Zener diodes are diodes that act as regulators. By using the reverse breakdown state of the PN junction, the current of the Zener diodes can be changed within a wide range while the voltage is unchanged.

I Principle of Zener diodes

Zener diodes are diodes that act as regulators. By using the reverse breakdown state of the PN junction, the current of the Zener diodes can be changed within a wide range while the voltage is unchanged. This diode is a semiconductor device with very high resistance up to the critical reverse breakdown voltage. At this critical breakdown point, the reverse resistance is reduced to a very small value. In this low resistance region, the current increases while the voltage remains constant. The Zener diode is binned according to the breakdown voltage. Because of this characteristic, the Zener is mainly used as a voltage regulator or voltage reference element. Zener diodes can be connected in series for use at higher voltages, and higher stable voltages can be obtained by connecting in series.

The forward characteristic of the volt-ampere characteristic curve of the Zener diode is similar to that of an ordinary diode. The reverse characteristic is that when the reverse voltage is lower than the reverse breakdown voltage, the reverse resistance is very large and the reverse leakage current is extremely small. However, when the reverse voltage approaches the critical value of the reverse voltage, the reverse current suddenly increases, which is called breakdown. At this critical breakdown point, the reverse resistance suddenly drops to a very small value. Although the current varies in a large range, the voltage across the diodes is stable near the breakdown voltage, thereby achieving the voltage stabilization of the diodes. Semiconductor diodes prevent reverse current, but if the applied reverse voltage becomes too high, premature breakdown or damage can occur.

Zener diodes are the same as standard PN junction diodes, but they are specially designed to have a low and specified reverse breakdown voltage. It makes use of any reverse voltage applied to it. The Zener diode behaves like an ordinary general-purpose diode, which is made of silicon PN structure. When forward biased, the anode is relative to its cathode, and it behaves like a normal signal diode passing a rated current. However, unlike conventional diodes, which prevent any current from flowing through themselves when reverse biased, the cathode becomes more positive than the anode, and once the reverse voltage reaches a predetermined value, the Zener diode begins to conduct reversely. This is because when the reverse voltage across the Zener diodes exceeds the device's rated voltage, a process called Avalanche Breakdown occurs. The semiconductor depletion layer and current begin to flow through the diodes to limit the voltage increase.

II IV characteristics of Zener diodes

Figure 1. IV characteristics of Zener diodes

Zener diodes are used in "reverse bias" or reverse breakdown mode, where the anode of the diode is connected to the negative supply. It can be seen from the above IV characteristic curve that the reverse bias characteristic region of the Zener diode is almost a constant negative voltage, which has nothing to do with the value of the current flowing through the diode, and it remains almost unchanged even if the current varies greatly. The Zener diode current remains between the breakdown current I Z (min) and the maximum rated current I Z (max).

This ability to control itself can be used to regulate or stabilize the voltage source to prevent power or load changes. The fact that the voltage across the diode in the breakdown region is almost constant has proven to be an important feature of Zener diodes because it can be used in the simplest voltage regulator applications.

The regulator should provide a constant output voltage to the load connected in parallel. Despite the supply voltage fluctuation or load current change, the Zener diode will continue to adjust the voltage until the diode current falls below the minimum IZ (min) value in the reverse breakdown region.

III Zener diode regulator

Zener diodes can be used to produce a stable voltage output with low ripple under varying load currents. Bypassing a small current from the voltage source through the diode, through a suitable current limiting resistor (RS), the Zener diode will conduct enough current to maintain the voltage drop Vout.

Remember that the DC output voltage of a half-wave or full-wave rectifier contains a ripple superimposed on the DC voltage and the average output voltage when the load value changes. By connecting a simple Zener stabilizer circuit as shown below to the rectifier output, a more stable output voltage can be generated.

Figure 2. Zener stabilizer circuit

The resistor RS is connected in series with the Zener diode to limit the current through the diode, and VS is connected in the combination. The regulated output voltage Vout is taken from a Zener diode. The cathode terminal of the Zener diode is connected to the positive rail of the DC power supply, so it is reverse biased and will operate in its breakdown state. Then choose the resistor RS to limit the maximum current flowing in the circuit.

Without a load connected to the circuit, the load current will be zero (IL = 0), and all circuit current passes through the Zener diode, which in turn consumes its maximum power. When a small part of the load resistance RLRS will result in a larger diode current connection because this will increase the power dissipation requirements of the diode. Selecting the appropriate series resistance value so that no load or high impedance conditions Do not exceed the maximum rated power of Zener.

The load is connected in parallel with the Zener diode, so the voltage on RL is always the same as the Zener voltage (V <sub>-[R = V <sub> ž). There is a minimum Zener current for which the stabilization of the voltage is effective, and the Zener current must always remain higher than this value operating under a load in its breakdown region. The upper limit of the current depends of course on the rated power of the device. The power supply voltage VS must be greater than VZ.

One small problem is the same as the Zener diode stabilizer circuit. The diode sometimes generates electrical noise on top of a DC power supply because it tries to stabilize the voltage. This is usually not a problem for most applications, but it may be necessary to add a large value decoupling capacitor at the Zener output to achieve smoothing.

