Document Number: 83858 For technical questions, contact: optocoupleranswers@vishay.com
www.vishay.com
Rev. 1.3, 28-Apr-09 1
VISHAY SEMICONDUCTORS
Optocouplers and Solid-State Relays
Application Note 56
Solid-State Relays
APPLICATION NOTE
INTRODUCTION
Vishay offers a full line of miniature MOSFET solid-state
relays (SSRs) for use in telecommunication, industrial
control, security, and instrumentation applications.
MOSFET SSRs feature an optocoupler construction, but
have a pair of MOSFETs on the output instead of a
phototransistor. A pair of source-coupled MOSFETs emulate
an electromechanical relay by providing bidirectional switch
capability and a linear contact. No output power supply is
required.
Fig. 1 - SSR Internal View
The advantages of the MOSFET contacts are solid-state
reliability and long life as well as very low thermal switch
offset, extremely high off-resistance, and lack of contact
bounce. Thermal switch offset is actually a misnomer for
SSRs. Any contact offset voltage is photo induced. This
photo-induced voltage is extremely low and typically runs
about 0.1 µV. These attributes make SSRs a significant
contender in applications historically served by reed relays.
In some designs, however, the user must consider the
contact on-resistance and capacitance. Because MOSFET
on-resistance is dominated by the bulk resistivity in the n-drift
region and there is no bipolar junction, no diode offset exists
in the MOSFET SSR I-V characteristics and the
on-resistance is extremely linear. The contact capacitance of
a MOSFET SSR is higher than an open contact of an
electromechanical relay.
The majority of Vishay SSRs have LED inputs and monolithic
switch outputs (figure 1). The switch is built using BiCMOS
technology. Individual components are fabricated in
dielectrically isolated tubs. A fully integrated die has many
advantages. Higher reliability is achieved due to a reduction
in the number of wire bonds. Finer control over circuit
operating parameters is realized for higher-performance
circuits like current limiting.
These miniature SSRs are offered in 6 pin packages, DIP or
surface mount, with single pole, normally open (1 form A) or
normally closed (1 form B) contacts. They also come in 8 pin
DIP or surface-mount packages with two normally open (dual
or 2 form A) or normally closed (dual form B) contacts. Some
SSRs are also available in a low-profile, small-outline
package (SOP).
FUNCTIONAL DESCRIPTION
The infrared light emitted by a gallium-aluminum-arsenide
(GaAlAs) LED within the relay, controls the switch output.
The LED is placed over the output-control switch, directing
light downward onto a stack of photodiodes.
Both input and output silicon are fully encapsulated in a
translucent inner-mold compound that passes light while
providing a reliable, sustaining dielectric barrier in the
thousands of volts. A dark outer mold compound, with a
matched thermal expansion coefficient to the inner-mold
compound, is then called an over under double molded
design.
In a basic schematic for an optically coupled MOSFET SSR,
the photodiode array acts as a floating power source for the
MOSFET switches (figure 2). Each diode is fabricated in its
own dielectrically isolated tub. Current-transfer ratios are
small, and dielectric isolation provides optimum light
reception with no leakage to the substrate. Each diode acts
as a 0.6 V battery when illuminated by the LED. With 20 to
30 diodes, ample voltage is generated to turn on the
MOSFET pair, even at high operating temperatures where
LED and photodiode output drop.
Fig. 2 - SSR Functional Diagram
To turn-on the relay, current is applied to the LED. The LED
emits light, illuminating the inner mold and the photodiode
array. The amount of light emitted is dependent upon the
5 mA
LED
+
-
PDA
Current-
limiting
circuitry
MOSFET
MOSFET
R
SH
17280