Principle and Application of Photoelectric Coupler

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Principle and Application of Photoelectric Coupler

I. Principle and composition of Photoelectric Coupler

1. Principle. Photoelectric Coupler, usually referred to as photocoupler. The basic principle is to transmit electrical signals using light as a media. In some special application scenarios, the input/output end is required to achieve electrical isolation, so that traditional electronic devices cannot be used to transmit electrical signals. photocoupler is born to adapt to such scenarios. The input end of the photocoupler uses a light-emitting diode, which uses an electrical signal to drive the Light-Emitting Device (usually infrared light). The receiving end is a render tube, which converts the received optical signal into an electrical signal output. Through electric-> light-> electric conversion, both signals can be transmitted and electrical isolation is achieved. For example:

2. In order to realize the conversion of electricity-> light, the light emitting diode is required, while the conversion of light-> requires a render tube. To eliminate the interference of the outside light, generally, the sender/receiver is encapsulated with opaque materials to connect the input/output PIN. To achieve high electrical isolation strength, the encapsulation material must have a high insulation strength. Usually, the photocoupler device consists of the following components: Light Emitting Diode, render tube (usually photosensitive Transistor), packaging material, input/output PIN. For example, an X-ray photo of an optical coupling:

Ii. Manufacture of optical coupler

The photocoupler is usually enclosed by DIP or SMD. The actual process is to encapsulate the Infrared LEDs and silicon phototransistor and form the input/output pins. This process is similar to the IC Encapsulation, the difference lies in the selection of materials and key process control. Generally, the process of optical coupling is as follows:

Ceramic base making → thick film Circuit making → chip (ir led, PD and ASIC) test → chip sintering and Pressure Welding → Medium Test → coupling alignment → mounting → encapsulation → leak detection → Medium Test → aging screening → final test.

According to the process flow, the industry summarized some technological factors that affect the reliability of the optical coupling by analyzing the anatomy of the defective product in the production of the optical coupling and combining the characteristics of the optical coupling:
1. Effects of silver glue on Reliability
The mounting frame adopts the silver paste adhesive process, which can meet the requirement that the chip has better ohm contact and lower forward pressure drop. However, different brands of silver glue have different adhesion strength with various chip materials and brackets, there are also many differences in storage and service life. Improper use may result in poor bonding between the chip and the bracket, affecting the bonding and even generating the chip.
2. Impact of hand frame and bonding quality on Reliability
The infrared chip has a length and width of only 0.3mm, and the material is fragile. Therefore, hand-mounted tool holders are prone to quality problems. Therefore, automatic mounting and keybonding are usually used.
3. Effects of inner encapsulation and injection molding outsourcing sealing on Reliability
When the needle encounters the gold wire, it will generate poor conditions such as virtual welding, broken wires, and inverted wires in the colloid when the coupling is encapsulated in the gel. Due to the elasticity of the encapsulated materials, it is difficult to filter out defective products, affecting product reliability. In addition, there is a big difference between the thermal matching of inner packaging materials and plastic packaging materials. The combination of inner packaging materials and plastic packaging materials is not very close after outsourcing sealing. Due to the small bonding part between the plastic seal and the bracket on the optical coupling structure, the band deformation will occur when the die is encapsulated. Because the inside of the plastic seal is still completely solidified when the die is started, there will be gaps after the stress, this causes loose of the pin, which causes the internal lead to be broken in the frame of the lead, forming a intermittent open circuit. Therefore, the combination of good plastic sealing materials and lead frame can be used to improve the process and methods to prevent internal leads from being broken during encapsulation, so as to improve product sealing and device stability.

Definition of Photoelectric Coupler Parameters

The technical parameters of optical coupling can be divided into several parts: input, output, transmission, and isolation.

3.1 input features

The input characteristics of the photocoupler are actually the characteristics of its internal light emitting diode. Common parameters include:

1. Forward Current If (Forward Current)

If is the forward current value of the LED When it emits normally. The maximum current allowed by different LEDs varies.

2. Forward pulse Current Ifp (Peak Forward Current)

IFP refers to the forward pulse current value flowing through the LED. To ensure the service life, LEDs are usually driven in the form of pulses. Generally, IFP in the LED specification is calculated based on the Pulse Current of 1/10 MS pulse width and duty cycle.

