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Expanding application areas spurred by various innovationsBasics of photorelays and their usage considerations as replacements for mechanical relays

Relays are widely used in various electronic devices to control a load while providing electrical isolation between two circuits. There are various types of relays, each with different principles of operation.
Toshiba Electronic Devices & Storage Corporation (“Toshiba”) provides an extensive portfolio of semiconductor relays called photorelays.
The following describes the recommended applications of photorelays while providing answers to frequently asked questions and examples of Toshiba’s photorelays.

Relays have a long history.
Even now, the type of relay that springs to mind for many engineers is the electromagnetic relay (mechanical relay) that emits a distinctive buzzing sound.
Although mechanical relays are still widely used, they have many constraints and problems to be addressed with respect to implementation and operation.
Lately, mechanical relays are being replaced by semiconductor relays.
Photorelays, a type of semiconductor relays, are major candidates as replacements for mechanical relays.
Toshiba has more than 40 years of experience with photocouplers.
Indeed, our photocouplers have held the largest market share for eight consecutive years since 2010*1.
The following describes several things to consider when replacing mechanical relays with photorelays.

Photorelays, major candidates as replacements for mechanical relays having many constraints and problems

CASE : 01

―― I am now using mechanical relays, which are not easy to use because of their large size and short life.

Mechanical relays have a long history.
Even now, in the 21st century, mechanical relays are widely used in industrial electrical systems owing to their proven track record.
However, mechanical relays have many drawbacks.
For example, it is difficult to reduce their size because of their structure.
Mechanical relays also have a short service life as their contacts wear out over time.
In addition, mechanical relays require special design considerations since they are susceptible to external magnetic interference, shock, and vibration because of the noise generated by contact arcing and bouncing as well as the back-EMF from the electromagnetic coil that throws a contact.
For example, to arrange dozens of mechanical relays on a board, it is necessary to allow sufficient space between them in order to avoid mutual magnetic interference.

Therefore, photorelays are attracting lots of attention as an alternative to mechanical relays.
Mechanical relays and photorelays have the same functionality in that they are used to control a load while providing electrical isolation between the input and output circuits.
However, photorelays have none of the constraints and problems described above.
Photorelays are physically smaller, provide longer service life, and allow board assembly with higher density than mechanical relays.
Therefore, replacing mechanical relays with photorelays helps reduce the size and improve the maintainability of systems.

Classification of Relays
Classification of Relays

Advantages of photorelays over mechanical relays

CASE : 02

―― I would like to know more about photorelays.

A photorelay consists of an LED on the input side and, for example, a photodiode-MOSFET pair on the output side and is housed in a plastic package.
A photorelay provides electrical isolation between the input and output circuits, and transfers an electrical signal between the two circuits using light so as to control a load.

Because of this principle of operation, photorelays have many advantages over mechanical relays.
For example, composed of extremely small devices, a photorelay can be housed in a very compact package.
Since photorelays are free from magnetic interference, many photorelays can be arranged in close proximity.
In addition, unlike mechanical relays, photorelays generate no electromagnetic radiation, electric noise, acoustic noise, or vibration.
Photorelays are also free from concerns about the contact service life and hot switching (i.e., opening or closing of a relay while carrying a signal, which adversely affects the service life of a mechanical relay) and no special care is required for the neighboring circuits.
A photorelay allows faster switching and dissipates less power than a mechanical relay because a photorelay consists of only semiconductor devices such as an LED and a photodiode.

However, for example, photorelays compare unfavorably with mechanical relays in terms of output capacity.
It has been difficult to use photorelays for high-voltage, high-current applications such as industrial equipment.
In recent years, however, Toshiba has been releasing photorelays with increasingly higher output capacity using innovations in MOSFETs and packages, expanding their application areas.
Accompanying the increase in output capacity, photorelays will enter widespread use in industrial equipment.

Structures of Photorelays
Structures of Photorelays

Advantages of Toshiba’s photorelays in the photorelay market

CASE : 03

―― What are the features of Toshiba’s latest photorelays?

Toshiba provides an extensive portfolio of high-capacity photorelays for industrial and automotive applications that are suitable as replacements for mechanical relays.
Photorelays with even higher output capacity are under development.
Regarding these application areas, our focus is on increasing photorelays’ maximum output current by using MOSFETs fabricated with the latest process.
New photorelays also incorporate protection functions, which make them easier to use for system designers. Moreover, Toshiba provides extensive technical support for system designers who are considering replacing mechanical relays with photorelays. (See the last question and answer.)

