Types of optical semiconductors

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 device (combination of light-emitting element and light-receiving element)---Optocoupler, optical fiber coupler.

LED lighting principle

The light-emitting principle of a light-emitting diode (LED) is to apply a forward current to the pn junction of a compound semiconductor.

When forward current passes through a light-emitting diode, 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 is released in the form of light. The emitted light depends on the energy gap (Eg) of the compound semiconductor.

Note: Traditional silicon diodes do not emit light because the recombination energy becomes heat.

LED wavelength range

LED emits different wavelengths of ultraviolet light and even infrared light. The emission wavelength will be expressed by the following equation using the energy gap (Eg) of the compound semiconductor material

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., use GaAs (gallium arsenide) material; for red/green indicator LEDs, use GaP (gallium phosphide) or InGaAlP (indium aluminum gallium phosphide); for blue LEDs, Use InGaN (Indium Gallium Nitride) or GaN (Gallium Nitride).

Luminous colors of LEDs of different materials (by material)

What is an optocoupler?

An optocoupler is a device that integrates a light-emitting diode (LED) and a photodetector in a single package. Unlike other optics, light is not emitted outside the package. Their appearance is similar to non-isolator/solid state relays. Although an optocoupler is an optical device, it does not process light, but rather electrical signals.

Optocoupler operation example:

(1) LED is on (0⇒1).

(2) LED light enters the phototransistor.

(3) The phototransistor is turned on.

(4) The output voltage changes from 0⇒1.

(1) LED turns off (1⇒0).

(2) LED light stops entering the phototransistor.

(3) The phototransistor is turned off.

(4) The output voltage changes 1⇒0.

Why do you need an optocoupler?

In an optocoupler, the primary side (LED side) and secondary side (light-receiving device side) are electrically insulated. Therefore, even if the potentials of the primary and secondary sides (even the GND potential) are different, the primary electrical signal can be transmitted to the secondary side.

In an inverter application like the one shown on the right, the control unit (such as a microcontroller) usually operates at low DC voltages. On the other hand, IPM and IGBT will drive high voltage loads (for example requiring 200V AC). High-voltage system components can be controlled directly from the microcontroller via couplers.

Type of optocoupler

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

Transistor Output - A phototransistor is a type of detector. Darlington types are also available.

IC output - We have products that use 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.

Triac/Thyristor Output - Photothyristor or phototriac is used for the output. They are mainly used for the control of AC lines.

Type of optocoupler (package)

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

Insulation Creepage Distance - The shortest distance along the surface of an insulator between two conductors (primary and secondary).

Gap - The shortest distance between two conductors measured through air.

Insulation Thickness - The minimum distance of insulation between two conductors.

Isolation voltage - The isolation voltage between two conductors.

According to UL regulations, AC voltage will not destroy the insulation even if applied for 1 minute.

Type of optocoupler (internal structure)

Optocouplers have different types of internal packaging structures due to limitations such as required insulation performance, package size, and internal chip size.

Single Mode Transmissive - Frame-mounted LED and frame-mounted photodetector come in face-to-face molded packages. The light-transmitting part between the LED and the photodetector is made of silicone material.

Single-mode transmissive type with film - To increase the isolation voltage, a polyimide film can be inserted between the LED and photodetector.

Dual-mode transmissive type - In this transmissive structure, the inner mold is white and the outer mold is black. Resin with high infrared transmittance is used for white molds of light-transmitting parts.

Reflective - The frame-mounted LED and frame-mounted photodetector are in the same plane. The LED light reflects in the silicone and reaches the photodetector. Therefore, it is called reflective type.

Safety standards for optocouplers

Optocouplers may need to comply with different safety standards when they are installed in electrical equipment to protect humans from electric shock.

There are various regulations and standards in place to ensure safety.

From a design and manufacturing perspective, safety standards can be divided into "set standards" and "parts standards".

Setting standards is the basis for designing and manufacturing equipment such as televisions, video recorders and power supply units. "Complete machine standards" vary according to equipment type, isolation method and level, driving voltage, etc.

In addition, items that must be maintained for insulated parts (dielectric strength (insulation voltage), creepage distance, clearance, etc.) are designated as "parts standards".

Main safety standards

Parts standards

UL1577 (cUL)

Isolation voltage standard (1 minute)

Approved Organization: UL (Underwriters Laboratories, Inc.)

EN60747-5-5

Maximum operating isolation voltage and maximum overvoltage standards

Approved organization: VDE (Venation of German Electrical Engineers)

Approving organization: TUV (Technical Inspection Association)

Machine standard

(Approved organizations: BSI (UK) SEMKO (Sweden), etc.)

EN60950

Telecom network equipment (workstations, PCs, printers, fax resistors, modems, etc.) standards

EN60065

Standards for household appliances (TVs, radios, video recorders, etc.)

Characteristics of Optocoupler (Current Transfer Ratio: CTR)

Current transfer ratio of a transistor coupler: It is expressed by the amplification ratio of the output current relative to the input current, such as the transistor hFE.

Current transfer ratio = CTR = IC / IE = output (collector) current / input current × 100 (%)

For example:

When the output is IF=5mA, IC=10mA is obtained.

CTR＝IC /IF＝10mA/5mA×100（%）＝200%

Main characteristics of optocouplers (triggering LED current)

"Trigger LED current" refers to "the LED current that changes the trigger state".

IFT, IFH, IFLH, IFLH, etc. are used as symbols.

The triggering LED current shown in the specification sheet represents the guaranteed current value of the product. For stable operation, designers must ensure that at least the trigger LED current (maximum value) flows when designing.

The input LED current IF gradually increases from 0mA. If the output switches to the on state at 1mA, then IFT = 1mA.

In the spec sheet below, the maximum IF required to switch the output to the on state is 3mA.

Optocoupler aging change data

The light output of light-emitting elements (LEDs) diminishes over time. In optocouplers, the aging changes in LED light output are more obvious than the aging changes in the light-receiving device. Therefore, designers need to use the aging change data of the optocoupler used to estimate the decreasing trend of the luminous level. Designers will calculate the change in light output of an LED based on the usage environment of the device and the total operating time of the LED. This value must be reflected in the initial value of the LED forward current (IF).

*For example, when the duty cycle (light-emitting duration) is 50% and the operating time is 1000 hours, the total operating time is calculated to be 500 hours.

How to use an optocoupler

These issues are explained on the next page with the following steps.

Step 1: Design LED input current IF and input side resistor RIN

Step 2: Calculate the output current based on IF and CTR

Step 3: Design the output side resistor RL

Step 4: Check Each Design Constant

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