3. Forward Operating Voltage Vf (Forward Voltage)

VF refers to the pressure drop of the LED itself under a given operating current. Common low-power LEDs usually use if = 20mA to test the forward operating voltage. Of course, the test conditions and test results are different for different LEDs.

4. Reverse Voltage Vr (Reverse Voltage)

The maximum reverse voltage that an LED can withstand. exceeding this reverse voltage may damage the LED. When using an AC pulse to drive the LED, pay special attention not to exceed the reverse voltage.

5. Reverse Current Ir (Reverse Current)

It usually refers to the reverse current flowing through the LED under the maximum reverse voltage.

6. Allow Power Consumption Pd (Maximum Power Dissipation)

The maximum power consumption that the LED can afford. If the power consumption exceeds this threshold, the LED may be damaged.

7. center wavelength λ p (Peak Wave Length)

It refers to the center wavelength value of the LED light. The wavelength directly determines the color of the light. for dual-color or multicolor LEDs, there will be several different center wavelength values.

3.2 output features

The input Characteristics of the optical coupling are actually the characteristics of the internal photosensitive transistor, similar to the common transistor. Common parameters include:

1. Collector Current)

The current of the phototransistor collector, usually the maximum value.

2. Collector-emitter Voltage Vceo (C-E Voltage)

Collector-the voltage that the emission pole can withstand.

3. Emission pole-collector Voltage Veco (E-C Voltage)

Emitter _collector Voltage

4. Reverse cut-off current Iceo

The open circuit of the light emitting diode. When the voltage between the collector pole and the emission pole is set, the current flowing through the Collector is the reverse cut-off current.

5. C-ESaturation Voltage Vcd (sat) (C-E Saturation Voltage)

When the operating current of the light emitting diode and the collector current IC are set to a specified value, and the voltage drop between the collector and the emission pole is maintained when the IC/IF is equal to or less than CTRmin (CTRmin is specified in the tested tube technical conditions.

3.3Transmission features:

1. Rising time Tr
(Rise Time )&
Drop Time Tf (Fall Time)

Under the specified working conditions, the optical coupler inputs the specified IFP pulse wave, and the output tube outputs the corresponding pulse wave, from 10% to 90% of the output pulse frontier, the time required is the pulse rise time tr. From 90% to 10% of the amplitude of the output pulse, the time required is the pulse descent time tf.

2. Current Transfer ratio CTR (Current Transfer Radio)

When the operating voltage of the output tube is set, the ratio of the output current to the forward current of the light emitting diode is the current transfer ratio CTR.

3.4 isolation features

1.Isolation Capacitance):

Capacitance value between the input and output ends of the optical coupling device

2. Inbound and Outbound isolation resistance Rio :( isolation resistance)

Insulation resistance value between the input and output ends of the semiconductor optical coupler.

3. Isolated voltage vio (isolation voltage)

Insulation voltage value between the optical coupler input and output.

Other parameters, such as operating temperature and dissipation power, are not described in detail.

Optical coupler classification and Application

4.1 category

According to the transmission characteristics, the optical coupler is generally divided into two categories: Non-linear optical coupling and linear optical coupling. The current transmission characteristic curve of Nonlinear photocoupler is non-linear. This type of photocoupler is suitable for transmitting Switching signals and is not suitable for transmitting analog data. The current transmission characteristic curve of the linear photocoupler is close to a straight line, and the performance is good when the signal is small. It can be isolated and controlled based on linear characteristics. Therefore, it is usually necessary to transmit analog signals. For example, linear optical coupling is required when the output feedback signal of the switching power supply is isolated from the input signal, for example, nonlinear optical coupling is usually used between the sampling signal and the single-chip microcomputer input. The isolation principle of linear photocoupler is no different from that of ordinary photocoupler. It only slightly changes the single-node single-circuit mode of the ordinary photocoupler and adds an optical receiving circuit for feedback. In this way, although both optical receiving circuits are non-linear, the non-linear characteristics of the two optical receiving circuits are the same. The non-linearity of the feedback channel can be used to offset the non-linearity of the pass-through channel, so as to achieve linear isolation. According to the structure type, optical coupling can be divided into: general type (also divided into two types: no base pole lead and base pole lead) dington, Schmidt, high-speed, optical integrated circuit, optical fiber, photosensitive Thyristor (also divided into unidirectional thyristor, bidirectional Thyristor), photosensitive field effect tube, in addition, there are two channels (two internal tubes), high-gain type, ac-DC input type and so on. The following is an example of general and dington output models. In the general type optical coupler, the receiving end is a silicon photoelectric tube, so there is only one silicon pnjunction between the B-E; darington type optical coupling is not, it consists of a composite tube, two silicon PN knots are combined to form the emission junction of the composite tube, so the dington photocoupler has a greater current transmission ratio (usually the ctr of the dington photocoupler is 1 ~ larger than that of the general type photocoupler ~ 2 orders of magnitude), so as to have greater drive capabilities, such.

