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Adequate heatsinking, including
consideration of air temperature and air flow, is essential to the proper
operation of a solid state relay (SSR or SCR or IGBT package). It is necessary
that the user provide an effective means of removing heat from the package. The
importance of using a proper heat sink cannot be overstressed, since it directly
affects the maximum usable load current and/or maximum allowable ambient
temperature. Lack of attention to this detail can result in improper switching
(lockup) or even total destruction of the SSR. Up to 90% of the problems with
SSRs are directly related to heat. VB-Controls has created several
customer-specific heatsink designs where overall size, fin geometry, fin angle /
spacing, and draw down were optimized for the particular application.
All solid state relays develop heat as a result of a forward voltage drop
through the junction of the output device. Beyond a point, heat will cause a
lowering (or derating) of the load current that can be handled by the SSR.
Heatsinks are used to create a method of removing heat away from the relay, thus
allowing higher current operation.
With
loads of less than 4 amperes, cooling by free flowing convection or forced air
currents around the unit is usually sufficient. Loads greater than 4 Amps will
require heat sinks. SSR units are to be mounted to some heatsinking object,
material heat conductivity should be kept in mind. Heatsinks are approximately
equivalent, in heat dissipation, to a sheet of aluminum 1/8" thick by the
dimensions shown:
12" X 12" =
approximately 2.1 degrees C per watt thermal rise
15" X 15" = approximately 1.5 degrees C per watt
thermal rise
18" X 18" =
approximately 1.0 degrees C per watt thermal rise
(Hint: the lower the C/W rating, the better
the heat sink is at dissipating the heat, given proper ventilation and ambient
temperature.)
In comparison, twice
the amount of steel and four times the amount of stainless steel would be needed
to achieve the same effect. Units should not be mounted in an enclosed area
without proper air flow. Units should also never be mounted to a plastic base or
to painted surfaces. The heat sink should be positioned with the fins in a
vertical position with an unimpeded air flow. The vertical mounting will aid in
heat dissipation in that heat may rise from the heat sink unobstructed. Any
panel mount Solid State Relay must be mounted to a clean, bare (non-painted)
surface that is free of oxidation.
Silicone (thermal) grease should be placed on the metal
base of the relay before mounting to a metal surface. Heat Transfer is affected
by the thickness of the thermal compound, uniformity of application and how
firmly the relay is attached to the heatsink. We suggest an evenly applied
0.002" thick layer of Dow Corning 340™ or equivalent and torque of 10
inch-pounds on both of the SSR mounting screws. Note that a thicker layer of
thermal compound actually decreases heat transmission.
Care must be taken when mounting multiple SSRs in a
confined area. SSRs should be mounted on individual heatsinks whenever possible.
Panel mount SSRs should never be operated without proper Heat Sinking or in Free
Air as they will THERMALLY SELF DESTRUCT UNDER LOAD. A simple Rule-Of-Thumb for
monitoring temperature is to slip a thermocouple under a mounting screw. If the
base temperature does not exceed 45 degrees Celcius under normal operating
conditions, the SSR is operating in an optimal thermal environment. If this
temperature is exceeded the relay's current handling ability must either be
thermally improved by the use of a heatsink, or greater air flow must be
provided over the device through the use of a fan. ANY moving air in an
installation, greatly improves the thermal transfer from the heatsink to the
air. If the actual internal SSR device ever achieves an internal temperature of
105 - 125C, it will be permanently destroyed. Therefore, the total engineering
requirement is to: provide a slow heat rise internal SSR and then to provide a
heat sinking capability that draws the internal heat rise away at a faster rate
than the SSR's internal heat rise.
Some cases may require the selection of a higher current
output SSR and thermally derating the device accordingly. VB-Controls has
specialized in designing SSR packages that optimize the amps vs thermal rise
application. By using a variety of SSR die sizes and copper bonding techniques,
we have created packages that generate significantly less thermal rise than is
seen in similar packages.
