What is zener diode and how it works?

 

A Zener diode is a type of semiconductor diode that is designed to operate in the reverse breakdown region. It is similar to a standard diode but with a unique ability to maintain a stable, specified reverse voltage when a reverse bias is applied. Unlike regular diodes, which block current flow in the reverse direction, Zener diodes are made to conduct when the reverse voltage exceeds a specific threshold known as the Zener voltage (Vz). This makes Zener diodes ideal for voltage regulation applications.

How a Zener Diode Works

A Zener diode behaves like a regular PN junction diode when it is forward biased (anode connected to the positive terminal, and cathode to the negative terminal). In this configuration, it allows current to flow just like any normal diode. However, when it is reverse biased (anode connected to the negative terminal), it blocks current flow until the reverse voltage reaches a certain point.

Once the reverse voltage exceeds the Zener voltage (Vz), the diode enters a breakdown region called avalanche breakdown. In this region, the diode starts to conduct current, and it can maintain a nearly constant reverse voltage regardless of fluctuations in the input voltage, as long as the current is within a specified range.

A semiconductor diode blocks current in the opposite direction, but will fail prematurely if the reverse voltage applied across the terminals becomes too high. However, Zener diodes or "breakdown diodes" as they are sometimes called, are essentially the same as standard PN junction diodes, but specially designed to have a specified low reverse breakdown voltage taking advantage of any reverse voltage is applied. to her. 

 

The Zener diode works like a regular general purpose diode made of PN silicon junction and when it is forward biased. That is, anode to its cathode it behaves like a normal signal diode carry rated current.  However, unlike a conventional diode which blocks all current flowing through itself when reverse biased i.e. cathode becomes more positive than anode, as soon as reverse voltage reaches a predetermined value, Zener diode start to lead in the opposite direction. This is because when the reverse voltage applied across the Zener diode exceeds the rated voltage of the device, a process called Avalanche Breakdown occurs in the depletion layer of the semiconductor and a current begins to flow through the diode to limit it. this voltage rise. 

 

The current flowing through the Zener diodes increases suddenly to the maximum value of the circuit (usually limited by a series resistor) and when this reverse saturation is reached, this reverse saturation current remains fairly stable. over a wide range of reverse voltages. The voltage point at which the voltage across the zener diode becomes stable is known as the "zener voltage", (Vz) and for zener diodes this can range from less than one volt to several hundred volts.  The point at which the zener voltage triggers current through the diode can be controlled very precisely (within a tolerance of 1%) in the doping step of the diode's semiconductor construction giving the diode a specific zener breakdown voltage. possible, (Vz) for example, 4.3V or 7.5V. This Zener breakdown voltage on the IV curve is almost a vertical line.

Zener Diode I-V Characteristics

The I-V characteristic of a Zener diode shows that, in the reverse bias region, the voltage across the diode remains relatively constant despite changes in the current. This behavior is what makes Zener diodes useful for voltage regulation. The voltage at which this constant voltage is maintained is called the Zener voltage (Vz). The Zener voltage can be precisely controlled during manufacturing, allowing diodes to be made with specific breakdown voltages ranging from less than one volt to several hundred volts.

Zener diodes are used in "reverse bias" or reverse breakdown, i.e. the anode of the diodes connects to the negative supply. From the above IV characteristic, we can see that the Zener diode has a region in its reverse bias characteristic where a negative voltage is almost constant regardless of the amount of current flowing through the diode.  This voltage is virtually constant even with large current variations provided that the current of the Zener diodes remains between the breakdown current I_Z (minimum) and the rated current  I_Z maximum (maximum). The self-control of this Zener diode can be used well in regulating or stabilizing a voltage source against changes in supply or load. The fact that the voltage across the diode in the breakdown region remains virtually constant proves this is an important property of the Zener diode as it can be used in the simplest types of voltage regulation applications.  The function of a voltage regulator is to provide a constant output voltage to a load connected in parallel despite ripples in the supply voltage or variation in load current. The Zener diode will continue to regulate its voltage until the current holding diode drops below the minimum value  I_Z (min) in the reverse breakdown region.

