Understanding the Diode Datasheet

The article explains how semiconductor datasheets provide critical information such as electrical characteristics, operational limits, and application guidance for components like diodes. It emphasizes key parameters like reverse breakdown voltage, maximum forward current, forward voltage drop, and maximum reverse current, which are essential for circuit designers and maintenance personnel to ensure proper component selection and reliable operation.

A datasheet, or specification sheet, lists important parameters and operating characteristics for semiconductor devices. Also, essential information such as case styles, pinouts, testing procedures, and typical applications can be obtained from a component’s datasheet. Semiconductor manufacturers generally provide this information in data books or on the manufacturer’s website. This information can also be found on the Internet by companies that specialize in cross-referencing or component substitution.

Much of the information on a manufacturer’s datasheet is obscure and of use only to circuit designers. These specifications are useful to know when replacing or substituting for a failed device.

Reverse Breakdown Voltage

Let us start with the datasheet for a 1N4001, a rectifier diode used in power supplies (circuits that convert ac voltage to dc voltage). Figure 1 shows a datasheet for the 1N4001 to 1N4007 series of diodes: seven diodes that have the same forward characteristics but differ in their reverse characteristics. We are interested in the 1N4001 member of this family. The first entry under “Absolute Maximum Ratings” is this:

Symbol 1N4001
Peak Repetitive Reverse Voltage VRRM 50 V

 Example of a Diode Datasheet with Diagram - 1N4001–1N4007 Diode.

 Example of a Diode Chatacteristics Curves

Figure 1. Example of a Diode Datasheet – 1N4001–1N4007 Diode.

The breakdown voltage for this diode is 50 V. This breakdown occurs because the diode goes into avalanche when a huge number of carriers suddenly appears in the depletion layer. With a rectifier diode like the 1N4001, breakdown is usually destructive.

With the 1N4001, a reverse voltage of 50 V represents a destructive level that a designer avoids under all operating conditions. This is why a designer includes a safety factor. There is no absolute rule on how large to make the safety factor because it depends on too many design factors. A conservative design would use a safety factor of 2, which means never allowing a reverse voltage of more than 25 V across the 1N4001. A less conservative design might allow as much as 40 V across the 1N4001.

On other datasheets, reverse breakdown voltage may be designated PIV, PRV, or BV.

Maximum Forward Current

Another entry of interest is average rectified forward current, which looks like this on the datasheet:

Symbol Value
Average Rectified Forward Current @ TA=75°C IF(AV) 1 A

This entry tells us that the 1N4001 can handle up to 1 A in the forward direction when used as a rectifier. You will learn more about average rectified forward current in the next chapter. For now, all you need to know is that 1 A is the level of forward current when the diode burns out because of excessive power dissipation. On other datasheets, the average current may be designated as I0.

Good To Know

For critical safety reasons, failed semiconductors in circuits should normally be replaced with the exact specified parts.

Again, a designer looks upon 1 A as the absolute maximum rating of the 1N4001, a level of forward current that should not even be approached. This is why a safety factor would be included—possibly a factor of 2. In other words, a reliable design would ensure that the forward current is less than 0.5 A under all operating conditions. Failure studies of devices show that the lifetime of a device decreases the closer you get to the maximum rating. This is why some designers use a safety factor of as much as 10:1. A really conservative design would keep the maximum forward current of the 1N4001 at 0.1 A or less.

Forward Voltage Drop

Under “Electrical Characteristics” in Figure 1, the first entry shown gives you the following data:

Characteristic and Conditions Symbol Maximum Voltage
Forward Voltage Drop (IF) = 1.0 A, TA =25°C VF 1.1 V

As shown in Figure 1 on the chart titled “Forward Characteristics,” the typical 1N4001 has a forward voltage drop of 0.93 V when the current is 1 A and the junction temperature is 25°C. If you test thousands of 1N4001s, you will find that a few will have as much as 1.1 V across them when the current is 1 A.

Maximum Reverse Current

Another entry on the datasheet that is worth discussing is this one:

Characteristic and Conditions Symbol Maximum Voltage
Reverse Current IR
TA =25°C 10 μA
TA =100°C 50 μA

This is the reverse current at the maximum reverse dc-rated voltage (50 V for a 1N4001). At 25°C, the 1N4001 has a maximum reverse current of 10 μA. But notice how it increases to 50 μA at 100°C. Remember that this reverse current includes thermally produced saturation current and surface-leakage current. You can see from these numbers that temperature is important. A design that requires a reverse current of less than 10 A will work fine at 25°C with a 1N4001, but will fail in mass production if the junction temperature reaches 100°C.

Diode Datasheet Key Takeaways

Understanding the details outlined in semiconductor datasheets is crucial for engineers and technicians in various applications. These datasheets provide precise specifications like maximum voltage ratings, current capabilities, and thermal characteristics, ensuring components are used within safe operating limits. For instance, knowing the maximum forward current helps prevent overheating and component failure, while understanding reverse breakdown voltage ensures circuits are protected from voltage spikes. Such information is essential for designing robust circuits, troubleshooting issues, and ensuring long-term reliability in electronic systems across industries from power supplies to consumer electronics.