Zener diode – also called stabilizer or stabilistor. It is a variant of the semiconductor diode that is often applied in stops systems (including frequency modulation (FM) systems, pulse selection systems), voltage stabilizers and as an element that secures and protects circuits from power surges.
Its crucial parameter is the breakdown voltage of the “p-n junction”. When diode is forward-biased it works like typical semiconductor diode. It means that it starts conducting current after crossing the threshold voltage – for a silicon diode approx. 0.7V. However, when it is reverse-biased, one can notice that Zener diode is able to conduct electricity after exceeding the specified voltage at the junction, so called breakdown voltage without damaging the element. After exceeding this voltage, current value rapidly increases. For small voltages (up to 5V) Zener effect plays a crucial part, in the range of 5-7V, the Zener effect and avalanche breakdown, and after exceeding 7V – only avalanche breakdown. It is worth mentioning that Zener breakdown occurs in heavily doped junctions, whereas avalanche breakdown in weakly doped. We can assume that the voltage breakdown of diode is not relevant with the current that flows through the diode (only in cases of large breakdown current changes very slight changes may be noticed – low dynamic resistance)
Zener diode, like the other diodes has its own unique characteristics. At the same time it inherits some of them. Down below there is list of main static and dynamic elements of this item:
- UF – forward voltage at fixed forward current IF,
- IR – reverse current at given UR reversing voltage (most often UR value is 1V)
It should be noted that these parameters may be irrelevant when diode is working in “breakdown area”.
- UZ – Zener voltage, corresponding to the contractual current value of stability (information available in the data sheets of individual diodes)
- TKUZ – factor temperature of stability voltage (proportion of relative change of stability voltage to the absolute change in ambient temperature at a certain current stabilization).
Zener diode dynamic parameter is the dynamic resistance expressed by the formula:
Zener diode is characterized by the following acceptable parameter limits:
- IFMAX – the maximum, constant current conduction,
- IFMMAX – peak, permitted current of the diode conduction (usually given for low and medium power diodes)
- IZMAX – the maximum, permitted current stabilization, in most cases determined from the quotient of Pad dissipation rate and voltage stabilization Pad UZ,
- PMAX – Maximum loss power.
Exercise to compute:
Design a parametric stabilizer system with an output voltage Vout = 10V for load current ranging from Iout = 0 ÷ 35 mA. Input voltage is ranging from Vin = 17 ÷ 23 V.