Toshiba’s photorelays have a dominant market share, particularly in the field of semiconductor testers.
Toshiba has strength in photorelays because it develops and manufactures all the components of photorelays such as LEDs, photodiodes, and MOSFETs and is committed to the innovation of each of these components as well as photorelay packages.
Toshiba takes pride in its industry-leading technology as typified by long-life LEDs, low-input-current photodiodes, MOSFETs with low overall power loss, and low-profile packages.
In particular, Toshiba’s proprietary low-profile, long-creepage package can be mounted on the backside of a printed circuit board.
Photorelays in this package are ideal for semiconductor tester applications since they can be arranged with unparalleled density.

Photorelay Roadmap
Photorelay Roadmap

Example of replacing mechanical relays with photorelays: HVAC

CASE : 04

―― In what fields are mechanical relays being replaced by photorelays?

For example, thanks to an increase in the output capacity, photorelays can now be used for heating, ventilation, and air conditioning (HVAC) applications for building automation.
More specifically, high-capacity photorelays are used to transmit thermostat signals and control damper motors for valve actuator control.
In the past, mechanical relays were primarily used for these applications.
However, as the output capacity of photorelays increases, mechanical relays in these applications are being replaced by photorelays.
Photorelays emit no acoustic noise during operations and provide longer service life.
The long service life of photorelays, in turn, helps reduce the frequency of system maintenance, which benefits system manufacturers, maintenance companies, and users.

Photorelays are also replacing mechanical relays in programmable logic controllers (PLCs) that provide the foundation for various industrial control systems.
Mechanical relays are mainly used for PLCs since they need to switch relatively high current.
Because of the increasing output capacity, photorelays have begun to be used for PLCs.

Photorelays are also used as contact-output relays in smart electricity meters for external communication as well as in the cell group monitoring circuits of power supply systems incorporating batteries.

Example of an HVAC System Using Photorelays instead of Mechanical Relays
Example of an HVAC System Using Photorelays instead of Mechanical Relays

Photorelay usage considerations (1):
Output failure caused by external surge

CASE : 05

―― I heard about a case of photorelay failure caused by external surge.
Why did it happen? How can you prevent photorelay failure?

As described above, the output side of a photorelay incorporates a MOSFET, which could be damaged owing to various factors.
One of the major causes of MOSFET damage is external surge.
For example, the output circuit of a photorelay could fail in the event that the induced impulse noise on a power line or an electrostatic discharge (ESD) surge is superimposed on the load power supply.
There are short- and open-circuit failure modes.
A short-circuit failure causes a load to turn on despite the absence of an input whereas an open-circuit failure causes a load not to turn on despite the presence of an input.

Photorelays are susceptible to failure due to voltage surge when they are used in systems that operate from the AC mains power supply or when an ESD cannot be blocked completely.
In such cases, photorelay protection is necessary.
For example, adding a varistor is effective for photorelay protection.
In the event of excessive voltage being applied, a varistor shunts current away from the following circuit For ESD protection, multilayer chip varistors are generally used.

For the protection of a photorelay’s output stage, a varistor with a rated voltage lower than the off-state output terminal voltage (VOFF) of the photorelay should be used. (We recommend using a varistor with a rated voltage equal to roughly 70% of VOFF.)
See the following table for the selection of a varistor and where it should be connected.

Example of Varistor Selection to Protect a Photorelay’s Output Stage from External Surge
Example of Varistor Selection to Protect a Photorelay’s Output Stage from External Surge

Photorelay usage considerations (2):
Output device failure caused by back-EMF

CASE : 06

―― Are there any other considerations for using photorelays?

Back-EMF from an inductive load can also cause a photorelay’s output stage to fail.
When the output of a photorelay to the inductive load switches off, the inductor current drops to zero instantaneously.
A sharp change in current causes high back-EMF voltage to appear across an inductor.
A photorelay might fail in the event of the back-EMF voltage exceeding its off-state output terminal voltage.

If high back-EMF voltage is expected to be induced by an inductive load, a photorelay should be protected from excessive voltage by adding a protective component or circuit.
For example, photorelays can be protected by:

  • adding an external diode to release the back-EMF energy
  • adding a snubber to absorb the back-EMF energy
  • adding a varistor to suppress excessive voltage

The effect of a protective component varies, depending on its distance from the load or the photorelay.
If the protective component is far away from the load or the photorelay, it might not protect the photorelay.
We recommend placing a protective component as close to a photorelay or a load as possible.

Mechanism of Back-EMF and its Approximate Voltage
Mechanism of Back-EMF and its Approximate Voltage

E-books and other information about technical support and an evaluation board gift campaign

CASE : 07

―― I want to use photorelays. Are there any evaluation tools or support websites?

Toshiba provides various types of information about photorelays, most of which are available on its product website. An e-book is available for engineers who are transitioning from mechanical relays to photorelays.
This e-book details considerations for using photorelays that are not covered herein.
We hope that you will find it informative and convenient.

*1 Source: Market Share: Semiconductors by End Market Worldwide, 2017 from Gartner (April 4, 2018)