Components Summary: Working Principle and Failure Characteristics of Zener Diodes

The voltage stabilization principle of the Zener diode:

The characteristic of the voltage zener diode is that after breakdown, the voltage across its two ends remains basically unchanged. In this way, when the voltage zener diode is connected to the circuit, if the voltage at each point in the circuit changes due to fluctuations in the power supply voltage or other reasons, the voltage across the load will remain basically unchanged.

In circuits, Zener diodes are often represented by "ZD" plus a number, such as: ZD5 represents the Zener diode numbered 5.

Fault characteristics:

The failure of the voltage regulator diode is mainly manifested in open circuit, short circuit and unstable voltage value. Among these three kinds of failure, the former is manifested by the increase of power supply voltage; the latter two are manifested by the power supply voltage dropping to zero volts or unstable output.

The models and voltage values ​​of commonly used voltage regulator diodes are shown in the following table:

The voltage regulator is also a crystal diode. It works by using the characteristic of the PN junction breakdown region to stabilize the voltage. The voltage regulator is widely used in voltage stabilizing devices and some electronic circuits. We call this type of diode a voltage regulator to distinguish it from diodes used in rectification, detection and other unidirectional conductive occasions. The voltage-ampere characteristic and its symbol of the voltage regulator are shown in the figure.

(1) Stable voltage Uz Uz is the breakdown voltage of the PN junction, which varies slightly with the operating current and temperature. For the same type of Zener diode, the voltage value has a certain degree of discreteness.

(2) Stable current Iz is the reference current value of the voltage regulator when it is working. It usually has a certain range, i.e., Izmin-Izmax.

(3) Dynamic resistance rz It is the ratio of the voltage change at both ends of the voltage regulator to the current change, as shown in the figure above, that is, this value changes with the working current. Generally, the larger the working current, the smaller the dynamic resistance and the better the voltage regulation performance.

(4) Voltage temperature coefficient: This is a coefficient used to explain the effect of temperature on the stable voltage value. Different types of Zener diodes have different stable voltage temperature coefficients, which can be positive or negative. For Zener diodes with a stable voltage value lower than 4V, the stable voltage temperature coefficient is negative; for Zener diodes with a stable voltage value higher than 6V, the stable voltage temperature coefficient is positive; for Zener diodes between 4V and 6V, the stable voltage temperature coefficient may be positive or negative. In situations with high requirements, two tubes with opposite temperature coefficients can be connected in series for compensation (such as 2DW7).

(5) Rated power consumption Pz As mentioned before, the larger the working current, the smaller the dynamic resistance and the better the voltage regulation performance. However, the maximum working current is limited by the rated power consumption Pz. If it exceeds P2, the voltage regulator will be damaged.

When selecting a Zener diode, you should pay attention to the following: the current Iz flowing through the Zener diode cannot be too large, and Iz ≤ Izmax should be maintained, otherwise it will exceed the allowable power consumption of the Zener diode; Iz cannot be too small, and Iz ≥ Izmin should be maintained, otherwise the output voltage cannot be stabilized, which will limit the range of input voltage and load current. The figure below shows the dynamic equivalent circuit of the Zener diode when it is working, and the diode in the figure is an ideal diode.