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PhotoMOS (semiconductor relay) – a component that lies concealed inside industrial and automotive devices (Vol. 1)

In this article, we will introduce our PhotoMOS (PhotoMOS relay), which is a lesser-known Panasonic product with a high market share. Being used mainly in industrial equipment, this product is not so mainstream. It is nevertheless a major player in the industrial equipment field, and we are proud to tell you that Panasonic PhotoMOS ranks number one in the global market share.


What are PhotoMOS?

Perhaps the name "PhotoMOS" readily reminds you of an electronic semiconductor component, and the PhotoMOS is a semiconductor component indeed. In short, PhotoMOS is a semiconductor relay containing an LED serving as the input element, and a MOSFET as the output element.

Historically, mechanical relays have been more prevalent in the market, however semiconductor relays have many advantages over mechanical relays.
The signature features of a semiconductor relay is a long service life and high contact reliability. To know the features of the semiconductor relay in detail, visit the Panasonic product page.

Now, let's dissect the internal structure of PhotoMOS semiconductor relays.
The following diagram shows the basic structure of PhotoMOS, which is composed of three basic components consisting of an LED, a photoelectric element, and a MOSFET.
The diagram can be used to visually explain how the PhotoMOS functions. An electric signal input to the primary side is (1) converted into light by the LED. The LED's light is then (2) received by the photoelectric element to generate an electromotive force. Finally, the generated electromotive force (3) switches the MOSFET on.

Internal structure of the PhotoMOS

Selection drawing1

Selection drawing2Internal structure of the PhotoMOS

Through this process, the incoming electric signal to the input side switches the MOSFET on and off on the output side. Simply put, the PhotoMOS functions as a switching element (i.e., a relay) in which the input side is electrically insulated from the output side through a light transmission path.

The PhotoMOS operation principle is relatively easy. In practical applications, the process and implementation of this technology, requires an elaborate know-how to manufacture. Panasonic has been researching and developing PhotoMOS products for over 30 years. Based on many years of experience and accumulated knowledge, the company retains the industry's highest standard of design technology. Panasonic has the flexibility to meet various customers' requests such as custom-designed products.

Differences compared to a Photocoupler

When explaining the basic operation of PhotoMOS, a frequently asked question comes up which asks, "What difference is there between the PhotoMOS and the photocoupler?"

The PhotoMOS is similar in operation to the photocoupler but has been developed based on a product concept which is different. What distinguishes these two components is that the PhotoMOS is capable of bidirectional current control while the photocoupler is used for relatively simple pulse signal transmissions or DC control. The PhotoMOS can be used for both AC control and DC control. Additionally, PhotoMOS have an advantage of a lower offset voltage which is useful for controlling small signals which may be clipped or distorted from using a Photocoupler or Phototriac.

In what circuit are the PhotoMOS used?

The example below can be used to demonstrate a type of circuits which commonly uses PhotoMOS.
In recent years, as the mobility equipment market expands further, designs using PhotoMOS in battery management systems (BMS) have been steadily increasing. Specifically, in these such cases, PhotoMOS are used in circuits that monitor the remaining battery capacity. As shown in the following figure, four PhotoMOS are used for singular or multiple battery cells in a basic circuit configuration.

In what circuit are the PhotoMOS used

First, PhotoMOS connected to the battery cell are switched on (closed) to charge capacitors.

At this time, other PhotoMOS which are connected to the measuring system are kept open to maintain isolation between the battery cell and the measuring system. When the capacitors are completely charged, the PhotoMOS connected to the battery cell are opened, followed by the PhotoMOS connected to the measuring system which are then closed (switched on). This process allows one to measure the voltage of the battery cell safely. Monitoring of the remaining battery capacity is carried out frequently as long as the device is powered. This frequent switching used during the monitoring of the battery system would pose a problem for mechanical relays because of their limited service life. Adopting PhotoMOS (semiconductor relays) eliminates the problem caused by frequent switching. PhotoMOS are needed particularly for products that consume a large amount of power, such as electric vehicles carrying many battery cells that must be monitored and controlled individually. Another reason for the large adoption of PhotoMOS in this market is that they have a much smaller in size compared to mechanical relays allowing for higher density mounting.

The BMS is an example of a system to which the PhotoMOS is applied. We encourage you to see how the simple design and extreme versatility of PhotoMOS can be advantageous for your own applications.

Related information:
PhotoMOS-related information provided in the Pearls of Wisdom on the Panasonic website.

Panasonic is a leading company in semiconductor relay production and mechanical relay production, offering a wide range of product lineups.
Panasonic PhotoMOS product lineup page

To see the next article, visit this link to Vol. 2

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