Zener diodes always operate under reverse bias conditions. A Zener diode can be used to design a voltage regulator circuit to maintain a constant DC output voltage across the load in the event of a change in input voltage or load current. The Zener voltage regulator consists of a current-limiting resistor RS in series with the input voltage V S. Under this reverse bias condition, the Zener diode is connected in parallel with the load RL. The stable output voltage is always chosen to be the same as the breakdown voltage VZ of the diode.

Example

5.0V stable power is needed from 12V DC power input. Zener diodes have a maximum rated power PZ of 2W. Calculated using the Zener regulator circuit above:

a). The maximum current flowing through the Zener diode.

b). The minimum value of series resistance, RS

c). Load current IL, if a 1k&Omega; load resistor is connected across the Zener diode.

d). Zener current IZ, at full load.

IV Zener diode voltage

In addition to generating a single stable voltage output, Zener diodes can also be connected in series with ordinary silicon signal diodes to produce a variety of different reference voltage output values as shown below.

Zener diodes connected in series

Figure 3. Zener diodes connected in series

The value of each Zener diode can be selected to suit the application, while silicon diodes always drop approximately 0.6-0.7V under forwarding bias. The supply voltage Vin must, of course, be higher than the maximum output reference voltage, which in the example above is 19v.

The typical electronic circuit of a typical Zener diode is 500mW, BZX55 series or 1.3W, BZX85 series. For example, C7V5 is a 7.5V diode and the diode reference number is BZX55C7V5.

The 500mW series Zener diodes have a voltage range of approximately 2.4 to 100 volts and usually have the same sequence of values for the 5% (E24) resistor series. These small but very useful diodes have separate voltage ratings, as shown in the table below.

V Zener diode clamp circuit

So far, we have studied how a Zener diode regulates a constant DC power supply. But how the Zener diode reacts to a changing signal if the input signal is not a steady-state DC but an AC-ac waveform.

The diode limiting and clamping circuit are used to form or modify the input AC waveform (or any sine wave) and produce different shaped output waveforms according to the circuit arrangement. Diode limiter circuits are also called limiters because they limit the positive (or negative) part of the input AC signal. Since Zener clamp circuits limit or cut off part of the waveform, they are mainly used for circuit protection or waveform shaping circuits.

For example, if we want to clip the output waveform to +7.5 V, we will use a 7.5V Zener diode. If the output waveform attempts to exceed the 7.5V limit, the Zener diode will "cut off" the overvoltage from the input, producing a waveform with a flat top and still keeping the output constant at + 7.5V. Please note that under forward bias conditions, the Zener diode is still a diode. When the AC waveform output is lower than -0.7V, the Zener diode will "conduct" like any normal silicon diode and limit the output to -0.7V, as shown below.

Figure 4. Zener diode clamp circuit

Back-to-back connected Zener diodes can be used as what the AC voltage regulator produces what is nicknamed "Poor's Square Wave Generator". With this configuration, we can cut the waveform between the positive value of + 8.2V and the negative value of -8.2V for the 7.5V Zener diode

So, for example, if we want to clip the output waveform between two different minimum and maximum values, such as + 8V and -6V, we only need to use two Zener diodes with different ratings. Note that the output limits the AC waveform to between + 8.7V and -6.7V due to the increase in forwarding biased diode voltage.

In other words, the peak-to-peak voltage is 15.4 volts instead of the expected 14 volts because the forward bias voltage drop across the diode increases by 0.7 volts in each direction.

This type of limiter configuration is quite common for protecting electronic circuits from overvoltages. Two Zener diodes are usually placed on the power input terminals. During normal operation, one of the Zener diodes is "off" and the diode has little effect. However, if the input voltage waveform exceeds its limit, the Zener diode turns "ON" and clamps the input to protect the circuit.

VI Application of Zener diodes

1. The typical series regulator circuit

Figure 5. The typical series regulator circuit

In this circuit, the base of the transistor T is stabilized at 13V by the Zener diode D, then its emitter will output a constant voltage of 13-0.7 = 12.3V. Within a certain range, whether the input voltage increases or decreases, regardless of The load resistance changes and the output voltage remains unchanged. This circuit is used in many situations. 7805 is a series integrated voltage regulator circuit that can output 5V. 7805-7824 can output 5-24V voltage. It has applications on many appliances.

Figure 6. 7805 series integrated voltage regulator circuit

2. Overvoltage protection circuit in TV

Figure 7. Overvoltage protection circuit in TV

115V is the main power supply voltage of the TV. When the output voltage of the power supply is too high, D is turned on and the transistor T is turned on. Its collector potential will change from the original high level (5V) to the low level. The voltage through the standby control line puts the TV into standby protection.

3. Arc suppression circuit

Figure 7. Arc suppression circuit

When an appropriate Zener diode is connected in parallel to the inductor coil (the principle can also be connected to an ordinary diode), and the coil is cut off in the on the state, the high voltage generated by the release of its electromagnetic energy is taken by the diode.  So when the switch is turned off, the arc of the switch is eliminated. This application circuit is used more in industry, such as some larger power electromagnetic control circuits.