Remember
that the heatsink removes the heat from the Solid State Relay and transfers that
heat to the air in the electrical enclosure. In turn, this air must circulate
and transfer its heat to the outside ambient. Providing vents and/or forced
ventilation is a good way to accomplish this. Heatsinks should always have at
least one inch below them, so air can enter the finned heat sink area. Heatsinks
should always have empty space above them so the warm air can exit the heat sink
area. If horizontal, plastic, wiring trays are used above the heat sink, then
the empty space should be greater than the depth of the plastic tray. For
example, if you use 4 inch deep wiring trays, leave >4 inches of empty space
above the relay. All Solid State Relays are capable of running at full rated
power (with proper heatsink), however it is strongly suggested that they be used
at no more than 80% power, to provide a safety margin in case of higher than
expected voltage, temperature, dirt on the heatsink, poor air flow, etc.
PROTECTIVE MEASURES / ELECTRICAL NOISE
SSRs generally do not fail due to electrical noise, unless they happen to
mistrigger during a point in the line cycle when an excessivly high current
surge might occur. Usually, a malfunction due to noise is only temporary, such
as turning on when the SSR should be off, and vice-versa. By its very nature,
noise is difficult to define, being generated by the randomness of contact
bounce and arcing motor commutators, etc. Noise, more properly defined as
ElectroMagnetic Interference (EMI), affects the SSR by feeding signals into the
sensitive parts of the circuit, such as the SCR. A built-in snubber RC network
across the output is effective in reducing sensitivity to noise, especially at
lower frequencies.
MOVs
The metal oxide varistor was developed about
the same time as the SSR and has subsequently become a trustworthy companion of
the SSR, providing much needed protection in some of its more hostile
environments. An MOV can be used as follows: across the incoming line to supress
external transients before they enter the system; across the load to supress
load generated transients; or more frequently, across the SSR to protect it from
all transient sources. In the latter case, the MOV can be conveniently mounted
to the same SSR output terminals as the load wiring. An MOV can be used
effectively across such loads as transformers and switching power supplies where
spikes too fast to be absorbed by the transformer itself may be fed back into
the primary (SSR load) winding. Used within its ratings, the MOV will most
likely outlive its associated equipment and provide low cost protective
insurance for the SSR.
SURGE
RATINGS
There are very few
completely surgeless SSR loads. Next to improper heatsinking, surge current is
one of the most common causes of SSR failure. Overstress of this type can also
seriously impair the life of the SSR. Therefore, in a new application, it would
be wise to carefully examine the surge characteristic of the load and then
select a device that can adequately handle the inrush as well as the steady
state condition, while also meeting the lifetime requirements. In addition to
the actual surge ratings given for SSR's, the rate of rise of surge current (di/dt)
is also a factor in AC thyristor types. Exceeding its value may result in
destruction of the device. As a guide, the amperes-per-microsecond (di/dt)
withstand capabilities are typically in the order of their single cycle surge
ratings. The highest surge current rating of an SSR is typically 10 times the
steady-state RMS value, and it is given as the maximum peak current for one line
cycle. It should be noted that a surge of this magnitude is allowable only 100
times during the SSR's lifetime. Furthermore, control of conduction may be
momentarily lost due to a surge. This means that it may not be possible to turn
off the SSR by removal of control power both during and immediately after the
surge. The output thyristor must regain its blocking capability and the junction
temperature allowed to return to its steady state value before reapplication of
the surge current, which may take several seconds. It should be noted that the
preceding cautionary notes apply only to the extreme limits where the SSR should
not be designed to operate anyway. Generally, DC SSRs do not have an overcurrent
surge capability, since the output transistors are usually rated for continuous
operation at their maximum capacity. The tendency is for the DC SSR to cut off
(current limit), thus impeding the flow of excessive current. However, the
resultant overdissipation may destroy the relay if the surge is prolonged.
