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Optical-coupled MOS FET Structure and Features
November 2004
An optical-coupled MOS FET is a full solid state relay which consists of a light-emitting diode (LED) for the input side and MOS FETs for the contact point.
Therefore, it is usually called SSR(Solid State relay).
Compared to a traditional mechanical relay, an optical-coupled MOS FET is not only smaller and lighter weight, but easier to drive and high speed.
And it also generates little noise.
These features make it an ideal relay.
The structure and the features are described below.
1. STRUCTURE
As shown in Figure 1, an optical-coupled MOS FET consists of a light-emitting diode (LED) in the input side, a photo-voltaic diode (PVD) facing to the LED, and MOS FETs that serves as the contact.
Figure 1. Internal Perspective Diagram
(Colors are provided to facilitate understanding and bear no relation to actual colors.)
The input light-emitting diode (LED) emits an infrared light when a forward current flows through it. As this light shines on the photo-voltaic diode (PVD), a voltage is generated in the PVD.
When this voltage is applied to the gates of the MOS FETs, the MOS FETs switch the drain current on and off.
There are two MOS FETs connected serially in reverse, so they can switch alternating current also.
Further, since there are two types of MOS FETs, the normally OFF type (Enhancement type) and the normally ON type (Depletion type), it is possible to make the former an A contact (Make contact) type relay by using a connection that gives a positive gate voltage, and make the latter a B contact (Break contact) type relay by using a connection that gives a negative gate voltage.
Moreover, optical-coupled MOS FET of us has an original drive controller internally. The controller accelerates discharge from the MOS FET's gate, when the LED stops emitting light.
Thereby it realizes smooth high-speed switching operation.
Thanks to this structure, the input and output are completely insulated electrically, while the output side contact realizes conductivity with good linearity and high cut-off characteristics both in the forward and reverse directions.
Moreover, as shown in the following figure, in optical-coupled MOS FET of us, the input LED and the PVD in the output side are insulated from each other with a highly insulating resin with high transparency, and the exterior is further covered with black-colored resin with high darkness.
Thereby, it operates strictly and stably regardless of ambient brightness.
Figure 2. Molding Structure
There are two types of molding structures, the basic structure standard type (left figure) and the high-insulation type with a longer insulating distance (right figure).
2. FEATURES
As described above, optical-coupled MOS FETs are full solid state relays consisting of semiconductors, and have the following features compared with conventional mechanical relays or photocouplers.
- Compact size and light weight
Ultra-compact products that do not exist among mechanical relays, such as small outline package (SOP) and ultra-small flat-lead type optical-coupled MOS FETs, are being developed one after another, and are ideal for equipment for which a premium is placed on compact size and light weight, such as laptop computers, mobile information terminals, and various adapter cards, as well as semiconductor testing equipment that requires high-density mounting.
 Figure 3. External Appearance Photograph (Each square measures approximately 1 x 1 cm.) |
- Excellent shock and vibration resistance
As optical-coupled MOS FETs do not have both metal contacts and moving parts that suffer the effects of physical vibrations and shocks, they are particularly suitable for portable information terminals such as portable audio devices and laptop computers, machine tools, and automatic testers, which require resistance to shocks and vibration. - Noiseless
Optical-coupled MOS FETs do not generate operation noise unlike mechanical relays, and thus are very effective for reducing noise levels in offices, plants, homes, and so on.
Furthermore, optical-coupled MOS FETs do not cause contact sparks unlike mechanical relays.
Therefore, they do not cause radio frequency interference in surrounding audiovisual equipment, and there are no worries about extra stress applied to parts undergoing testing. - High-speed operation
Since optical-coupled MOS FETs operate with light and electrons, their operating speed is more than 10 times faster than that of mechanical relays.
Therefore, they are suitable for semiconductor testing equipment and so on which requires high-speed operation. - Extremely low malfunction rate
Optical-coupled MOS FETs are free of the chattering that occurs in mechanical relays, and compared with general photocouplers, have a greater tolerance to fluctuations in electric potential between the input side and the output side.
As a result of them, malfunctions due to these factors are extremely rare. - High insulation, high reliability
Since optical-coupled MOS FET uses an optical connection between the input and output, they are completely insulated electrically.
Moreover, while being relays, optical-coupled MOS FETs are semiconductor devices like microprocessors or memories, and thus do not suffer mechanical wear and contact degradation caused by switching operations.
This makes them suitable for continuous high-speed switching applications such as semiconductor testing equipment. - High sensitivity and low power consumption
Since optical-coupled MOS FET usually has an extremely low driving current of just a few mA, and no induction on the input side unlike coils, they can be directly driven by CMOS logic gates.
As a result of them, the external circuit is very simple, and as optical-coupled MOS FETs have low power consumption, they are effective for reduction of heat radiation and power saving in battery-driven laptop computers and portable information terminals, as well as semiconductor testing equipment where they are used in large numbers. - Capability for high voltage, large current, and alternating current compared with photocouplers
Photocoupler has a photo detector and a contact element on a chip together.
On the other hand, optical-coupled MOS FET has a photo detector and MOS FETs for contact on different chips each other.
Therefore, it is easy to make optical-coupled MOS FETs which can control either high voltage or large current through selection of the MOS FET chip.
Moreover, since two MOS FETs are serially connected in reverse for the contact, not only direct current but also alternating current can be switched on and off. - Little temperature dependency and high linearity of the characteristics
The MOS FETs used for the contact on the output side have good linearity in both the forward and reverse directions, making it ideal for controlling minute analog signals.
Moreover, since the ON-resistance of the MOS FET has little fluctuation within the wide temperature range required of laptop computers, PDAs, etc., stable characteristics can be obtained.
 Figure 4. Characteristics Examples |
Based on these various features, we offer rich lineups of optical-coupled MOS FETs that support from minute signals to large current control, allowing users to select the ideal MOS FET for each one of their applications.
