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SSR Technicalities

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An electronic switch made up of solid state components.No mechanical contacts or moving parts.

What is a Solid State Relay?

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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.

The SSR is powered by the AC Line itself, by connecting the 2 gates of the output SCR’s through a controlled switch.

Basic Switching Element of the SSR

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Isolation (Optical)

InputOutput

EMR SSR

Input

Isolation (Magnetic)Isolation (Magnetic)

Output

Form, Fit, and Function Comparison…

Electro-Mechanical Relays vs. Solid State Relays

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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 noiseHigh surge current capability for severe inrush loads

Capability to >1500 Apk for 1 cycle.

Advantages of SSR’s over EMR’s

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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

The printed circuit board shown is a complete 1200 V type which was assembled by an automated robotic machine

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SSR Output Devices - AC

TriacSingle silicon chip device. Switches both polarities of the AC line. Economical, but consideration needs to be given to inductive loads that might produce self- commutating effects. (dv/dt)

Dual SCR’s2 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 TransistorEconomical, but drawbacks include relatively slow turn-off and high power dissipation.

MOSFETWith 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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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.

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.)

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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

Time

Volta

ge

ControlSignal

SwitchedRelay

Output toLoad

1

4

3

2

AC Supply

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Types of Switching (cont.)

NON ZERO-CROSS SWITCHING

1. When the input signal is activated,2. The output immediately turns on

since there is no zero-cross detector circuit,

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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Non Zero-cross (Random)

Time

Volta

ge

AC Supply

ControlSignal

SwitchedRelay

Output toLoad

1

2

3

4

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Using Random SSR’s for Phase Controlling

Time

Volta

ge

ControlSignal

Voltageacross load

AC Supply

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Dimming Control

Time

ControlSignal

Outputvoltageto load

AC Supply

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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

CMR CKR

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.

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OvercurrentOvervoltageTransients

SSR Protection Methods

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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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Isolation (Optical)

Input

Output

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.

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Micro-Controlled SSR’s

Crydom has released a series of SSR’s incorporating various Microprocessor controlled functions built in to one package.

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

Series MCTC•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

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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