FUSING
Fast, "Semiconductor Fuses" are the only reliable way to protect SSR's. They are
also referred to as current-limiting fuses, providing extremely fast opening
while restricting let-through current far below the fault current that could
destroy the semiconductor. This type of fuse tends to be expensive, but it does
provide a means of fully protecting SSRs against high current overloads where
survival of the SSR is of prime importance. An I2T fuse rating (ampere-squared
seconds) is useful in aiding in the proper design of SSR fusing. This rating is
the bench mark for an SSRs ability to handle a shorted output condition.
Continental Industries advocates circuit protection through the use of a
properly selected I2T (semiconductor fuse). Devices such as electromechanical
circuit breakers and slow blow fuses cannot react quickly enough to protect the
SSR in a shorted condition and are not recommended!! Fast blow type fuses may be
appropriate for some applications. Every SSR has an I2T rating (see SSR
specifications on this Web site). The procedure is to select a fuse with an I2t
let-through rating that is less than the I2T capability of the solid state relay
for the same duration. I2T relates directly to the published fuse
characteristics. System designers who are considering using I2T fuses should
consult a good technical manual dealing with the application of these fuses when
designing their systems (i.e. Littlefuse's semiconductor fuse catalog).
SSR APPLICATIONS
Solid State Relays (SSRs) cannot always be applied in exactly the same way as
Electromechanical (EMRs) and when such is the case, caution should be taken.
Inductive Loads
While most SSR loads, even lamps, include
some inductance, its effect with resistive loads is usually negligible. Only
those loads that utilize magnetics to perform their function, such as
transformers and chokes (windings), are likely to have any significant influence
on SSR operation. These loads can create large current surges and the SSR should
be derated accordingly. ***Please call Technical Support for additional
Suggestions.
Transformer Switching
Extremely high current surges are commonly associated with transformers,
especially those with a penchant for saturation. The zero voltage turn on
feature of standard SSRs can increase this possibility and might require that
special precautions be taken. The zero current turn off characteristic of SSRs,
while minimizing the problem, will not prevent it. From the practical and
economic standpoint, the best choice may still be a standard SSR, overrated to
withstand huge surges. ***Please call Technical Support for additional
Suggestions.
Motor Switching
Dynamic loads such as motors and solenoids,
etc., can create special problems for SSRs. High initial surge current is drawn
because their stationary impedance is usually very low. As a motor rotates, it
develops a back EMF that reduces the flow of current. This same back EMF can
also add to the applied line voltage and create 'overvoltage' conditions during
turn off. Most of the surge reducing techniques discussed earlier can also be
applied to motors. It should be noted that overvoltage caused by capacitive
voltage doubling or back EMF from the motor cannot be effectively dealt with by
adding voltage-transient suppressors. Suppressors such as Metal Oxide Varistors
(MOVs) are typically designed for brief high voltage spikes and may be destroyed
by sustained high energy conduction. It is therefore important that SSRs are
chosen to withstand the highest expected sustained voltage excursion. ***Please
call Technical Support for additional Suggestions.
Lamp Switching
The inrush current characteristic of
incandescent (tungsten filament) lamps is somewhat similar to the surge
characteristic of the thyristors used in AC SSR outputs, making them a good
match. The typical ten times steady state ratings which apply to both from a
cold start allow many SSRs to switch lamps with current ratings close to their
own steady state ratings. CAUTION: Using SSRs for driving mercury, fluorescent,
or HID lamps should be avoided. If they must be used, the SSR must be severly
derated and thoroughly tested in the specific application.
Some of the information shown is based on
data from Continental Industries. Many of the personnel at VB-Controls designed,
managed, or worked in other roles at Continental. For more information on the
newest products, please visit:
http://www.power-io.com for solid state relays, din rail relays, I/O
modules, or IGBT based products. |
Back to Top
An SSR (solid state relay) or SCR (silicon controlled
rectifier) is a solid state switching device that can provide fast, infinitely
variable proportional control of electric power. This provides the best control
of your heat process, and it can extend heater life many times compared to
other control methods. Since the SCR is a solid state device, it can be cycled
on and off over without any wear and tear resulting in virtually an indefinite
life IF the consumer addresses the 3 "over" conditions. That is: avoid
over-temperature, over-amperage, and over-voltage conditions. If you fail to
plan for the three "over" conditions, the SSR or SCR is guaranteed to fail.
