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Do You Know Anything About Fiber Optic Coupler?

Do You Know Anything About Fiber Optic Coupler?

November 10 , 2023 0

I.Optical Semiconductor Type

Common types of optical semiconductors are as follows:

(1) Light emitting devices · · Visible LED, infrared LED, ultraviolet LED, laser diode
(2) Light receiving devices · · Light sensors, solar cells, CMOS sensors
(3) Composite devices (combination of light-emitting elements and light-receiving elements) · · Optocoupler and fiber coupler

II.The Luminous Principle OF LED

The Luminous Principle OF LED

The luminous principle of a light-emitting diode (LED) is to apply a forward current to the pn junction of a compound semiconductor.
When a forward current passes through a light-emitting diode, charge carriers (electrons and holes) move. The holes in the P-type region move to the N-type region, and the electrons in the N-type region move to the P-type region. The injected carriers recombine, and the energy difference before and after recombination will be released in the form of light. The emitted light depends on the energy gap (Eg) of the compound semiconductor.
(Note: conventional silicon diodes do not emit light because the recombination energy is turned into heat.)

III.Wavelength Range OF LED

Leds emit different wavelengths of ultraviolet light and even infrared light. The emission wavelength will be represented by the equation below using the compound semiconductor material energy gap (Eg).

λ (nm) = 1240/Eg (eV)

Materials with larger energy gaps emit shorter wavelengths, and materials with smaller energy gaps emit longer wavelengths.
For infrared leds used in TV remote controls, etc., GaAs (Gallium arsenide) materials are used; For red/green indicator leds, use GaP (Gallium phosphide) or InGaAlP (indium gallium phosphide); For blue leds, use InGaN (indium gallium nitride) or GaN (Gallium nitride).

Different materials LED light color (by material)

IV.What Is Optocoupler?

Optocoupler is a device that integrates light-emitting diodes (leds) and photodetectors in a single package. Unlike other optical devices, light is not emitted outside the package. It looks similar to a non-isolator/solid-state relay. Although the optocoupler is an optical device, it does not deal with light, but with electrical signals.
Example operation of optocoupler:
(1) The LED is on (0⇒1).
(2) LED light enters the phototransistor.
(3) The phototransistor is switched on.
(4) Output voltage change 0⇒1.

(1) LED off (1⇒0).
(2) LED light stops entering the phototransistor.
(3) The phototransistor is off.
(4) Output voltage change 1⇒0.

V.Why Do You Need Optocouplings? The Responsibility OF Optocoupler

In an optocouple, the primary side (LED side) and the secondary side (light receiving device side) are electrically insulated. Therefore, even if the potentials (even GND potentials) of the primary and secondary sides are different, the electrical signal of the primary side can be transmitted to the secondary side.
In the inverter application shown on the right, the control unit (such as a microcontroller) usually operates at low DC voltage. On the other hand, IPM and IGBT will drive high voltage loads (such as 200V AC). High voltage system components can be controlled directly by a microcontroller via couplers.

VI.Type OF Optocoupler

Leds are used for optocoupler input. On the other hand, there are various devices available for output.

Transistor output
A phototransistor is a detector. The Darlington type can also be used.
IC output
We have products for photodiodes as light receiving devices, output products such as logic, high-current output products for IGBT and MOSFET gate drives, and high-function products such as isolation amplifiers.
Bidirectional thyristor/thyristor output
Photothyristor or photothyristor is used for output. They are mainly used for AC line control.
Optical relay (MOSFET output)
The photovoltaic array (photodiode array) drives the gate of the MOSFET to turn the output on/off. With this operation, it can be used as a relay switch for the MOSFET output.

VII.Type OF Optocoupler (package)

The optocoupler must have a package shape and dielectric strength that meets safety standards. When designing according to safety standards, the following items need to be checked.

Insulation creepage distance
The shortest distance between two conductors along the insulator surface (primary and secondary sides).
gap
The shortest distance between two conductors measured by air.
Insulation thickness
The minimum distance of an insulator between two conductors.
Isolation voltage
The isolation voltage between two conductors *
According to UL, the AC voltage will not damage the insulation even if applied for 1 minute.

VIII.How To Use Optocoupler

Step 1: Design LED input current IF and input resistance RIN

Step 2: Calculate output current based on IF and CTR

Step 3: Design the output resistor RL

Step 4: Check each design constant