Applications of Zener Diodes

Zener diodes are primarily used for voltage regulation. They are widely employed in power supply circuits to maintain a stable output voltage, even if the input voltage fluctuates or if the load current changes. A Zener diode voltage regulator is often used to provide a constant output voltage to sensitive devices that require a steady DC supply, such as in battery charging circuits or low-voltage power supplies.

Zener Diode Voltage Regulator

Zener diodes can be used to generate stabilized voltage outputs with less ripple under a variety of load and current conditions. By passing a small current from the voltage source through the appropriate current limiting resistor (R_S) to the diode, the Zener diode draws enough current to maintain the Vout voltage drop. 

We know that the DC output voltage of a half-wave or full-wave rectifier contains ripples superimposed on the DC voltage, and the average output voltage changes as the load value changes. A simple Zener stabilizer circuit, as shown below, can be connected to the output of the rectifier to produce a more stable output voltage.

Resistor, R_S is connected in series with the zener diode to limit the current flow through the diode with the voltage source, V_S being connected across the combination. The stabilized output voltage Vout is taken from across the zener diode. The zener diode is connected with its cathode terminal connected to the positive rail of the DC supply so it is reverse biased and will be operating in its breakdown condition. Resistor R_S is selected so to limit the maximum current flowing in the circuit. 

With no load connected to the circuit, the load current will be zero, (I_L = 0), and all the circuit current passes through the zener diode which in turn dissipates its maximum power. Also a lesser value of the series resistor R_S will result in a greater diode current when the load resistance R_L is connected and large as this will enhance the power dissipation requirement of the diode so care must be taken when selecting the appropriate value of series resistance so that the zener’s maximum power rating is not exceeded under this no-load or high-impedance condition. The load is connected in parallel with the zener diode, so the voltage across R_L is always the same as the zener voltage, ( V_R = V_Z ). 

There is a minimum zener current for which the stabilization of the voltage is effective and the zener current must stay above this value operating under load within its breakdown region at all times. The higher limit of current relying on the power rating of the device. The supply voltage V_S must be larger than V_Z. 

In zener diode stabilizer circuits, diode can sometimes produce electrical noise on top of the DC supply as it tries to stabilize the voltage. Usually this is not a problem for most applications but the addition of a large value decoupling capacitor across the zener’s output may be required to give extra smoothing.
Zener diodes are always reverse biased. Therefore, Zener diodes can be used to design simple voltage regulator circuits to maintain a constant DC output voltage across the load regardless of input voltage fluctuations or load current fluctuations.  

The Zener voltage regulator consists of a current limiting resistor RS connected in series with the input voltage V_S and a Zener diode connected in parallel with this reverse biased load R_L. The regulated output voltage is always selected to be equal to the diode breakdown voltage V_Z.   

Conclusion

Zener diodes are essential components in voltage regulation circuits, offering a stable and controlled output voltage even in the presence of varying input voltage or load conditions. Their ability to conduct in reverse breakdown and maintain a constant voltage makes them highly useful in power supply applications. By selecting the appropriate Zener voltage and current-limiting resistor, a Zener diode can provide efficient voltage regulation for various electronic systems.

FAQ

1. What is the Zener voltage?
The Zener voltage (Vz) is the reverse breakdown voltage at which the Zener diode begins to conduct in reverse bias. This voltage remains nearly constant, even with variations in current.

2. How is a Zener diode different from a regular diode?
Unlike a regular diode that blocks current in reverse bias, a Zener diode allows current to flow once the reverse voltage exceeds its Zener voltage, maintaining a stable voltage.

3. What is the role of the series resistor in a Zener diode regulator circuit?
The series resistor limits the current flowing through the Zener diode, protecting the diode from excessive current and ensuring stable voltage regulation.

4. Can Zener diodes be used for high-power applications?
Zener diodes are typically used in low to medium power applications for voltage regulation. For high-power applications, other types of regulators may be more appropriate.

5. What happens if the current through a Zener diode is too low?
If the current is too low, the Zener diode will not be able to maintain its breakdown voltage, causing the output voltage to fluctuate and fail to regulate properly.