SSR/Relay replacement (transistor SW drive) Insulated FET driver Si8751/52

Si8751, Si8752, SSR, Solid State Relay replacement, EMR: Electro Mechanical Relay or Electro Magnetic Relay Mechanical relay replacement, no secondary power supply required

This article describes the isolated FET driver "Si875x", which can replace SSR/relay and does not require a secondary power supply.

table of contents

・Overview and features of the product

・Comparison with mechanical relay/SSR + photocoupler

Product overview and features

・ A MOS relay can be easily configured with the SI875x and an external FET.
Easy system downsizing!

・ Superior MOS relay configuration compared to mechanical relays and SSRs
Achieving miniaturization and noise reduction

・ Compact SOIC-8 package lineup

・ Low power consumption

・ Dielectric strength of 2.5 kVrms

The Si875x is an isolated FET driver that can be driven without a DC power supply on the secondary side.

It can also be used in applications such as replacing electromechanical relays (EMR) with FET switches or replacing solid state relays (SSR). High-speed switching is not possible, but it can operate up to about 7.5KHz.

In addition to reducing switching noise, which is a problem with mechanical relays, it also has advantages in terms of power, life, and heat, as it reduces the PCB area.

It also has advantages over solutions using photocouplers, etc. in terms of life, operating temperature range, etc. Moreover, the simple Miller clamp circuit is also built in and the performance is raised.

Inputs include the digital CMOS input "Si8751" and the photocoupler equivalent diode emulator input "Si8752".

Comparison with mechanical relay/SSR + photocoupler

The Si875x has an edge over mechanical relays. In addition, it is superior to SSRs that use photocouplers in terms of thermal design and life.

	Si875x＋FET	SSR + photo coupler	mechanical relay
Life Reliability	No contacts or moving parts Long life	Restricted by the life of the photocoupler part	Wear due to arc discharge Life is short due to deterioration of contacts and mechanical parts
temperature range	125℃ support possible	125°C support is possible short life due to heat	125℃ support possible
SW part impedance	no change	no change	Increases due to aged deterioration of contacts
Operation sound of SW	none	none	can be
contact bounce (chattering)	none	none	can be
Noise such as EMI	almost none	almost none	There is a possibility that noise may occur in the SW drive circuit, etc.
device size	small	small	Big
external circuit	Bypass C+FET	LED drive circuit, etc. required	surge protection diodes and other drive circuits It is necessary to suppress the surge generated from the coil
power consumption	Input current is Lower than most SSRs	low	high
speed	High speed (below 7.5KHz)	high speed	low speed
surface mount	Surface mountable	Surface mountable	Mounting Surface mounting cannot be used in many cases
Mounting reliability due to vibration shock	Strong (low profile)	Strong (low profile)	Weak High height due to mechanical parts
Reliability of electrical contacts (SW) due to vibration shock	Strong (Electric)	Strong (Electric)	Weak (mechanical)
important point	Heat dissipation measures for external transistors self-heating	heat life electric power cost	Electrical endurance curve EMC

* This is an opinion based on general characteristics.

motion technology

Si8751 application circuit

Si8751 Typical Turn-On Time vs.

Capacitance and TT (90% of Output)

The power sent to the secondary side changes depending on the resistance value connected to TT. The resistance to be connected can be used from 0Ω to Open.

By controlling the input, the output (Gate) voltage will be around 10V. Since it is around 7V at least, I think that most FETs can be driven.

For example, when the TT pin is open, the time to reach 90 % output for a gate capacitance of 200pF is about 1ms (see the light blue line).

The arrival time can be shortened by shorting the TT pin to GND (0 Ω). The maximum frequency that can be driven is about 7.5KHz.

Product lineup

CMOS input Si8751

LED emulator input Si8752

Application example

BMS example

Battery monitor system, etc. It is possible to reduce power consumption by connecting only when monitoring voltage.
It can also be used for earth leakage detection by connecting to the housing.