Zener Diode Applications: Circuit Protection

Key Takeaways

• The Zener diode can safely conduct in the reverse bias direction, unlike the standard diode.

• The Zener diode’s ability to operate in the reverse bias direction lends itself to circuit protection applications.

• A Zener diode is also suitable for waveform reshaping or voltage sensing.

Zener diode applications leverage the ability of the diode to conduct bidirectionally.

Diodes are the most fundamental semiconductor component, constructed of a single p-n junction. Unsurprisingly, many variations of this simple component build on its central function as a polarized conductor. A diode strongly impedes current flow from cathode to anode until breakdown voltage conditions modify the underlying material, but this is not true for all diodes. The Zener diode is an exception that intentionally allows current flow under reverse bias conditions, provided this voltage meets a minimum intrinsic value to overcome standard reverse bias impedance. Because of this selectively backward conductance, Zener diode applications can protect components with rudimentary circuit control.

Comparing Zener and Standard Diode Operating Regions

What Is a Zener Diode?

The Zener diode is a special class of diodes. Like a standard diode, its I-V curve is similar in the breakdown, reverse-biased, and forward-biased regions. It differs in operation: in the water analogy, diodes are one-way valves that control the current flow with an idealized zero resistance in the forward direction and infinite resistance in reverse. Of course, neither resistance value is practically achievable; instead, it’s acceptable to consider the diode conducting or nonconducting in the forward or reverse direction (respectively) unless circuit conditions affect the material properties of the device. Consider the extremes of voltage biasing on a standard diode for either end of its operating spectrum:

• At large negative voltages greater than the reverse bias conditions (in the absolute value sense), reverse breakdown occurs. In this region, the diode conducts current much like it was forward-biased. However, the process effectively undoes the doping of the p-n junction, irreversibly damaging or destroying the device. 

• At large positive voltages, the material resistance of the diode dominates, and the forward bias region's exponential curve bends toward the device's bulk resistance. Conduction is less than that of the forward-biased region.

Therefore, diode usage where large negative voltages may develop across the terminals can be inefficient or dangerous depending on the circuit configuration. The Zener diode addresses the breakdown region concern by allowing for a reverse current flow once the minimum reverse voltage (known as the Zener voltage) develops, and its construction can safely withstand a wide range of operating currents. Functionally, the Zener voltage acts like the threshold voltage in the forward-bias region, enabling bidirectional (but not simultaneous) current flow after meeting either voltage minimum. The reverse operation of the Zener diode and associated voltage are similar to an avalanche diode – both Zener and avalanche effects are present in either diode type – with the Zener voltage occurring at a lower threshold. 

The Multitude of Zener Diode Applications

The Zener diode offers additional functionality over the standard diode applications. As mentioned, Zener diodes can hold a steady voltage for a wide range of currents when reverse-biased – this quality makes them highly suitable for use as regulators. A simple circuit model uses a resistive load on the cathode end (the negative side) of the Zener diode and the input and output voltages to determine current through the device. Even as the voltage fluctuates on the input, the output voltage remains steady, with the resistance value ensuring a minimum reverse current such the diode remains in reverse breakdown operation without exceeding maximum power ratings. With the resistance properly calibrated, any load parallel to the Zener diode sees a constant voltage.

This characteristic offers tremendous possibilities. Consider some of the common uses of Zener diodes in circuit design:

• Waveform clipper/Zener limiter - A variation of the above model that uses two Zener diodes in series with their cathode ends facing. The configuration limits the voltage maximum for both signal polarities, protecting against positive and negative overvoltage conditions. Alternatively, the clipping action allows for waveform reshaping, typically transforming sinusoidal waves into squares.

• Voltage sensing - A useful functionality for circuit actuation and measurement, voltage sensing can be as simple as a resistor and Zener diode network or incorporate transistors. Moderate-to-high voltage sensing creates larger error bars during measurement, but a Zener diode can cut down on this error by reducing the range of the reading. For example, an input voltage near 30 volts can use a 20V Zener diode to cut the measurement range down to 10V – higher voltage Zener diodes further cut down on the range and improve accuracy at the potential cost of fluctuations exceeding the measurement levels.

• Overvoltage and circuit protection - Input signals can experience perturbations leading to transients that increase the voltage over safe levels. By selecting an appropriate Zener diode at a voltage slightly above the intended operating voltage, circuit designers can create something analogous to a pressure relief valve. After a signal passes through a current-limiting resistor, it will continue to its input pin if it is below the specified voltage level. However, should the signal exceed safe levels for the input pin, it meets the reverse voltage of the Zener diode and finds a lower resistance path to ground.

Cadence Solutions Combine Safety and Performance in Design

Zener diode applications are vast due to the device’s unique ability to selectively conduct bidirectionaly, making it highly suitable for protecting vulnerable circuitry. Safely regulating voltage protects both equipment and users from what can be dangerous overvoltage conditions; as with all safety measures, simulation and modeling before physical confirmation of compliance is key. To that end, Cadence offers premier simulation features with its  PCB Design and Analysis Software suite that improve product outcomes while accelerating development. By seamlessly integrating simulation results into the fast and powerful OrCAD PCB Designer, ECAD has never been easier.

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