4.2 Applications

Due to the outstanding advantages of optical coupling: for example, one-way signal transmission, the input end and the output end are completely isolated and isolated from each other. The output signal has no effect on the input end, strong anti-interference ability, stable operation, no contact, and long service life, high transmission efficiency, and its structure and variety are very wide, so it is widely used in the following scenarios:
(1) Applications in Logical Circuits
The Photoelectric Coupler can form various Logical Circuits. Because the anti-interference performance and Isolation Performance of the optical coupler are better than those of the transistor, the logical circuit composed of the photoelectric coupler is more reliable. It should be noted that the optical coupling transmission rate can meet the needs of the logic circuit.
(2) solid switch application
In a switch circuit, a good electrical isolation is often required between the control circuit and the switch, which is difficult for general electronic switches, but it is easy to implement with a photoelectric coupler. In addition to good isolation performance required for optical coupling, switching time is also very important for Switching circuits.
(3) Application in the Trigger Circuit
The Photoelectric Coupler is used in The bistability output circuit. Because the Light Emitting Diode can be respectively stringed into two tube emitter pole circuits, the output and load isolation problems can be effectively solved. Due to the influence of temperature on the dark current of the photocoupler and the stability of the LED, pay attention to the use environment of this type of circuit.
(4) Application in pulse amplification circuit
The Photoelectric Coupler is used in digital circuits to enlarge the pulse signal.
(5) Application in Linear Circuits
Linear photocoupler is used in Linear Circuits and has high linearity and excellent electrical isolation performance. This type of circuit is generally used to simulate signal transmission, such as switching power supply circuit.

(6) applications in other special scenarios
The Photoelectric Coupler can also be used in high-voltage control, replacing transformers, replacing contact relays and a/d circuits. It can also be used for industrial control to drive high-power devices with weak signals.

The following is an example.

For example, single-chip microcomputer and RS485 communication ports are isolated by optical coupling, so as to protect the single-chip microcomputer from external signal interference and burst impact. Because the communication between RS485 interface and single-chip microcomputer is a digital signal, the requirement on linearity is not very high, but the baud rate of communication is relatively high, it will become the bottleneck of the entire communication system. Therefore, select high-speed optical coupling. In addition, the resistance value of the photocoupler-matched resistor needs to be optimized. For example, if the resistance R2 and R3 are selected to be large, the photocoupler luminous tube enters saturation slowly from the end. If the selected value is too small, the exit saturation will also be slow, therefore, the values of these two resistors should be carefully selected, which can be determined by experiments.

Key Points of Selection: 1. CTR and linearity; 2. Isolation characteristics; 3. increase/decrease speed; 4. Isolation capacitor; 5. driving capability; 6. input current.

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Ii. Supplier Introduction

SHARP (Japan's SHARP photoelectric Branch was established in 1960 and has factories and R & D centers in SHINJO, TENRI, and overseas. Its sales of optoelectronic semiconductor products have ranked first in the world for 20 years .)

TOSHIBA

FAIRCHILD xiantong

AGLIENT anjet (released from HP in 1995)

NEC

VISHAY weishi

BRIGHT baihong

COSMO crown West

MOTOROLA

PHILIPS Phillips

EVERLIGHT yiguang

PANASONIC