

• Source voltage (V_S) must exceed Zener breakdown voltage (V_z) for proper operation.

• A series resistor (R_s) limits Zener current below the maximum rating to avoid burnout.

Problem 1

• Zener Breakdown Voltage: 10V; supply varies from 20V to 40V.

  • Minimum Zener current when supply voltage at minimum (20V): Voltage across resistor is 20V - 10V = 10V.

  • Maximum current when supply voltage at maximum (40V): Voltage across resistor is 40V - 10V = 30V.

Problem 2

• Zener Breakdown Voltage: 10V, Zener Resistance: 8.5Ω.

  • Load voltage when Zener current is 20mA: V = 10V + 0.17V = 10.17V.

Loaded Zener Regulator

• Zener diode maintains load voltage (V_L) constant regardless of source voltage (V_S).

• Thevenin's Voltage (V_TH) must exceed Zener voltage for breakdown operation.

Loaded Zener Regulator Operations

• Load voltage equals Zener voltage due to parallel placement with the load resistor.

• Kirchhoff’s current law is applied, and Ohm’s law can provide necessary calculations for current in resistors.

Problem 3

• Check if Zener diode is in breakdown region:

  • Thevenin voltage (14.2V) > Zener voltage (10V), indicating breakdown region operation.

  • Zener current (I_Z) calculation shows 19.6mA exceeding the nominal Zener current.

Problem 4

• Circuit analysis required to find:

  • Output voltage (if V_Z > 50V, in ON state).

  • Voltage drop across series resistance.

  • Current through Zener diode.

Regulated Power Supply

• An electronic circuit providing stable DC voltage irrespective of load changes, also referred to as linear power supply.

Block Diagram of Regulated Power Supply

• Components:

  • Step-down Transformer: Reduces AC voltage for rectification.

  • Rectifier Circuit: Converts AC to pulsating DC.

• Full-wave rectifiers provide enhanced technical advantages over half-wave rectifiers.

Filter Circuit

• DC filter circuits transform high ripple AC into smooth DC voltage.

Voltage Regulator

• Voltage regulators control and correct fluctuations in output voltage ensuring constant DC voltage against input and load variations.

Types of Voltage Regulators

• Zener diode shunt regulator

• Transistor shunt regulator

• Transistor series regulator

• Fixed and variable IC voltage regulators.

Regulatory Performance Characteristics

• Line Regulation: Maintains constant output voltage when input voltage varies.

• Load Regulation: Maintains output voltage constant under varying load conditions.

Applications of Zener Diodes

• Mobile chargers

• Oscillators

• Amplifiers

• Testing circuits

• Electronic computers.