A Zener diode is designed to operate in reverse breakdown at a specified test current. It can clamp a node or provide a simple reference, but it is not an ideal voltage source. The actual voltage changes with current, device tolerance and temperature, so reliable circuits require a current-limiting element and worst-case calculations.
Forward and reverse operation
In forward bias, a Zener diode behaves broadly like another silicon PN-junction diode. Its defining use is in reverse bias. Below breakdown, only leakage flows. When the reverse voltage reaches the breakdown region, current can rise sharply while voltage changes more slowly.
ROHM's Zener diode overview explains the basic behavior and common voltage-regulation use. The nominal Zener voltage is specified at a particular test current. At a much lower current, the diode may be near its knee and regulate poorly; at a higher current, voltage rises because of dynamic resistance and the device dissipates more power.
Calculate the series resistance
A basic shunt regulator uses a series resistor from the supply to the Zener and load. The resistor must limit current at the highest input voltage and lightest load while still supplying enough Zener current at the lowest input voltage and heaviest load.
For a simple circuit, resistor current is approximately:
IR = (VIN − VZ) / R
The resistor current splits between the Zener and the load. Use worst-case input voltage, Zener tolerance, resistor tolerance and load current. Confirm that the minimum Zener current remains in the intended regulation region and that the maximum current does not exceed the diode or resistor limits.
Check Zener and resistor power
Zener power is approximately voltage multiplied by Zener current. Resistor power is current squared multiplied by resistance. Apply suitable margin and account for ambient temperature, PCB copper, airflow and package thermal behavior.
A diode's headline power rating may assume a defined lead length, pad area or ambient condition. Use the datasheet derating curve for the actual package. Short pulses may be permitted under a separate pulse-power curve, but repetitive events also affect average temperature.
Tolerance and dynamic resistance affect accuracy
The marked voltage usually has an initial tolerance. Dynamic resistance causes additional voltage change with current. The source impedance and load variation therefore determine the output regulation of a shunt circuit.
For precision references, low noise, wide load range or low standby current, a dedicated reference or regulator IC may be more suitable. A Zener remains useful when its accuracy and current consumption fit the application.
Temperature coefficient is not universal
Breakdown mechanism and temperature behavior vary with Zener voltage. Lower-voltage parts and higher-voltage avalanche parts can have different temperature coefficients, with values near the transition region often smaller. Use the curve or coefficient for the exact device instead of assuming that every Zener voltage falls with temperature.
Include self-heating: current changes junction temperature, which can shift breakdown voltage. Measure the circuit across the required ambient and operating range.
Clamping and protection applications
A Zener can limit a low-energy node, protect an input or create a bias voltage. Determine the available fault current and energy. For electrostatic discharge, lightning surge or other defined transients, a TVS diode designed and rated for the waveform may be more appropriate.
Placement and current return paths matter. Keep the protection path short and prevent the clamped current from flowing through sensitive ground or signal conductors.
Selection checklist
- Define the desired clamp or reference voltage and tolerance.
- Calculate minimum and maximum diode current.
- Check breakdown voltage at the relevant current.
- Include dynamic resistance and temperature coefficient.
- Verify steady-state and pulse power with derating.
- Confirm package, polarity, footprint and assembly process.
- Test the circuit at supply, load and temperature limits.
For sourcing, provide the full manufacturer part number, voltage, tolerance, power, package and qualification requirements. Browse our diode sourcing page or submit the bill of materials through our contact page.
