Zener Diode is a general purpose diode, which behaves like a normal diode when forward biased. But when it is reverse biased above a certain voltage known as zener breakdown voltage or zener voltage or avalanche point or zener knee voltage the voltage remains constant for a wide range of current.
Clarence Zener is the scientist who discovered this electrical property and the device is named after him.
Ordinary diodes will not have any significant current (only leakage current ) when reverse biased below its reverse breakdown voltage. When the reverse bias is increased beyond reverse breakdown voltage its potential barrier breaks down. This may damage the diode due to excess heat produced by the high current flow through the diode unless the current is limited. Zener diode also exhibits similar properties except that it is designed to have lower breakdown voltage. Ordinary diodes have breakdown voltages in the order of 100 or above.
There are two breakdown mechanisms :
- Zener Breakdown
- Avalanche Breakdown
Zener Breakdown or Zener Effect
Zener Breakdown is a type of electrical breakdown mechanism occurs in reverse biased P-N Junctions. This breakdown occurs due to the high electric field density across the depletion region breaks some of the covalent bonds leading to large number of minority carriers.
Zener effect is predominant up to 5.6 V for silicon diodes and has negative temperature coefficient.
Avalanche Breakdown or Avalanche Effect
Avalanche breakdown is distinct from zener breakdown. It is caused due to collision. Minority carriers in the depletion region gets accelerated by the reverse biased electric field. This may make energy of minority carriers sufficient to release electron hole pairs from covalent bonds through collision.
Avalanche effect is predominant above 5.6 V for silicon diodes and has positive temperature coefficient.
Thus in a 5.6 V zener diode both zener and avalanche effects are equally predominant thus its positive and negative temperature dependency cancels each other. Thus naturally 5.6 V zener diode is the component of choice in temperature critical applications.
Zener Diode is commonly used for making reference voltages for Voltage Regulators and to protect other electronic devices from voltage surges.
Zener Diode as Voltage Regulator
Zener Diodes are widely used as Shunt Voltage Regulators to regulate voltage across small loads. Zener Diodes have a sharp reverse breakdown voltage and breakdown voltage will be constant for a wide rang of currents. Thus we will connect the zener diode parallel to the load such that the applied voltage will reverse bias it. Thus if the reverse bias voltage across the zener diode exceeds the knee voltage, the voltage across the load will be constant.
In the above circuit diagram excess voltage (Vin – Vz) will drop across Rs thus by limiting the current through Zener. For the proper designing of the regulator we should know,
- Unregulated Input Voltage Range
- Required Output Voltage
- Max Load Current Required
The value of resistance Rs should satisfy the following conditions,
- The value of Rs must be small enough to keep the Zener Diode in reverse breakdown region. The minimum current required for a Zener Diode to keep it in reverse breakdown region will be given in its datasheet. For example, a 5.6 V, 0.5 W zener diode has a recommended reverse current of 5 mA. If the reverse current is less than this value, the output voltage Vo will be unregulated.
- The value of Rs must be large enough that the current through the zener diode should not destroy it. That is the maximum power dissipation Pmax should be less than IzVz.
Thus we should find Rsmin and Rsmax. To find the value of Rsmin we should consider the extreme condition that Vin is minimum and load current is maximum.
Is = Izmin + ILmax
Izmin = Please Refer Datasheet
Vs = Vinmin – Vz
Rsmin = Vs/Is
To find the value of Rsmax we should consider the extreme condition that Vin is maximum and load current is minimum (ie, no load connected).
Is = Izmax + ILmin
Izmax = Pmax/Vz
Vs = Vinmax – Vz
Rsmax = Vs/Is
Standard Zener Diode Voltages
|BZX55 Zener Diode 500mW|
|BZX85 Zener Diode 1.3W|
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