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Thermal Management
Crouzet Solid State Relays Introduction
Heat sinks are required to ensure the proper operation and reliability of solid state relays. AC output solid state relays by nature create thermal energy (heat) at roughly the rate of 1 to 1.5 watts per ampere of load current carried through the relay. This internally generated heat, coupled with the ambient temperature inside the panel and the available heat sink for the relay, determines the operating temperature within the solid state relay.

Heat and thermal fatigue created by the changing temperature within a solid state relay reduce the life expectancy and reliability of the product. Lower operating temperatures and/or thermal excursions within the relay improves overall reliability. Conversely, higher operating temperatures and/or thermal excursions within the relay reduce overall reliability. Therefore, utilizing the proper heat sink improves the overall reliability of a solid state relay by reducing internal temperature and the thermal excursions.

This document covers the basic variables involved in determining the proper heat sink for most solid state relay applications. Morever, it provides information on how to account for duty-cycle, forced airflow, and the key parameters necessary for making these calculations. For convenience, a glossary of terms and definitions is available at the bottom of this text.

Crouzet Solid State Relays SSR / Heat Sink Structure
Heat sinks are made of thermally conductive material(s) and are intended to reduce the temperature of a heat generating element mounted to it's surface. Generally, heat sinks are made from high thermal conductivity materials such as aluminum or copper. They are available in a variety of standard extrusion profiles or may be custom extruded per the customer's specific requirements.

Heat sink performance is rated by thermal resistance measured in degrees C per watt (°C/W), which is the inverse of thermal conductivity. A heat sink rating of 2.0°C/W means that any heat generating element mounted to the heat sink will operate at a temperature above the ambient equal to 2 times the wattage being dissipated in the element. The lower this number, the better the heat sink is at dissipating heat into the ambient air.

Solid state relays, which have semiconductor outputs, dissipate power and generate heat in the conduction state because there is a finite voltage drop across the semiconductor during current conduction. Generally speaking, SSRs can operate satisfactorily up to approximately 5 to 8 amps without benefit of integral or external heat sinking. Beyond that, power dissipation may be sufficient to heat the solid state relay to such a degree that it will fail.

The basic structure of a solid state relay includes an internal power semiconductor mounted to an electrical insulator, which in turn is mounted to the SSR's base plate. The relay's thermal impedance rating (Rjb) represents the temperature difference between the semiconductor and the SSR's base plate due to the thermal resistance of the materials in the thermal path (insulator, base plate, the solder used in the assembly, and the SCR die themselves). Likewise, the thermal interface material used between the base plate and heat sink has a thermal impedance rating, which also has an effect on heat transfer.

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Crouzet Solid State Relays Required Information
In order to calculate and then select a heat sink for a given application, or to ascertain if a pre-selected heat sink/SSR combination is adequate for a given application, certain information about the application and the solid state relay is required. The following list indicates the minimum required information:
  • Determine the maximum ambient temperature (°C) inside the panel where the solid state relay will be utilized. The heat dissipated by the relay inside the panel must be taken into consideration when estimating ambient.
  • Determine available airflow. Airflow may be forced or natural convection, depending upon the enclosure. Forced air is the most effective way to cool an assembly, but requires the use of air-handling components. Convection airflow is the most common and cost effective method, but requires that the panel allow for air to easily circulate around the relay and heat sink.
  • Determine the maximum load current (Iload) that will flow through the output of the solid state relay and the duty-cycle (if known). The duty-cycle is the ratio of the solid state relays on-time to it's off-time.
  • After selecting the appropriate solid state relay, determine it's actual or typical forward-voltage drop (Vf) and it's thermal impedance (Rjb).
  • Determine the impedance of the thermal interface material (Rti) that will be used with the solid state relay and heat sink. The thermal interface material used in solid state relay / heat sink assemblies should be as thin as possible, yet still thick enough to fill any voids that may exist between the relay's base plate and the surface of the heat sink.
  • Determine the maximum allowable base plate temperature of the solid state relay (Tbpmax). This is typically 100°C for most relays, and can be assumed as such if not specified for the product.