Since the damage from the 3 "over" conditions is an irreversible cummulative
effect, it is imperative that the installation be designed properly from the
first day.
- Over Temperature occurs
since the SSR/SCR is generating internal heat at the rate of approximately 1.1
- 2.2 watts of heat per amp switched (depending upon the vendor's
specification). If that heat is not removed, the internal device will achieve a
temperature of 125C and will fail. It is critical that the internal heat rise
is kept below this failure point. VB-Controls specializes in SSR / SCR / IGBT
die design + ceramic copper bonding techniques + heat sink transfer. By doing
this, we minimize the initial thermal rise rate and then effectively transfer
that heat in a highly efficient manner to the appropriate heat
sink.
- Over amperage occurs due to
exceeding the SSR's amperage rating, when at operating temperature. In general,
you should design an application to use no more than 80% of a SSR's rating,
when at 40C, in order to anticipate short term fluctuations in the load,
supply, or application. As the ambient temperature rises, or if air flow is
"restricted or blocked", then the SSR must have the amperage capability derated
according to the published graph for that particular model.
- Over voltage occurs due to
voltage surges in an installation. These are typically the result of turning
off a nearby inductive load (a motor, pump, coil, or any other inductive
source). The resulting surge on the power line is damaging to other products
that share that same power line. Over voltage damage is the most difficult
problem to troubleshoot so it is to the customer's advantage to anticipate it
in advance.
ZERO Crossing Switched controls proportionally turn on and off
each full sine wave of the power line. By varying the number of AC power line
cycles, the SCR provides power to the heaters. For example: if 33% power is
required, when using on a one second cycle time controller (PID controller, or
PLC, or similar controller), then 33% = 20 sine waves "on" and 40 sine waves
"off", 20 sine waves "on" and 40 sine waves "off" etc. Since modern PID
temperature controllers can create an overall cycle time of as little as 0.2
seconds, in this same example, the same SCR would now operate at 3 sine waves
"on", 9 sine waves "off", 3 sine waves "on", etc. On small, low mass IR
heaters, you might see the elements slightly flickering. On larger heaters or
resistive heaters, then you would not see any flickering. By using a short
on/off time, the thermal expansion and contraction the heater is reduced and,
therefore, the heater life is increased.
PHASE Angle fired
turns on a percentage of each power line half sine wave. By delaying the
"start" until the sine wave has already crossed the initial zero mark, this
method can provide infinitely variable application of power to the load. This
method has the draw back of generating electric noise so it is usually used
only with loads that require a soft start such as variable resistance heater
loads, transformer coupled heater loads, and some IR heater loads.
ON/OFF Control is usually used with mechanical or mercury
relays. The load stays on or off for an extended period of time such as 20-60
seconds minimum. This extended time is required to prevent the mechanical or
mercury contactor from failing too soon since they have a projected life time
from 250,000 - 6 million cycles, when under ideal conditions. Unfortunately to
extend the life of the relay, you may decrease the life of the heater. The
longer on vs. off cycles cause a dramatic increase in thermal fatigue of the
heater so it will fail sooner.
IGBT Control, as used
in the exclusive VB-Controls design, is similar to phase angle control. An IGBT
turns on at the zero cross mark and turns off later in the same half sine wave.
By delaying the "end" until later in the sine wave, this method can provide
infinitely variable application of power to the load. The advantage in IGBT
technology is that by activating the load at the zero crossing mark, electronic
noise is minimized, the load receives the gradually increasing power reducing
shock to the load (such as eliminating element "sing" in lamp circuits), and
shutdown strategies can by developed to protect the IGBT from failure.
