SSR Technicalities
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What is a Solid State Relay?
An electronic switch made up of solid state
components.
No mechanical contacts or moving parts.
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Basic Switching Element of the SSR
The SSR is powered by the AC
Line itself, by connecting the
2 gates of the output SCR’s
through a controlled switch.
When S1 is closed, current from the AC supply flows into
the gate of the forward biased SCR triggering it into
conduction.
As long as S1 is closed, this action continues, reversing
every half cycle of the AC supply.
When S1 is opened, the SCR presently in conduction will
continue to conduct until the zero current point is reached at
which time the SSR will be off.
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Electro-Mechanical Relays vs.
Solid State Relays
Form, Fit, and Function Comparison…
Output
Input
Output
Input
Isolation
(Magnetic)
EMR
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Isolation
(Optical)
SSR
Advantages of SSR’s over EMR’s
Unlimited Life when properly selected and applied
No moving contacts to burn, stick, arc, or bounce
Very low input current required
Very fast response time
Ability to switch at AC zero-cross point or randomly
Inherent characteristic of turn-off at zero current point
No audible noise
High surge current capability for severe inrush loads
Capability to >1500 Apk for 1 cycle.
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Considerations for SSR Use
The Vf drop of the switching silicon will produce
internal heating that must be considered in the
system design.
Off-state leakage. In contrast to EMR’s, there is a
leakage current through the output in the off state of
typically 100 microamps to several milliamps
depending upon the specific model.
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SSR Construction
The two back-to-back, high
voltage, SCR assemblies
show the smallest and
largest SCR chips used by
Crydom in its wide range of
panel mount solid state
relays
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The printed circuit board
shown is a complete 1200 V
type which was assembled
by an automated robotic
machine
SSR Output Devices - AC
Triac
Single silicon chip device. Switches both
polarities of the AC line. Economical, but
consideration needs to be given to
inductive loads that might produce selfcommutating effects. (dv/dt)
Dual SCR’s
2 physically separate silicon devices
connected
in
an
inverse
parallel
configuration. Much better dv/dt ratings
than the Triac.
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SSR Output Devices – DC
Bi-polar Transistor
Economical,
but
drawbacks
include
relatively slow turn-off and high power
dissipation.
MOSFET
With very low Rds-on values available, (vs.
the constant Vf of Bi-polar devices), less
internal heating is produced.
MOSFET based SSR’s can be easily
paralleled for very large current loads.
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The Application of SSR’s
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Basic Specifications
Each SSR series datasheet contains the electrical, mechanical,
available options, and unique derating curves for each relay.
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Primary Criteria..
Current Rating – .(General rule of thumb… derate
to 70% of maximum current desired.) Don’t forget
to also consider the minimum current rating also.
SSR’s need this minimum to function properly.
Package Desired – (e.g. PCB, Panel, or Din Rail
mounting.)
Line Voltage – Consider in harsh electrical
environments using an SSR with a line voltage
rating a step above the application voltage.
Control Voltage – DC, low AC, or High AC.
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Secondary Criteria…
These parameters may require a revisit with the Primary SSR
selection criteria. (I.e. Use of a higher current rated SSR when
the Thermal Environment is near a limit.)
Random or Zero-Cross – (Peak Switching is also
available with the PSD series for highly inductive,
saturable loads.)
Single Cycle Surge Current Rating – Most tungsten
loads upon a cold start can draw 10x to 15x their
normal hot load for a few cycles. Capacitive loads also
require careful analysis.
Leakage Current – Models are available without internal
snubber networks, (R-C across the output used to
improve commutating dv/dt), for low-leakage needs.
Thermal Environment – Use of the derating curves to
determine the heatsinking required, maximum safe
current, or limit of ambient temperature.
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Types of Switching
ZERO-CROSS SWITCHING
1. When the input signal is activated,
2. The internal zero-crossing detector
circuit triggers the output (Triac or
AC Switch) to turn on as the AC load
voltage crosses zero,
3. The load current is maintained by the
thyristors after the input signal is
deactivated,
4. The thyristor is turned off when the
load current crosses zero.
