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
17279
5 mA
LED
+
-
PDA
Current-
limiting
circuitry
MOSFET
MOSFET
R
SH
17280
Solid-State Relays
www.vishay.com For technical questions, contact: optocoupleranswers@vishay.com
Document Number: 83858
2 Rev. 1.3, 28-Apr-09
Application Note 56
Vishay Semiconductors
APPLICATION NOTE
amount of forward current applied. For high-temperature or
high load current operation, more LED current is required.
The photodiode voltage biases the normally open,
enhancement-mode MOSFET gates positive with respect to
their sources. For a normally closed depletion-mode
MOSFET, the photodiode array would be wired to bias the
gates negative with respect to their sources.
Figure 2 portrays a current-limiting circuit. This circuit is a
unique feature on many of Vishay form A SSRs. When
current through the MOSFETs becomes greater than the
SSRs‘ rated value, the integrated current-limit circuit is
activated. This circuit increases the impedance of the
MOSFET switches thereby regulating the amount of current
flowing through the SSR. When LED current is removed, the
gate-to-source shunt resistance (R
SH
) turns the MOSFET
switches off by providing a discharge path for the gate
charge. Vishay SSRs use either a JFET or MOS circuit for
the gate-to-source shunt resistance in order to achieve fast
turn-off. This control circuitry ensures a smooth “click free”
turn-on and a slower turn-on than turn-off. This feature can
sometimes be used to achieve break-before-make operation
when using multiple relays.
OUTPUT OPERATION
Figure 3 shows the bidirectional or AC/DC I-V characteristics
of a current-limited SSR. Figure 4 shows the bidirectional I-V
characteristics of an SSR without current limiting. In
operation, the SSR is exceptionally linear up to the knee
current (I
K
). This linearity provides a distortion-free contact,
making it ideal for small-signal applications such as V.34
modems. Beyond I
K
, the incremental resistance decreases,
by approximately 35 %, thereby minimizing internal power
dissipation.
For SSRs with current limiting, overload currents are
clamped at I
LMT
by internal circuitry. The current-limiting
circuitry exhibits a negative temperature coefficient, thereby
reducing the current-limit value when relay temperature is
increased. An extended clamp condition, which increases
relay temperature, decreases the current-limit value,
resulting in a current-fold back characteristic. When the
overload current is removed, the relay immediately resumes
its normal I-V characteristics.
Most 6 pin SSRs can be used in a unidirectional or DC mode.
In this mode, on-resistance is reduced by 75 % and load
currents are doubled. For unidirectional applications, pins 4
and 6 become the positive output of the relay and pin 5
becomes the negative output of the relay. Only the LH1510
provides current limiting in this configuration.
Figure 5 shows the unidirectional I-V characteristics of the
LH1510 SSR. Figure 6 shows the unidirectional
characteristics of the SSRs without DC current limiting. Here
the SSRs are exceptionally linear up to and beyond their
rated load current.
Fig. 3 - Typical AC/DC On-Characteristics of a
Current Limited SSR-LH1540
Fig. 4 - Typical AC/DC On-Characteristics of a SSR
without Current Limiting
Fig. 5 - Typical DC Characteristics of the LH1510,
Pins 4 and 6 Shorted
Fig. 6 - Typical DC Characteristics of an SSR without
DC Current Limiting, Pins 4 and 6 Shorted-LH1540
I
LMT
I
K
I
K
- V
+ I
I
+ V
17281
-
I
LMT
172
82
I
L(max.)
+ I
- I
I
K
- V
+ V
I
L(max.)
I
K
+ I
I
LMT
I
L(max.)
-I
- 0.8 V
- V
720 mA
350 mA
- 320 mA
Off-state
2.5
On-state
+ V
17283
+ I
I
L(max.)
- I
Off-state
On-state
- V
+ V
17284