A Zener diode uses the reverse breakdown of a heavily doped PN junction to maintain an almost constant voltage across its terminals. This behavior sets it apart from a signal or rectifier diode, for which reverse breakdown is a failure mode. Here, reverse breakdown is the normal operating mode.
Dynamic resistance and test current: what the datasheet doesn’t tell you clearly enough
The Zener voltage listed on a datasheet corresponds to a specific test current. Away from this operating point, the actual voltage can differ significantly from the nominal value.
Dynamic resistance explains this difference. In the breakdown region, any change in the current flowing through the diode causes a proportional change in the voltage across it. The lower this resistance, the tighter the regulation.
At low current, around the knee of the characteristic curve, regulation deteriorates significantly. A Zener biased at one-tenth of its nominal test current may have a voltage noticeably below the expected value. We recommend always checking the component’s full current-voltage curve rather than relying on a single tabulated value.
An article detailing how a Zener diode works confirms that dynamic resistance must be evaluated at the circuit’s actual operating point, not just under the manufacturer’s nominal conditions.
Zener diode vs. TVS diode: two components often confused
A common mistake is to use a standard Zener to protect a circuit against fast transients. A regulation Zener and a TVS diode are not interchangeable.
A Zener is characterized for continuous operation: its nominal voltage, power dissipation, and dynamic resistance are specified under static or quasi-static conditions. A TVS, by contrast, is specified for short-duration pulses, with a clamping voltage and waveform defined according to specific immunity standards.
Criteria for choosing between a Zener and a TVS
- A Zener is suitable for voltage regulation, voltage references, and biasing active components in steady-state operation
- A TVS is designed to absorb transient energy spikes (ESD, switching surges) without exceeding its clamping voltage during the pulse
- The selection parameters differ: choose a Zener based on its nominal voltage and continuous power rating, and a TVS based on its standoff voltage, clamping voltage, and ability to dissipate the energy of a standardized pulse
Using a Zener where a TVS is required exposes the circuit to an inadequately controlled clamping voltage and risks rapid component failure if the pulse exceeds its continuous power rating.
Designing a shunt regulator circuit with a Zener diode
The shunt regulator is the most common application of the Zener diode. The diode is placed in parallel with the load, while a series resistor limits the current from the power supply.
Parameters to determine when sizing the circuit
The series resistor must be calculated so that the current through the Zener stays within its allowable current range: above the minimum regulation current (the knee of the characteristic curve) and below the maximum current set by power dissipation.
The regulated output voltage is the Zener voltage, minus the voltage drop across the dynamic resistance multiplied by the change in current. The more current the load draws, the less current flows through the Zener, and vice versa. If the load is disconnected, all the current flows through the Zener: this is the worst-case scenario for thermal dissipation.
The power dissipated by the Zener with no load determines the component’s power rating. A Zener in an SOD-323 package cannot dissipate as much power as one in a DO-41 package. Undersizing for this condition causes excessive heating, which changes the breakdown voltage and can destroy the component.
Temperature coefficient and Zener voltage stability
A Zener’s thermal behavior is not a minor detail. Heating changes the breakdown voltage, and the direction of this drift depends on the dominant physical mechanism.
For diodes with a nominal voltage below about five volts, the pure Zener effect (tunneling) dominates. The temperature coefficient is then negative: the voltage decreases as the temperature rises. Above this value, avalanche breakdown takes over, with a positive coefficient.
Around five volts, the two mechanisms partially offset each other, resulting in components with very low thermal drift. This is why voltage reference diodes are often specified in this range.
Applications in voltage references
In analog-to-digital conversion circuits or precision power supplies, reference stability determines the system’s overall accuracy. A Zener selected in the temperature-compensation region, combined with a stable bias current, provides a usable voltage reference without an external compensation circuit.
For applications where the remaining drift is too high, integrated voltage references (bandgap references) are preferred, but Zeners remain a practical choice in cost-conscious discrete circuits.
Choosing a Zener diode depends less on its nominal voltage than on all its parameters at the actual operating point: dynamic resistance, test current, power dissipation, and temperature coefficient. A component correctly sized for its thermal and electrical conditions can provide reliable regulation for decades. Overlooking even one of these parameters can turn a regulator into a source of intermittent faults.
