Fundamentals of Zener Diodes

Introduction to Special Purpose Diodes

• Rectifier Diodes:

  • Commonly used for converting AC voltage to DC in power supplies.

• Beyond Rectification:

  • Diodes serve in various applications beyond rectification.

• Zener Diodes:

  • Optimized for breakdown properties.

  • Essential for voltage regulation in circuits.

• Other Diodes Covered:

  • Optoelectronic Diodes (e.g., LEDs)

  • Schottky Diodes (known for low forward voltage drop)

  • Varactor Diodes (used for variable capacitance)

  • Additional special-purpose diodes for specific applications.

The Zener Diode

• Operation:

  • Unlike small-signal and rectifier diodes, Zener diodes are designed to operate in the breakdown region.

  • Backbone of voltage regulators, maintaining a nearly constant load voltage despite fluctuations.

• Schematic Symbol:

  • Two variations of Zener diode symbols resemble a “z” for "Zener."

  

• Breakdown Voltages:

  • Vary from 2V to over 1000V, depending on doping levels.

I-V Characteristics of Zener Diode

• Operating Regions:

  1. Forward Region:

     • Conducts at \(\sim0.7~\mathrm{V}\), like a regular silicon diode.

  2. Leakage Region:

     • Small reverse current between zero and breakdown.

  3. Breakdown Region:

     • Sharp knee followed by a near-vertical increase in current.

     • Voltage stays approximately constant \((V_Z)\) in the breakdown region.

  

• Safe Operation:

  • Maximum reverse current: \(I_{ZM}\).

  • Exceeding \(I_{ZM}\) leads to diode destruction.

  • A current-limiting resistor is essential to protect the diode.

Zener Resistance

• Third Approximation of a Diode:

  • Forward voltage = Knee voltage + Voltage across bulk resistance

  • Reverse voltage = Breakdown voltage + Voltage across bulk (Zener) resistance

• Zener Resistance:

  • Defined as the inverse slope of the I-V curve in the breakdown region.

  • More vertical breakdown region = smaller Zener resistance

• Effect on Reverse Current:

  • Increase in reverse current leads to a slight increase in reverse voltage.

  • Typically, this voltage increase is only a few tenths of a volt.

Zener Regulator

• Constant Output Voltage:

  • Zener diode maintains a constant output voltage even as current changes.

  • Requires reverse bias for normal operation.

  

• Operating Conditions:

  • Source voltage (\(V_s\)) must be greater than Zener breakdown voltage (\(V_Z\)).

  • A series resistor (\(R_s\)) is used to limit current and prevent burnout.

Zener Regulator – Current Calculation

• Current through Series Resistor:

  • Voltage across the series resistor (\(R_s\)) = Source voltage \((V_S)\) - Zener voltage \((V_Z)\).

  • Current through the resistor: \[I_S = \frac{V_S - V_Z}{R_S}\]

  • \(I_s\) is equal to the Zener current since the circuit is in series.

  • Important: \(I_s\) must be less than \(I_{ZM}\) to prevent damage.

Ideal Zener Diode Approximation

• Ideal Zener Diode:

  • For troubleshooting and analysis, the breakdown region can be approximated as vertical.

  • Voltage is constant even with varying current (ignoring Zener resistance).

• Circuit Simplification:

  • A Zener diode in the breakdown region acts like a constant voltage source \((V_Z)\) .

  • This simplifies analysis as you can mentally replace the Zener diode with a battery of voltage \(V_Z\).

Problem-1

• The breakdown voltage of the zener diode shown in the Fig. is 10 V. What are the minimum and maximum zener currents?

• Applied voltage \(20-40~\mathrm{V}\) (vary)

• Zener diode replace with a battery of \(10~\mathrm{V} \Rightarrow\) output voltage is \(10~\mathrm{V}\) (fixed)

Loaded Zener Regulator

• Operation:

  • Zener diode holds load voltage constant in the breakdown region.

  • Load voltage \((V_L)\) stays fixed at the Zener voltage \((V_Z)\) despite changes in source voltage or load resistance.

• Voltage Divider & Thevenin Voltage:

  • Thevenin voltage (\(V_{TH}\)) facing the diode: \[V_{TH} = \frac{R_{L}}{R_{S} + R_{L}}V_{S}\]

  • Breakdown occurs only if \(V_{TH} > V_Z\).

Current Calculations in Loaded Zener Regulator

• Series Current \((I_S)\): \[I_S = \frac{V_S - V_Z}{R_S}\]

• \(I_S\) remains the same whether or not the load resistor is connected.

• Load Voltage \((V_L)\):

  • Ideally, \(V_L = V_Z\) because the load resistor is in parallel with the Zener diode.

• Load Current \((IL)\): \[I_L = \frac{V_L}{R_L}\]

Zener Current Calculation

• Total Current \((I_S)\): \[I_S = I_Z + I_L\]

• The sum of Zener current and load current equals the series current (Kirchhoff’s Current Law).

• Zener Current \((I_Z)\): \[I_Z = I_S - I_L\]

• \(I_Z\) no longer equals \(I_S\) as in an unloaded regulator; it’s reduced by the load current.

Summary of Loaded Zener Regulator Analysis

1. Calculate Series Current \((I_S)\): \[I_S = \frac{V_S - V_Z}{R_S}\]

2. Determine Load Voltage \((V_L)\): \[V_L = V_Z\]

3. Calculate Load Current \((I_L)\): \[I_L = \frac{V_L}{R_L}\]

4. Find Zener Current \((I_Z)\): \[I_Z = I_S - I_L\]