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Zero-cross
AC
Supply
1
Voltage
3
Control
Signal
2
4
Time
15
Switched
Relay
Output to
Load
Types of Switching (cont.)
NON ZERO-CROSS SWITCHING
1.
2.
3.
4.
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When the input signal is activated,
The output immediately turns on
since there is no zero-cross
detector circuit,
The load current is maintained by
the thyristors after the input signal
is deactivated,
The thyristor is turned off when the
load current crosses zero.
Non Zero-cross (Random)
AC
Supply
Voltage
1
3
Control
Signal
2
4
Switched
Relay
Output to
Load
Time
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Using Random SSR’s for
Phase Controlling
Voltage
AC
Supply
Control
Signal
Voltage
across load
Time
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Dimming Control
AC
Supply
Control
Signal
Output
voltage
to load
Time
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Heat Sinking & Thermal Management
The most common root cause of SSR
implementation problems
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Due to the forward voltage drop of the SCR’s, (the Vf
specification), SSR’s generate an internal power loss that is
a function of the load current. The specification is listed as
Vpk, but for normal load currents and 60 hz AC, this power
loss can be estimated at 1 Watt for every 1 Arms of load
current.
The “Thermal Resistance Junction to Case (R-theta)” value,
expressed in DegC/W is also useful in comparing the
different thermal performance of individual SSR’s, and is
also a needed parameter in calculations to determine how
much heatsinking is needed.
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The basic thermal system within the SSR is
illustrated as a simple impedance string…..
By knowing the three thermal impedance values, along
with the expected ambient air temperature, the system
designer can keep the Power Switch Junction
temperature below it’s critical limit of 125 deg.C. (Most
good designs are based on providing a 10 deg.C
margin, keeping the junction temperature to no more
than 115 deg.C).
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Using the spec’s for the CWD2450 mounted on a 1
deg.C/Watt heatsink, in an ambient environment of 60
degC, controlling a load of 30 amps, (30 watts
internally generated), as an example…
.2 DegC/W (from Spec’s)
.1 DegC/W (typ. With
compound or thermal pad)
1.0 DegC/W (HS spec.)
60 DegC Ambient
The heatsink temperature will be 90 degC, (1degC/W x 30W added to the ambient temp.)
The Relay baseplate will be 93 degC, (.1degC/W x 30W added to the heatsink temp.)
The Junction Temp will then be 99 degC, (.2degC/W x 30W added to the baseplate temp.)
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To determine what an adequate heatsink system is for a
particular application without going through the previous
calculations, derating curves are provided for each specific
SSR that were generated using the specification data, and
verified through extensive testing.
As in the case with the calculation
method, using the derating curves
can determine any of the variables
that the system designer wants to
deal with. (i.e. maximum current at
a particular ambient, what heatsink
to use for a set current and
ambient, what is the max. ambient
for a particular system, etc.)
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As an example, if a D1225 is mounted on a heatsink with a thermal resistance
of 1ºC/W and must operate in an ambient of 60ºC, the allowable current of 23A
may be determined by following the route A,B,C,D.
For a verification, the “Max Allowable Case Temperature”,
(baseplate temperature), scale to the right can be used to
double check that the selected system is operating as
expected. In this case, extending the horizontal line to point
“F” indicates that the baseplate temperature should be
expected to be no more than 88 to 89 degrees C.
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Keep in mind that Heatsink
ratings are not a constant.
Their efficiency improves,
(degC/W rating gets smaller),
as the power they are
dissipating increases.
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Integral SSR / Heatsink Units
These Din-rail mount
SSR’s have built in
heatsinks. This eliminates
heatsink selection, and
provides simplified
derating curves that are
strictly concerned with the
ambient operating
temperature.
CMR
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CKR
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SSR Protection Methods
Overcurrent
Overvoltage
Transients
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Overcurrent Protection
There are 2 specifications relevant for SSR’s in regard to
overcurrent
The Single Cycle Surge Rating
The I2T rating for fusing to protect against overloads
(Another current related specification, Di/Dt, is worth
mentioning only briefly, since in almost every application
the impedance of the power source limits the rate of
current rise vs. time to values well below the critical values
of modern SCR’s, even in short circuit conditions.)
