The selection of a voltage regulator diode (Zener diode) is a process that requires comprehensive consideration of multiple parameters. Here are some key selection parameters:
Core selection parameters of voltage regulator diode:
Nominal Zener Voltage (Vz):
Definition: This is the voltage value at which a diode breaks down and begins to stabilize under a specified test current. This is the most crucial parameter.
Selection key points: Select accurately based on the stable output voltage required by the circuit. Vz usually has standard values (such as 3.3V, 5.1V, 6.2V, 9.1V, 12V, 15V, 24V, etc.), and there are also different levels of accuracy (such as ± 1%, ± 2%, ± 5%, ± 10%). High precision applications (such as reference sources) require the selection of smaller tolerances.
Rated power consumption/power dissipation (Pz):
Definition: The maximum power that a voltage regulator can continuously dissipate without damage, usually measured in watts (W) or milliwatts (mW). Calculation formula: Pz=Vz Iz (max), where Iz (max) is the maximum allowable operating current.
Key selection points: crucial! It must be ensured that under actual working conditions (considering input voltage range, load variation range, ambient temperature), the actual dissipated power of the voltage regulator (Vz Iz) is less than and leaves sufficient margin for Pz. It is usually recommended to reduce the usage by 50% -70% to improve reliability. The power demand determines the packaging size and heat dissipation requirements.
Voltage regulation tolerance/accuracy (tolerance on Vz):
Definition: The allowable deviation range for nominal Vz (e.g. ± 5%).
Selection criteria: Choose according to the circuit's requirements for output voltage accuracy. Precision voltage regulation and reference sources require high precision (± 1% or better), while ± 5% may be sufficient for ordinary voltage regulation applications. Note that the tolerance is the value at a specific test current.
Test Current (Izt):
Definition: A specific current value used by manufacturers to measure and calibrate the nominal voltage regulation value Vz.
Key selection points: Understand that Vz is defined under this specific current. The actual working current should be as close as possible to Izt to achieve the best voltage stabilization effect (Vz is closest to the nominal value). The farther the working current deviates from Izt, the more the actual Vz may deviate from the nominal value (due to the influence of dynamic resistance).
Dynamic resistance/Zener impedance (Zzt or Rd):
Definition: Near the working point (usually at Izt), the ratio of the change in voltage regulation value Vz (Δ Vz) to the change in current that causes the change (Δ Iz), i.e. 'Zzt=Δ Vz/Δ Iz'. The unit is ohms (Ω). It reflects the ability of the voltage regulator to maintain voltage stability.
Key selection points: Key dynamic performance parameters! The smaller the Zzt, the better, which means that the output voltage fluctuation is smaller when the current changes, and the voltage stabilization effect is better. Low voltage regulators (such as<5V) typically have greater dynamic resistance than high voltage regulators. In applications with large load changes or high requirements for voltage ripple, models with smaller Zzt should be chosen.
Temperature Coefficient (TC or α z):
Definition: Represents the ratio of the voltage regulation value Vz to the ambient temperature, usually expressed in%/° C or mV/° C.
Key selection points:
Strong voltage dependence: around 5V-6V, with a temperature coefficient close to zero (due to the combined effects of Zener and avalanche effects). Below 5V is usually a negative temperature coefficient (Vz decreases with increasing temperature), while above 6V is usually a positive temperature coefficient (Vz increases with increasing temperature).
High precision or wide temperature range applications (such as automotive electronics, industrial equipment) must carefully consider TC. A 5.1V/5.6V tube close to zero TC can be selected, or a special voltage regulator with temperature compensation can be used.
Maximum current (Izm) and minimum current (Izk):
Izmax: determined by the rated power Pz: ` Izmax=Pz/Vz `. The actual working current must be less than Izmax.
Izk (or Izk Knee Current): The minimum current required to maintain the operation of the voltage regulator in the breakdown region (stabilized state). Below this current, the voltage regulator cannot effectively stabilize the voltage, and Vz will significantly decrease and become unstable.
Key selection points: Ensure that the current flowing through the voltage regulator is greater than Izk under all working conditions, especially under light or no-load conditions; Under the most adverse conditions (high input voltage, full load, or short circuit), the current flowing through the voltage regulator is less than Izmax. Usually, a series current limiting resistor is required to ensure the current range.
Reverse Leakage Current (Ir):
Definition: The reverse current flowing through a diode when the applied reverse voltage is lower than the nominal Vz.
Key selection points: In low-power applications (such as battery powered devices) or when the voltage is close to but does not reach Vz, excessive leakage current can lead to unnecessary power consumption. Choosing models with low Ir can improve efficiency.
Packaging and heat dissipation:
Definition: The physical appearance of a voltage regulator (such as DO-41, DO-15, SOD-123, SOT-23, TO-220, etc.).
Key selection points:
The packaging determines the power processing capability, size, and heat dissipation method.
Small power applications can choose small patch packaging (such as SOD-123).
When the power is high (>0.5W-1W), it is necessary to choose a package with heat sink or suitable for installation on the heat sink (such as TO-220), and design a reasonable PCB copper foil heat sink or external heat sink.
Consider assembly process (through-hole vs surface mount).
Summary of selection logic:
Fixed voltage: Determine the required stable voltage Vz.
Precision setting: Select Vz tolerance according to application requirements.
Calculate power: Analyze the power that the voltage regulator needs to withstand under the worst-case conditions (highest input voltage, minimum load, or short circuit), denoted as' Vin_max - Vz 'or' Iz_max '. Based on this, select a model with sufficient Pz and use it with a reduced rating.
Current testing range: Ensure that the current is greater than Izk under light load and less than Izmax under heavy load/short circuit (calculated from Pz/Vz). Design a current limiting resistor.
Check dynamic performance: If the load changes significantly or is sensitive to voltage ripple, choose a model with a smaller Zzt (dynamic resistance).
Check temperature impact: If the working temperature range is wide or the accuracy requirement is high, carefully examine the temperature coefficient TC, and prioritize selecting voltage points or temperature compensation types close to zero TC.
Consider power consumption and leakage: Low power applications focus on Ir (leakage current).
Package selection: Choose the appropriate package based on power, size, heat dissipation, and process requirements.
Refer to the manual: Finally, it is necessary to consult the detailed data manual of the selected model to confirm that all parameters meet the requirements under the application conditions, and pay attention to the testing conditions.
By systematically evaluating these parameters and combining them with specific circuit operating conditions (input voltage range, load current range, ambient temperature, accuracy requirements, power consumption limitations, space limitations, etc.), you can choose the appropriate voltage regulator diode for your application.