When choosing a voltage regulator (also known as a Zener diode), multiple key factors need to be considered to ensure its stable, reliable, and safe operation in practical circuits. Here are some core issues that need to be noted:
Core parameters: Basic requirements
Nominal voltage regulation value (Vz):
This is the primary consideration factor. You must choose a voltage regulator that meets your required reference voltage or clamping voltage requirements.
Note: The voltage regulation value is given at a specific test current (usually labeled Iz in the data manual). The actual working current will affect the accuracy of the voltage regulation value.
Rated power consumption (Pz) or maximum operating current (Iz max):
The voltage regulator consumes power (P=Vz Iz) during operation. It must be ensured that the power consumption generated by the working current Iz does not exceed its maximum rated power consumption (Pz max).
Calculate maximum allowable current: Iz max=Pz max/Vz`
Key point: Sufficient power margin must be left! The actual design power consumption should be much lower than Pz max (e.g. reduced to 50% -70% for use) to cope with factors such as power fluctuations, load changes, and environmental temperature increases, ensuring long-term reliability.
Voltage stabilization accuracy (tolerance):
The voltage regulation value is not absolutely accurate and there is a tolerance range (such as ± 5%, ± 2%, ± 1%). Select the appropriate accuracy level based on the voltage accuracy requirements of the application. Precision applications require smaller tolerances.
Dynamic characteristics: Performance performance
Dynamic impedance (Zzt or Zz):
Measure the ability of a voltage regulator to maintain voltage stability during current changes. The smaller the impedance, the smaller the voltage fluctuation when the current changes, and the better the voltage stabilization performance.
Dynamic impedance is usually given in the data sheet and measured at specific test currents. This value will vary with the operating current (usually the larger the current, the smaller the Zz).
For applications that require high voltage stabilization accuracy, such as precision reference sources, low Zz voltage regulators should be selected.
Temperature coefficient (TC):
Indicate the degree to which the voltage regulation value varies with temperature (such as mV/° C or%/° C).
Voltage regulators with a voltage stabilization value less than about 5.6V typically have a negative temperature coefficient (as the temperature increases, Vz decreases); Voltage regulators with a voltage greater than about 5.6V typically have a positive temperature coefficient (as the temperature increases, Vz increases). The temperature coefficient of the voltage regulator tube is the smallest near 5.6V.
In applications with a wide temperature range or high requirements for temperature stability, it is necessary to choose a voltage regulator with a small temperature coefficient or consider using a dedicated voltage regulator with temperature compensation.
Working conditions: environment and condition
Operating current range (Iz min Iz max):
Minimum operating current (Iz min/Knee Current): The voltage regulator needs to reach the minimum current required to achieve its nominal voltage regulation value. Below this current, the voltage stabilization effect deteriorates sharply, and the voltage will be significantly lower than Vz. When designing, it is necessary to ensure that the current flowing through the voltage regulator is greater than Iz min.
Maximum operating current (Iz max): determined by the rated power consumption Pz max and Vz (see point 1). The actual working current must be less than Iz Max.
When designing a current limiting resistor, it is necessary to ensure that the current flowing through the voltage regulator remains between Iz min and Iz max throughout the entire range of input voltage fluctuations and load changes.
Input voltage range (Vin):
The input voltage must be higher than the required voltage regulation value Vz (usually at least a few volts higher) to ensure sufficient voltage difference to drive current through the voltage regulator and current limiting resistor.
At the same time, the power consumption generated at the highest input voltage (calculated through a current limiting resistor) cannot exceed Pz max.
Working environment temperature (Ta):
The rated power consumption Pz max of the voltage regulator is usually given at 25 ° C or a specific temperature. As the ambient temperature increases, the maximum allowed power consumption will decrease (power derating curves are usually provided in data manuals).
Working in high temperature environments, it is necessary to recalculate the actual allowable maximum power consumption and maximum current based on the derating curve, and carry out stricter power derating design.
Reliability and Protection: Safe Operation
Failure mode: Understand the possible failure modes of the voltage regulator (usually short circuit, but may also be open circuit when overvoltage/overcurrent is severe). Designing protective circuits (such as fuses) requires consideration of the impact of their failure modes on other components.
Reverse leakage current (Ir): When the voltage is below Vz, the voltage regulator will still have a small reverse leakage current. In extremely high impedance or low-power applications, this current may become an error source.
Transient response and noise: For high-speed or noise sensitive applications, attention should be paid to the response capability of voltage regulators to fast transients and the noise level generated by them (Zener noise).
Current limiting resistor (Rs): This is a necessary component connected in series with a voltage regulator. Its function:
Limit the maximum current passing through the voltage regulator to prevent overcurrent burnout.
Share the voltage difference between the input voltage and the regulated value.
The resistance value must be carefully calculated to ensure that at the lowest input voltage and maximum load current, Iz > Iz min； At the highest input voltage and minimum load current (or no-load), Iz<Iz max (and power consumption<Pz max).
Summary of design suggestion steps:
Clear requirements: Determine the required voltage regulation value Vz, load current range, input voltage range (Vin min, Vin max), working environment temperature, and accuracy requirements (tolerance, temperature drift).
Preliminary selection of voltage regulator: Select models based on Vz and accuracy requirements.
Refer to the data manual: Obtain key parameters: Vz (and tolerance), Pz max, Iz max, Iz min (or Knee Current), Zz, TC. Pay special attention to the influence of temperature on Pz max.
Calculate the current limiting resistance (Rs):
Satisfy the condition of 'Iz>Iz min' (worst-case scenario: Vin min, maximum load current, minimum current flowing through the voltage regulator): 'Rs<(Vin min - Vz)/(Iz min+I2 load max)'`
Satisfy the conditions of 'Iz<Iz max (after derating)' and 'Pz<Pz max (after derating)' (worst-case scenario: Vin max, minimum load current or no load, maximum current flowing through the voltage regulator): 'Rs>(Vin max - Vz)/Iz x_rated' (Iz x_rated is the maximum allowable current considering derating)
Select a standard resistance value within the above two constraint ranges and calculate its rated power (power consumption of (Vin max - Vz) ²/Rs), which also needs to be downgraded for use.
Verify power consumption: Under the conditions of maximum input voltage and minimum load, calculate the actual power consumption of the voltage regulator, Pz=Vz ((Vin max - Vz)/Rs), ensuring that Pz<Pz x_rated (maximum power consumption after derating).
Consider dynamic performance and noise: If the application requires high speed or low noise, evaluate whether the Zz and noise characteristics of the selected voltage regulator meet the requirements.
Consider layout and heat dissipation: If power consumption is high, pay attention to PCB layout heat dissipation, and if necessary, choose power packaging or increase heat dissipation measures.
By systematically considering the above factors and carefully consulting the component data manual for parameter verification and calculation, you can select a suitable, reliable, and safe voltage regulator diode for your application.