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The Single Cycle Surge Rating
Next to improper heatsinking, surge current is one
of the more common causes of SSR failure.
A thorough understand of the initial few cycles of
the load demand can help in selecting an SSR with
a surge rating well in excess of an expected
demand. As stated earlier, a cold tungsten load for
example can draw 10 to 15 times its normal running
load for a cycle or two.
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The I2T rating for fusing
When specifying a fuse that will protect the SSR in addition to
the load, the “total clearing I2T” rating of the fuse selected
must be below the I2T rating of the selected SSR, and above
the expected “normal” current surges of the load.
“Semiconductor” type fuses must be used if protection of the SSR is
desired, (vs. solely protecting the load), as clearing time for
conventional fuses and circuit breakers is much too slow. In
general, protecting the SSR in this manner is seldom used since the
cost of the “Semiconductor Type” fuse usually exceeds the cost of
the SSR.
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Overvoltage and Transient
Protection
Even though AC output SSR’s operate well in a wide variety of
electrical environments, some conditions can produce fast
voltage transients, (spikes), that do not exceed the off-state
withstanding voltage but do affect the relay operation.
In most cases, the speed of these occasional transients that
exceed the dv/dt rating of the relay simply cause the power
SCR’s to conduct for the remaining half-cycle of the AC wave
and then with the absence of a control signal will turn off.
With most loads, that half-cycle conduction will not be
noticeable.
Occasionally, these conductions are not tolerable, so snubber
networks across the output of the SSR work to prevent this
condition by reducing the rate of rise that the output SCR’s
see.
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Output
Input
Isolation
(Optical)
Snubber
location
With the latest semiconductor technology, modern SCR’s
typically withstand upward of 1000 V/us dv/dt transients.
Consequently, the latest SSR series, (the CW), does not
automatically include snubber networks. This is available as
an option if so desired, but generally not necessary.
Examining the “Off-State Leakage” current specification will
reveal if a snubber network is present. Typical leakage
currents with snubbers present are in the 6 – 10 ma range at
maximum line voltage, while the leakage without snubbers is
normally less than 1 ma.
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Transient Overvoltages
Transient Overvoltages differ from fast transients in
that they exceed the “Transient Overvoltage” or
“Maximum Peak Withstanding” voltage specification,
and may or may not exceed the dv/dt rating.
In these instances, unprotected SSR’s may be
permanently damaged unless precautions are taken
to protect them.
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Suppression Devices for
Overvoltage Protection
MOV’s, (Metal Oxide Varistors), have been widely used
to protect sensitive circuit elements by shunting the
transient.
However, the characteristics of MOV’s
change each time they pass transient energy,
eventually failing.
TVS devices, are clamping or breakover diodes. These
devices have no “wear out” mechanism, but generally
cannot pass energy levels as high as the MOV. They
are therefore used within the circuitry as Crydom’s “P”
option, to gate the power devices into conduction
without breaking down the normal circuitry.
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The above circuit shows the location of the TVS device
that will gate the SCR’s on when a voltage transient
comes close to the maximum ratings on the SCR’s and
opto-isolator driver.
37
Micro-Controlled SSR’s
Crydom has released a series of SSR’s incorporating
various Microprocessor controlled functions built in to one
package.
Series MCTC
Series MCPC
•Microprocessor
based
burst-fire
controller / SSR
•Ratings from 25A to 90A @ 48-530 VAC
•Low-voltage, current, or potentiometer
control
•Output status indicator
•Separate output enable / disable control
•Two time-base periods available
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•Microprocessor based temperature controller / SSR
•Ratings from 25A to 90A @ 48-530 VAC
•Direct J or K thermocouple input
•Voltage or current controlled setpoint
•Separate output enable / disable control
•Open thermocouple protection
•LED status indicators
Series MCS
•Microprocessor based soft-start / soft-stop controller
•Ratings from 25A to 90A @ 48-530 VAC
•Low-voltage, current, or potentiometer control
•Output status indicator
•Adjustable ramp rates
For Reference:
Just about all of the information presented within
is available in several technical papers on the
Crydom
Website,
www.crydom.com
Don’t forget to visit our Tech Library and e-catalog
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