Zener Diode as Voltage Regulator

Zener Effect vs. Avalanche Effect

• Zener Effect:

  • Occurs in heavily doped diodes with a narrow depletion layer.

  • Electric field becomes intense, approximately 300,000 V/cm.

  • Field pulls electrons from valence orbits (known as high-field emission).

  • Dominates in breakdown voltages less than 4 V.

• Avalanche Effect:

  • Minority carriers are accelerated, dislodging other electrons.

  • Causes an avalanche of free carriers, resulting in large reverse current.

  • Dominates in breakdown voltages greater than 6 V.

• Between 4-6 V:

  • Both Zener and Avalanche effects are present.

• Naming:

  • Historically, all breakdown diodes are called Zener diodes, even though both effects may occur.

Temperature Coefficients in Zener Diodes

• Impact of Temperature on Zener Voltage:

  • Zener voltage changes with ambient temperature.

  • Listed in data sheets as the temperature coefficient:

    • Change in breakdown voltage per degree Celsius ($^{\circ}\mathrm{C}$).

• Breakdown Voltage $<$ 4 V:

  • Negative temperature coefficient (Zener effect).

  • Example: A 3.9 V zener diode may have a coefficient of -1.4 mV/$^{\circ}\mathrm{C}$.

    • For each $1^{\circ}~\mathrm{C}$ increase, breakdown voltage decreases by 1.4 mV.

• Breakdown Voltage $>$ 6 V:

  • Positive temperature coefficient (Avalanche effect).

  • Example: A 6.2 V zener diode may have a coefficient of +2 mV/$^{\circ}\mathrm{C}$.

    • For each $1^{\circ}\mathrm{C}$ increase, breakdown voltage increases by 2 mV.

• Between 4 and 6 V:

  • Temperature coefficient shifts from negative to positive.

  • Some diodes have a zero temperature coefficient, offering stability across temperature ranges.

    • Important for applications needing stable Zener voltage over wide temperature variations.

Problem-2

• Is the zener diode operating in the breakdown region?

$$V_{TH}=\frac{1 \mathrm{k}\Omega}{270 \Omega + 1 \mathrm{k}\Omega}(18 \mathrm{V})=14.2 \mathrm{V}$$

• The Thevenin voltage ($V_{th}$) is greater than the Zener voltage ($V_Z$).

• Operates in the breakdown region.

Problem-3

• What is the zener current?

$$\begin{aligned} I_{S}&=\frac{(18-10)\:\mathrm{V}}{270\:\Omega}=29.6\:\mathrm{mA}\\ I_{L}&=\frac{10\:\mathrm{V}}{1\:\mathrm{k}\Omega}=10\:\mathrm{mA}\\ I_{Z}&=29.6\:\mathrm{mA}-10\:\mathrm{mA}=19.6\:\mathrm{mA}\\ \end{aligned}$$

Problem-4

• What is the behaviour of this circuit?

  

• First Zener diode acts as a preregulator.

• Second Zener diode serves as a Zener regulator.

• Preregulator output: 20 V.

• Zener regulator output: 10 V.

• Provide the second regulator with a well-regulated input.

• Ensure the final output is extremely well regulated.

Problem-5

• Zener diodes are commonly used in voltage regulators, remaining in the breakdown region.

• Zener diodes can be utilized in circuits for waveshaping, as shown in the configuration with two back-to-back Zener diodes.

• Positive Half-Cycle:

  • Upper diode conducts; lower diode breaks down.

  • Output is clipped at $\text{Zener Voltage} + 0.7 \, \text{V}$.

• Negative Half-Cycle:

  • Lower diode conducts; upper diode breaks down.

  • Output approaches a square wave.

• Conclusion:

  • Larger input sine waves produce a better-shaped output square wave.

Problem-6

• Multiple DC Output Voltages:

  • Combination of zener diodes and ordinary silicon diodes used to generate multiple DC output voltages from a 20-V power supply.

  • Bottom diode produces 10 V output.

  • Forward-biased silicon diodes add small voltage steps: 10.7 V and 11.4 V.

  • Top zener diode with a 2.4 V breakdown voltage gives an output of 13.8 V.

  • Different combinations of zener and silicon diodes can yield various DC outputs.