Other diodes: Diode types
Datasheets state a variety of different parameters or specifications for Zener diodes.
When selecting a suitable Zener voltage reference diode for any given position in a circuit, it is necessary to ensure that it will fulfil its requirements. Understanding the datasheet specifications is key to selecting a suitable device.
Zener IV characteristics
The IV characteristic of the Zener / voltage reference diode is the key to its operation. In the forward direction, the diode performs like any other, but it is in the reverse direction where its specific performance parameters can be utilised.
Zener diode specifications
When looking at the specification sheet for a Zener diode there are several parameters that will be included. Each details a different aspect of the Zener voltage reference diode performance. Looking at each different characteristic it is possible to understand the performance of the diode and ensure it will operate correctly in any given circuit.
- Voltage Vz: The Zener voltage or reverse voltage specification of the diode is often designated by the letters Vz. Voltages are available over a wide range of values, often following the E24 ranges, although not all diodes are bound by this convention.
Values generally start at around 2.4 V although not all ranges extend as low as this. Values below this are not available. Ranges may extend top anywhere in the region of 47 V to 200 V, dependent upon the actual Zener diode range. Maximum voltages for SMD variants are often around 47 V.
- Current : The current, IZM, of a Zener diode is the maximum current that can flow through a Zener diode at its rated voltage, VZ.
Typically there is also a minimum current required for the operation of the diode. As a rough rule of thumb, this can be around 5 to 10 mA for a typical leaded 400 mW device. Below this current level, the diode does not break down adequately to maintain its stated voltage.
- Power rating: All Zener diodes have a power rating that should not be exceeded. This defines the maximum power that can be dissipated by the package, and it is the product of the voltage across the diode multiplied by the current flowing through it.
For example many small leaded devices have a dissipation of 400mW at 20°C, but larger varieties are available with much higher dissipation levels. Surface mount varieties are also available, but generally have lower dissipation levels in view of the package size and their ability for heat removal.
Common power ratings for leaded devices include 400mW (most common), 500 mW, 1W, 5W. Values for surface mount devices may be around 200, 350, 500 mW with occasional devices extending up to 1 W.
- Zener resistance Rz: The IV characteristic of the Zener diode is not completely vertical in the breakdown region. This means that for slight changes in current, there will be a small change in the voltage across the diode. The voltage change for a given change in current is the resistance of the diode. This value of resistance, often termed the resistance is designated Rz. The inverse of the slope shown is referred to as the dynamic resistance of the diode, and this parameter is often noted in the manufacturers' datasheets. Typically the slope does not vary much for different current levels, provided they are between about 0.1 and 1 times the rated current Izt.
- Voltage tolerance: With diodes being marked and sorted to meet the E12 or E24 value ranges, typical tolerance specifications for the diode are ±5%. Some datasheets may specify the voltage as a typical voltage and then provide a maximum and minimum.
- Temperature stability: For many applications, the temperature stability of the Zener diode is important. It is well known that the voltage of the diode varies according to temperature. In fact the two mechanisms that are used to provide breakdown within these diodes have opposite temperature coefficients, and one effect dominates below about 5 Volts and the other above. Accordingly diodes with voltages around 5 V tend to provide the best temperature stability.
It can be seen in the example given that there is a noticeable difference between the specification for the Zener diode reverse voltage at 0°C and 50°C. This needs to be taken into account if the circuit and equipment in which the Zener diode is to be used is subject to temperature change.
- Junction temperature specification: In order to ensure the reliability of the diode, the temperature of the diode junction is key. Even though the case may be sufficiently cool, the active area can still be very much hotter. As a result, some manufacturers specify the operating range for the junction itself. For normal design, a suitable margin is normally retained between the maximum expected temperature within the equipment and the junction. The equipment internal temperature will again be higher than the temperature external to the equipment. Care must be taken to ensure that individual items do not become too hot despite there being an acceptable ambient temperature outside the equipment.
- Package: Zener diodes are specified in a variety of different packages. The main choice is between surface mount and traditional leaded devices. However the package chosen will often define the package heat dissipation level. The choices available will be detailed in the Zener diode datasheet specification.
Example Zener diode datasheet characteristics
To give some idea of the datasheet characteristics to be expected from a Zener diode a real example is provided below. The main parameters that would be needed in a circuit design are given.
- BZY88 leaded Zener diode This diode is described as a miniature Zener diode for regulated power supply circuits, surge protection, arc suppression and other functions in a variety of areas. The 5V1 (5.1 Volt) version has been taken as an example.
|Typical BZY88 Zener Diode Characteristics / Specifications|
|DC Power dissipation||400||mW||@ Tl = 50°C: de-rate above 50°C 3.2 mW / °C|
|Junction temperature||-65 to +175||°C|
|Voltage Vz @ 5mA||4.8 min
|IR @VR||1 @ 2.0||µA|
The datasheet parameters given for this common Zener diode give a useful indication of specification given for a Zener diode. Although they are only for a small diode, the same sorts of data are given for other Zener diodes as well.
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