Supplement: Semiconductors Table 1 – Comparison of features in a mechanical and photorelay
Mechanical Relay (Signal Relay)
Lifetime Contact Capacity
Contact Resistance (On Resistance)
Contact Voltage (OFF Voltage)
Isolation Voltage
Operation / Release Time Operation Sound
Miniaturization
Input Power Consumption Contact Form
Leakage Current
What is a photorelay and how does it operate? A photorelay is a solid-state semiconductor device with an LED at its input stage and MOSFETs at the output stage as shown in Figure 1.
When current is applied to the LED, it emits light and this optical signal is converted into an electrical signal by the contact-side photodiode array (PDA). This electrical signal is applied as an input to the gate of the MOSFET, turning it on and thereby making the connection between the two output terminals. When the input current applied to the LED is removed, the input LED switches off, also causing the output MOSFET to switch off and breaking the circuit between the output contacts. This cycle is modeled for low frequency switching in Figure 2.
What is the difference between a mechanical and photorelay? Both devices perform the same function but photorelays offer better reliability because they have no moving parts that can become stuck or wear out after repeated switching cycles. Photorelays are gradually becoming smaller meaning they can help to save space and this is accelerating the transition away from mechanical relays. However, photorelays have a higher resistance (on-resistance or RON) than mechanical relays and also have a capacitance component
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∆ With contact limit (2A)
Ta 25°C / AC-DC applicable
About 0.1
(degraded by ON/OFF)
(ex: AC 250V, DC 30V) (ex: 1 KVrms)
∆ about 5ms ∆ (exist)
(ex: 60 mm2 )
x (coil) 100Mw~ 1c, 2c
Don't exist
(output capacitance or COFF) which is mainly attributable to the PN junction of the parasitic diode in the output side MOSFET. As signal frequency increases, the inductance (L) of a photorelay also becomes an issue. The high frequency model of a photorelay is shown in Figure 3.
Table 1 summarises the features of photorelays and mechanical relays and their relative merits.
Requirements for photorelays used in semiconductor ATE As the number of applications using semiconductors increases, there is also a growing requirement for semiconductor device inspection equipment. Automated Test Equipment (ATE) verifies that a semiconductor Device Under Test (DUT) is operating correctly by applying a voltage or current to specific pins (e.g. memory). Several relays are used to apply these signals to the DUT and traditional mechanical relays are generally not suitable in this application. When selecting a photorelay for use in an ATE application, the following features are important to take into account:
Voltage driven capability
The input side LED of a photorelay is a current driven device and therefore it is common to connect an external current limiting resistor to the LED terminal to
Photorelay
No contact limit (~5A)
Ta 25° C / VOFF basis
=60V
About 0.02-25 (Stable)
(ex : line up with 20V)~600V (max: 5 KVrms)
About 0.1ms (no sound)
S-VSON: 2.9mm2 1.45 x 2.0mm
-
(LED) ex: 0.5Mw~ 1a, 1b, 2a, 1a1b
(20pA~)
prevent damage from overcurrent if an excessively high voltage appears at the input. However, these resistors use up valuable board space which increases size and cost. It is also generally more desirable to drive a device directly with a voltage (instead of a current) source. Toshiba’s TLP3412SRLAA features an integrated resistor on the LED side, which enables voltage-controlled operation without the requirement for additional external resistors.
Low operating voltage for 1.8V system FPGA
The core operating voltage of Field Programmable Gate Arrays (FPGA) is continuously falling in line with the evolution of semiconductor process technology. The TLP3412SRLAA addresses this trend by offering the lowest voltage operation for any of Toshiba’s photorelay products. The maximum operating voltage VFON is 1.6V which comfortably enables the use of 1.8V system FPGA.
High operating temperature ratings A prober (semiconductor tester) is sometimes required to apply signals to a DUT at high temperatures. This results in photorelays being placed close to the high temperature chamber, and therefore they must have the ability to withstand high temperatures. The TLP3412SRLAA has a maximum operating temperature
rating of 125°C which allows it to survive challenging environments in high temperature probers.
Small packaging for high density mounting There are two major categories of test heads in ATE equipment – these are the device power supply (DPS) and the pin electronics (PE). ATEs use relays to switch power supply and signal to test a number of DUTs simultaneously. This makes the availability of high-density test boards critical to reduce the cost and improve the reliability of semiconductor ATEs. Conventional mechanical relays cannot deliver these requirements, whereas by comparison many photorelays can be mounted in a limited board space. The TLP3412SRLA uses an ultra-small S-VSON4T (1.45mm × 2.0mm (typ.), t = 1.4mm (max)) package to help further improve mounting density in ATEs.
Summary
Photorelays offer a range of benefits over mechanical relays in semiconductor ATE equipment. This article considered key attributes to consider when selecting a photorelay for this type of application. It highlights Toshiba’s TLP3412SRLAA as one of the most suitable options, given its performance and reliability.
https://toshiba.semicon-storage.com/eu Components in Electronics May 2024 47
High speed No noise
Smaller size Less power consumption High reliability Photorelay feature Long life
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