CHAPTER 20 (1): ZENER DIODES
Chapter 20: Special Purpose Diodes
Zener Diodes (Part 1)
Objectives
Understand the characteristics and operation of a Zener diode.
Determine Zener impedance.
Characteristics of Zener Diodes
Zener diodes behave like ordinary diodes when forward-biased.
Specifically designed to breakdown in a controlled manner at a predetermined voltage when reverse-biased.
Once breakdown occurs, the voltage across the diode is nearly constant.
Breakdown voltage is established by controlling the doping level during manufacturing.
Operating Regions
Reverse-Breakdown Region: Zeners operate in this region intentionally unlike junction diodes which may overheat and get damaged in breakdown.
Zener Breakdown: Occurs at low reverse voltage (typically < 5V) due to heavy doping; represents the ordinary working region of Zener.
Avalanche Breakdown: Happens at reverse voltages significantly above 5V, should be avoided to prevent damage.
Zener Diode Symbol
Cathode (K) and Anode (A) are clearly defined in Zener diode schematics.
Voltage-Current Characteristics
Zener-Breakdown Region: Identified as operating at a constant voltage (VZ).
VZ usually measured at IZT (Zener test current).
Forward-Bias Region: Typically around 0.7V for standard silicon diodes.
Practical Zener Diode Properties
An ideal Zener has a vertical breakdown characteristic, indicating constant voltage across varying current.
In practice, Zener voltage fluctuates slightly influenced by current changes.
Zener Impedance (ZZ): Assumed constant over the linear range of Zener current values, exhibiting resistive properties.
Modeling Practical Zeners
Practical Zener can be modeled as:
A voltage source (VZ)
A resistor-like property highlighting impedance.
Worked Examples
Example 20-1:
A Zener diode shows a change in VZ with a change in IZ between specified currents.
Use values to calculate Zener impedance.
Example 20-2:
For the IN4736 Zener diode (ZZT = 3.5Ω, VZT = 6.8V). Determine voltage at:
i) IZ = 50 mA
ii) IZ = 25 mA
Solution Strategy: Calculate the change in Zener voltage (ΔVZ) based on changes in Zener current (ΔIZ) multiplied by Zener impedance (ZZT).
Zener Diode Applications
Historically used for regulating D.C. voltages.
Now largely replaced by integrated circuit (IC) voltage regulators.
Important to note that the voltage across a resistive load (R2) connected to a Zener diode does not exceed VZT regardless of R1 and R2 values.
Summary
Operated normally in reversed-bias mode for maintaining constant voltage upon reaching reverse breakdown voltage.
Key characteristics visible on the Zener characteristics curve include Zener test voltage, Zener test current, and Zener maximum voltage.
Zener impedance is a crucial aspect of its operational characteristics.