Zener: – Potential, Region and Diode

Zener potential is the reverse biased potential that results in a dramatic change in the characteristics of a diode in the reverse biased. (Vz.)

SEE THE ABOVE IMAGE.

When the voltage across the diode increases in the reverse bias the velocity of the charge carriers also increases, which are responsible for the reverse saturation current Is. This velocity of theirs is associated with:

Wk= 0.5 * M * V^2;

This is sufficient to release additional carriers through collision which is otherwise in a stable state.

• Zener Breakdown mechanism is significant only for LOWER levels of (zener potential.).
• The maximum reverse bias potential that can be applied before entering the zener breakdown voltage is called the Peak Inverse Voltage or the Peak Reverse Voltage.
  • To increase the peak inverse voltage of a diode, connect another diode with same characteristics in series with the former one.
  • To increase the current capacity, connect them in parallel.
• At fixed temperature reverse saturation current increases with the reverse bias voltage.

Temperature Effects on Diode:

• In forward bias, the graph moves to left by 2.5mV for a one-degree centigrade rise in temperature.
• In reverse bias, the graph moves downwards as reverse saturation current of silicon diode doubles for a one-degree rise in temperature.

Zener Region:

Image.

• It is the region wherein we see a characteristic drop in an almost vertical manner at the reverse biased potential (Vz.).
• The fact that the curve drops down and away from the horizontal axis rather than up and away from the positive Vd region reveals that the current in the zener region has a direction opposite to that of forward biased.
• The slight slope of the characteristic curve is the indication of resistance in the circuit which is associated with Zener diode.

• This clearly represents that the direction of current flow in zener diode is opposite to that of a normal diode.
• Location of zener region can be controlled by the doping level.
  • And an increase in doping will increase the impurity and would decrease the zener potential.

Parameter values of Zener diode (AT 25-degree centigrade temperature):

1. Zener voltage: Vz = 10V.
2. Test Current: Izt = 12.5mA.
3. Max dynamic Impedance: Zzt = 8.5 ohms at Izt.
4. Max Knee Impedance: Zzk = 200 ohms at Izk = 0.25mA.
5. Max Reverse Current: Ir = 10uA. at Vr.
6. Test Voltage: Vr = 7.2V
7. Max Regulator Current: Izm = 32mA.
8. Temperature Coefficient (%/degree.Centigrade): +0.072.

The zener potential of a zener diode is very sensitive to the temperature of operation.

Tc = [(ΔVz  / Vz ) /  (Ti – To) ] * 100 (%/degree.Centigrade).

Zener as Voltage Regulator.

A Regulator is the combination of circuit elements in a way so as to get a fairly constant output voltage.

• Fixed Parameters:
  • Vi and Rl are fixed.
  • Determine the state of zener diode by removing it from the network and calculating the voltage across the resulting open circuit. V = Vl = (Rl * Vi)/ R + Rl; (Voltage Divider)
  • If V >= Vz then, Zener is ON.
  • If V < Vz then, Zener is OFF. Therefore, we substitute an open circuit in place of zener diode.
  • Substitute the appropriate equivalent circuit and solve for the desired unknown parameter.
  • Vl = Vz as both are in parallel connection.
  • In zener diode circuits current through it must be found out using KCL(Kirchhoff’s Current Law).
  • Power Dissipated: Pz = Vz * Iz.
  • If the zener is in ON state than the voltage across the diode is not V volts as No sooner we would turn on the network zener diode would reach Vz volts and would never reach the higher values of voltages like V volts. This is known as locking in the voltage to Vz.

• Fixed Vi, Variable Rl:
  • Due to the OffSet Value of Vz, there are limited values of Rl that can be taken (Hence, the load current is also controlled).
  • Small values of Rl will lead to low values of Vl and once the value of Vl is less than Vz zener diode turns OFF and thus enters into it’s Off state.
  • Minimum Rl can be found when zener diode is in its ON state.
  • Rl(min) = (R*Vz) / (Vi – Vz); this also gives Il(max).
  • Il(max) = (Vz) / (Rl(min));
  • Diode in ON state makes voltage across R fixed.
  • Vr = Vi – Vz;
  • Iz = Ir – Il;
  • Il(min) = Ir – Izm;
  • Rl(max) = Vz / Il(min);
• Variable Parameters:
  • Vi(min) = [ (Rl + R) * Vz ] / Rl;
  • Ir(max) = Izm + Il;
  • Vi(max) = Ir(max) * R + Vz;
  • Maximum value of Vi is limited by maximum zener current Izm in this case. (therefore, Second Equation).
  • For a fixed value of Rl, Vi most is sufficiently large to turn ON zener diode. (therefore, first equation).
  • ~Jay Mehta
    

    

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