Diodes

Introduction to Diodes: Basics, Types, Characteristics, Applications & Packages

Introduction

What is a diode? - The basic premise of a diode is to control the direction of current-flow as current can only flow in one direction. This is when the diode is forward biased, it allows current to flow in the natural direction.

For the average person, it’s easy to explain diodes as a “one way valve” where

If voltage of the anode > voltage of cathode, diode is “a short circuit”
Voltage of Cathode > Voltage of Anode, diode is reverse-biased and “open circuit

This is “kinda” true, but of course that’s a very simplistic model.

Common applications of diodes - Protecting other parts of circuit, making voltage regulators, rectifiers

Brief theory / how it actually works - https://www.electronicshub.org/semiconductor-diodes/

Diode Terminology and Parameters:

Forward Voltage – The amount of voltage required for current to flow through a diode

Maximum forward current – The maximum forward current that can flow through the device. If it is exceeded, it the diode will fail as it begins to heat up

Peak Inverse Voltage - The max amount of reverse voltage before the diode will break down

Total Power Dissipation – Maximum power dissipation of diode when it is forward biased

Maximum operating temperature – The maximum temperature allowable before the diode starts to stop working as intended

Reverse leakage current – how much current will leak when in reverse, temperature has a big role on this time

Reverse recovery time – The time when the diode switches from conducting state to the non conducting state

Reverse Breakdown – How much current it will allow to pass when in reverse mode

Zener voltage – is the predetermined voltage where the diode breaks down and current starts flow through the diode

Zener resistance – Zener diodes exhibit some resistance so be mindful of this when using them

Clamping Voltage (VC) - The clamping voltage is the voltage limited by the TVS diode in the event of a transient within the limit of the specified peak current.

Rated Standoff Voltage (VWM) - This indicates the limit within which the TVS diode will operate normally. Within the Rated Standoff Voltage, the TVS diode has a high impedance with only a small amount of leakage current.

Peak Pulse Power Dissipation (PPP) - The TVS diode needs to be able to safely dissipate the excessive current caused by the transient voltage. This is indicated by the Peak Pulse Power Dissipation.

Diode Configurations

Forward biased diodes (TBA)

When a voltage across the diode is applied it will allow the current to flow through the diode and only drop a small voltage

Reverse biased diodes

When the voltage across a diode is negative, ideal diodes will behave like an open circuit. It should be noted that there is actually a small amount of current that goes through one of these (called the leakage current) but it is typically ignored and will depend on the application.

When using a reverse biased diode, if the reverse bias voltage is too large the diode will breakdown

When using a reverse biased diode, a term that will typically come up is the Peak Inverse Voltage (PIV) {also known as the peak reverse voltage or simply the reverse voltage}. This value is the maximum reverse voltage that the diode can withstand before it will breakdown. Be mindful of this if you are using this in your boards!

Common types of diodes

Signal diodes

These are your typical low-cost diodes that have a medium forward voltage drop and a low max current rating.
Typical use cases - Protect other parts of the circuit from high voltages, also can be used as a flyback diode when used with a relay (should be connected in reverse direction) to protect the circuit from fluctuations
Common signal diodes; 1N4148, 1N914

Rectifier Diodes (Sometimes called power diodes)

A standard diode but it has a much higher max current but has a larger forward voltage
Some common applications:
- Rectify AC current to DC current (converting AC to DC)
Common rectifier diodes; 1N4001,

Schottky Diodes

A diode which has a low forward voltage and has fast switching actions

Common use cases

Reverse current protection
Can be used as rectifiers in switched mode power supplies
Useful to prevent transistor saturation

Advantages

Fast recovery time, it can switch between non conducting and conducting incredibly quickly
It will typically produce less noise than a typical diode
It will require less power than a standard diode so it will work for low voltage applications because of the low forward voltage

Parameters to consider

Forward voltage drop
Reverse leakage current
Reverse recovery time
Reverse Breakdown
Capacitance – Schottky diodes have a small capacitance, which is important for some applications
Operating temperature

Common Schottky diodes; 1N5711, 1N6263,

Zener Diodes

A diode that allows current to flow in either forward or reverse direction which depends on a certain voltage (Zener voltage). Zener diodes are typically connected in the reverse mode.

Zener diode parameters/metrics:

Zener voltage
The max current, the power rating, operating temperature
Zener resistance

Applications

Voltage regulator – load voltage equals the break down voltage so when a resistor is connected in series it will limit the current through the diode helping to alleviate the excess voltage

Over voltage protection – When input voltage is higher then Zener voltage, a short circuit will open up a fuse to disconnect the load from the supply. Alternatively, they can be used to limit the overvoltage to an acceptable value, especially for transient voltage suppression (TVS). For more about TVS read Designing Your PCB for Transient Voltage Suppression This will provide over voltage protection, but doesn’t require the fuse to open. The excess power will be dissipated by the diode, so the diode may be destroyed before the fuse opens (depending on the size of the diode and the current rating of the fuse). In some cases, a resistor can be added in series with the Zener diode, which will dissipate some of the power and limit the current, which will protect the diode.

Advantages

Can be low cost
Usable in small circuit
Good for over voltage protection

Example Circuit

Additional resources

Zener as a voltage regulator

Avalanche diode

A diode that is designed to break down at the avalanche breakdown (like how Zener diodes breakdown at the Zener voltage)

Applications

Used for voltage references because they are less affected by changing currents
Similar to the Zener diode it is typically used to protect components from high voltages. Its connected in the reverse biased mode so hat when the voltage goes to high, the avalanche effect takes place grounding the high voltage and saving the other parts of the circuit

Parameters

Forward voltage, breakdown voltage, reverse voltage

Other less commonly used diodes

Photodiodes

https://www.electronicshub.org/photodiode-working-characteristics-applications/

https://www.teamwavelength.com/photodiode-basics/

Some other examples of when to use diodes

Flyback diodes

Placed in reverse polarity from the power supply and in parallel to the relay’s inductance coil. The use of a diode in a relay circuit prevents huge voltage spikes from arising when the power supply is disconnected.