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Case 6: Using a solid-state relay to reduce the size and prolong the service life of power supply units for Programmable Logic Controller (PLC)

Programmable logic controller input output module

Reducing the size of a control relay unit, semiconductor, long service life, board size reduction, countermeasure for the effects of magnetic fields

Our customer was having difficulty in reducing the size of a control relay unit.
Its design team also had to reserve board space for devices necessary to protect against radiation noise and magnetic fields. Was there any solution?
Solving a problem by replacing a mechanical relay with a semiconductor device, namely, the TLP3906 photovoltaic-output photocoupler

Product Overview

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A photocoupler consists of an LED on the input side and a photovoltaic cell on the output side. Replacing a mechanical relay with a solid-state relay helped prolong the service life, reduce the size and achieve silent operation of the relay unit.

Features

  • Semiconductor devices eliminate the need to take a relay’s maximum cycles of make and break into consideration during the design process.
  • Photocouplers, which have no mechanical contact, generate no acoustic noise.
  • This photocoupler can offer sufficient isolation voltage where industrial control equipment requires 1500V to 2500V. And it can drive variety ofvarious MOSFETs.

Problem-Solving Examples

Solution for Company D, an industrial equipment manufacturer (Employee population: 600)

Company D started to design a new power supply unit for programmable logic controller (PLC) applications. Its design targets were a reduction in chassis size and a further improvement in efficiency. The project team of Company D improved and reduced the size of each section of the unit. The project was going smoothly until the design team faced the final challenge.
The team was using a mechanical relay, which is physically large and requires sufficient space to protect against radiation noise and the effects of magnetic fields. The team proceeded by trial and error and consulted a distributor’s field application engineers (FAEs). However, despite its best efforts, it was having difficulty in finding a solution.
Reprimanded by top management and unable to find a way to break the deadlock, the members of the team were growing increasingly anxious.

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Solving the problem using the TLP3906 photovoltaic-output photocoupler

The solution lay in the use of a photorelay. A sales representative of Toshiba introduced the project team to its field application engineer. He proposed replacing a mechanical relay with a solid-state relay, or more specifically, with a pair of the TLP3906 photovoltaic-output photocoupler and a MOSFET.
Mechanical relays control the make/break of an electrical signal using a metal contact whereas solid-state relays control a signal conduction passage using a MOSFET. Mechanical relays generate grinding or clicking noise and have shorter life expectancy than solid-state relays because of metal-to-metal wear and tear. While mechanical relays require maintenance every few years, solid-state relays, which have no mechanical parts, eliminate the need for regular maintenance. The team was concerned about whether solid-state relays can switch as high a current as mechanical relays. However, with the recent progress of microfabrication technology, the latest MOSFETs can handle a large current. Furthermore, MOSFETs are physically smaller. The TLP3906 photovoltaic-output photocoupler can generate electricity on its own using light from the integrated light-emitting diode, eliminating the need for an external gate power supply.
The project team assembled the TLP3906 photovoltaic-output photocoupler and a MOSFET on a printed circuit board. Consequently, the relay board exhibited satisfactory performance, meeting the design targets. The switching speed increased significantly while the chassis size became smaller than initially planned.

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