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Back to Top
| There are several types of
electrical switching products. From low cost to high performance, each method
serves a part of the market.
Mechanical relays or
mechanical contactors: The cost of a mechanical relay or mechanical
contactor is influenced by amperage and usage. As these increase, so does the
price. A mechanical product typically has two ratings: the number of mechanical
cycles (a high number) and the number of electrical cycles (a low number).
Therefore, mechanical products should be used when the final device is
activated infrequently. Mechanical relays are seldom used for heater
applications since the cycle time is too slow and the overall product life is
often too short. A mechanical device turns on or off when the coil is energized
which can result in noise spikes or surges since this is not at the zero
crossing point. An ideal cycle time for a mechanical device is often 30-60
seconds or more. Some applications will require snubbers across the power
terminals to help to suppress the voltage surge upon opening at the non-zero
point.
Mercury contactors or mercury
displacement relays (MDR): A mercury contactor uses a pool of mercury,
inside a glass sealed tube, to electrically connect the two contacts. This pool
may be more than 100 grams of mercury for a single contactor. Since mercury is
a cancer causing agent and is banned in many states, countries, or industries;
there are restrictions on shipping, storing, or cleaning up if a contactor
should leak or explode. A mercury contactor is not a zero crossing device so
there will be noise spikes or surges that may affect nearby electrical products
or PLCs. An ideal on/off cycle time for a mercury contactor is often 10-20
seconds and 3-8 million overall cycles. Mercury contactors are used frequently
for industrial heater control of resistive heaters. When used 24 hours a day in
a heater application, there may be 3 million on/off cycles in a single year.
Therefore, the mercury contactors might be replaced many times during the life
of the machine.
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Solid state relays
(SSR): The SSR products are usually packaged in the familiar cube shape
and are frequently referred to as "hockey puck relays". SSRs can use internal
triacs, or the better SSRs will use internal SCRs (a pair of silicon controlled
recifiers). Power-io only uses internal SCRs. SSRs turn on and off at the zero
crossing mark on a sinewave so electrical surges and noise spikes are greatly
reduced. Since there are no moving parts in a SSR, they can be cycled on and
off many times per second. SSRs can be an ideal product if you pay attention to
the three important application items. Those are: avoid over voltage (surges),
over current, and over temperature. For further information, visit:
Solid
state relay -- extended life and optimum performance.
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Power-io
has designed several unique features into our SSR family as part of our Maximum
Surge Survival™ technology. An ideal cycle time for a SSR is often as fast
as 0.2 - 2 seconds and virtually an unlimited number of cycles. SSRs are used
frequently for industrial control of resistive heaters, motor starters (due to
excellent short term amperage surge capability), PC based control applications
(due to minimum control signal input requirements), on/off lighting
applications (zero crossing extends the life of the lamp) and other general
purpose switching applications. When used for heater control, the fast on/off
cycles provide excellent temperature control and this dramatically improves the
heater life due to a reduction in thermal shock. For example: at 25% heater
demand, this may mean 0.05 seconds on, 0.15 seconds off, 0.05 on, .... This is
actually 3 sinewaves on, 9 sinewaves off, 3 on, 9 off, .... Since you are
switching full sinewaves, starting at the zero crossing mark and ending at the
zero crossing mark, there is no noise generated and the power is permitted to
start at zero and then ramp up. The heater stabilizes at a temperature range
and does not fluctuate, thus reducing thermal stress and, therefore, increasing
the heater's life. For information on Power-IO's SSR products, please visit the
"H" hockey puck products at:
solid state relay
products
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Solid state contactors
(SSC): When an industrially hardened SSR is assembled onto a properly
sized heat sink, the final product is frequently called a solid state
contactor. Power-IO uses over-sized internal SCRs (often 20-100% oversized), on
a DCB (Direct Copper Bonded ceramic/copper assembly) attached to a thermally
optimized heat sink design, finger-safe screw terminations, and an universal
mounting bracket. Installation size is minimized by our industry-leading
capability that permits you to install the product with NO spaces between
products on the same din rail. Therefore, the Power-IO "D" din rail family
offers the smallest installed product in the industry. As a finished contactor,
the product provides the fastest and most convenient way to install a complete
solid state switching product. For more information, please visit:
solid-state
relays
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For electric heater control, solid
state contactors are typically used with PLCs, PCs, network I/O, or PID
controllers that generate the pulsing vac or vdc control signal. This may be
called a time proportional output, TPO, or pulse wave modulation. When used
with some PLCs, it may be easier to reduce programming time by using an analog
output so Power-IO also has solid state contactors that accept a 4-20mA analog
control signal that then gets converted into the appropriate zero crossing
output. (4mA=off, 12mA=50%, 16.8mA=80%. 20mA=full on). These have a part number
starting with DMA (Din rail, Milliamp input). Visit:
4-20 mA input single phase or
three phase solid state relays
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"Intelligent" solid state
contactor: The solid state contactors can have additional functions
added for diagnostics and/or communications. The "C Family" contactors have the
ability to create an output based upon: SCR problems, load open, fuse open, and
thermal problems. In addition, they have the Power-io exclusive feature of
monitoring the SCR's health status independent of the control input which is
beneficial during machine start-ups. Intelligent contactors are one of the
fastest growing segments of power control since they integrate into larger
installations, un-staffed applications, and remote monitored applications for
on/off power control or on/off motor control. |
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"Zero Crossing" Silicon
Controlled Recifier (SCR): The Power-IO solid state relays and solid
state contactors use internal zero crossing silicon controlled rectifiers and
are sometimes called SCR products, SCR power controllers, SCR power packs, or
thyristors. The way that we activate and deactivate the zero crossing is
extremely precise, leading to an exceptionally clean, noise free control mode.
When activated by a vac control signal, we typically do NOT need burden
resistors on the control input. Power-IO products are usually for applications
from 1-100 amps. There are other SCR vendors who concentrate in SCR
applications up to 5000 amps.
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"Phase Angle" Silicon
Controlled Recifier (SCR): A minority of the heater applications use
transformer coupled loads or low cold resistance loads. Your heater
manufacturer will point out that these loads (such as silicon carbide heaters
or moly disilicide heaters) require phase angle SCR control plus current
limiting, automatic recalculation and correction for voltage fluctuations,
partial load imbalance, and/or other features. These units are typically much
more expensive but may be required for applications such as molecular beam
epitaxy equipment in semi-conductor fab facilities. Phase angle control means
the the electrical power is on for part of each sinewave. This permits precise
control but chopping every sinewave creates electrical noise that may impact
nearby equipment. For European applications or any country where CE regulations
are used, phase angle power controllers usually require customized noise
filters that may be as expensive as the phase angle controller that they are
attached to.
Insulated Gate Bipolar
Transistor (IGBT): An IGBT can be turned on AND off anywhere within an
AC sinewave. This also provides precise control capability, as well as the
ability to add performace features for a particular application. The
www.VB-Controls.com division of
Power-IO designs customized IGBT products for specific customer
applications. |
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Mosfet Solid State
Relay: When switching DC loads, the internal device inside the solid
state relay is a mosfet semiconductor (up to 200 vdc or an IGBT up to 600 vdc).
Power-io has a worldwide exclusive technology that permits the rapid 15Khz
switching times for 0-600 vdc applications. Rapid switching is required for PWM
dc servo control, automated test equipment applications, and other applications
where the need is for instanteous ON or OFF control. Mosfet solid state relays
are also used for vehicle or drone applications in order to avoid contact
bounce or other mechanical problems that are associated with electro-mechanical
relays. Most dc loads are somewhat inductive, so it is recommended that you
install a diode across the load to avoid EMF problems. For more information
visit: mosfet solid state
relays |
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