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Crouzet Solid State Relays #1 > Solid State Relay/Heat Sink Example
Determine the minimum heat sink rating required (Rs-a) in °C/W for an application in free air with a 100% duty cycle, based upon a known maximum allowed SSR base plate temperature (Tbpmax):
  1. Calculate power dissipation (Pd) by multiplying the maximum load current (rms) by the forward voltage drop of the solid state relay. Pd = Iload x Vf
  2. Determine the maximum ambient temperature inside the panel, taking into consideration the heat generated by the solid state relay assembly.
  3. Determine the maximum allowable base plate temperature of the SSR from published specifications.
EXAMPLE:
Ambient (Ta) = 55°C
Load Current = 12 amps
Vf = 1.2V
Tbpmax = 100°C

Calculate as follows:
Pd = Iload x Vf > 12 amps x 1.2 volts = 14.4 Watts
Min Rs-a = (Tbpmax - Ta) / Pd > (100°C - 55°C) / 14.4 Watts

Based upon on this calculation, the heat sink must be at least 3.125°C/W or better in order to keep the base plate from exceeding 100°C within the application.

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Crouzet Solid State Relays #2 > Solid State Relay/Heat Sink Example
Based on the information in the previous example, calculate the die temperature within the solid state relay to ensure that the maximum rating of 125°C is not being exceeded in the application. Power dissipation, ambient temperature, and the thermal impedance of the heat sink are now known.
  1. Determine the solid state relay's junction-to-base plate (Rjb) thermal impedance. This is usually readily available from the published specifications.
  2. Determine the thermal impedance of the interface material (Rti) being used between the solid state relays and the heat sink. Dow Corning 340 has a thermal impedance of approximately 0.1°C/W when applied properly.
  3. Determine the solid state relay's maximum allowable SCR die temperature (Tj). This is typically 125°C.
EXAMPLE:
Ambient (Ta) = 55°C (from example #1)
Tj = 125°C
Rs-a = 3.125°C/W (from example #1)
Pd = 14.4 Watts (from example #1)
Rjb = 0.4°C/W
Rti = 0.1°C/W

Calculate as follows:
Tj = Ta + Pd(Rs-a + Rjb + Rti)

You can see by the formula that the die temperature is calculated by multiplying the sum of the thermal impedance of each part of the assembly by the total power being dissipated, then adding the result to the ambient temperature. Using the information provided above, we get:

Tj = 55°C + 14.4W(3.125°C/W + 0.4°C/W + 0.1°C/W)
Tj = 55°C + 14.4W(3.625°C/W)
Tj = 55°C + 52.2°C
Tj = 107.2°C

In this application, the junction temperature of the SCR die in the solid state relay should not exceed 107.2°C. This is well below the maximum rating of 125°C, so the solid state relay selected is suitable for the application.

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Crouzet Solid State Relays Example Cautions
It is important to note that any change in the variables listed in these examples may significantly impact the actual temperature of the SCR die in the solid state relays. For example, assume that a 3.125°C/W heat sink was not readily available, so a more common 3.5°C/W heat sink was selected instead. The impact on the die temperature would be as follows:

Tj = Ta x Pd(Rs-a + Rjb + Rti)
Tj = 55°C + 14.4W(3.5°C/W + 0.4°C/W + 0.1°C/W)
Tj = 55°C + 14.4W(4.0°C/W)
Tj = 55°C + 57.6°C
Tj = 112.6°C

The SCR die temperature is still below the 125°C maximum rating. However, let's see how the change impacted the base plate temperature of the solid state relay:

Tbp = Ta + (Pd x Rs-a)
Tbp = 55°C + (14.4W x 3.5°C/W)
Tbp = 55°C + 50.4°C
Tbp = 105.4°C

While the die temperature is well below their maximum rating, the base plate of the solid state relay has exceeded the specified 100°C rating. Therefore, the efficiency of the heat sink must be improved in order to ensure the reliable operation of the solid state relay.

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Crouzet Solid State Relays Effect of Duty Cycle
In Eaxmple #1 and the related examples, a 100% duty cycle was a stated condition. Generally, a 100% duty cycle, or "full on" operation, creates the highest power dissipation and therefore the highest temperatures in the solid state relay/heat sink assembly (excluding the possible effects of repeated cycling of high current inrush loads). However, it also creates a stable temperature after a sufficient amount of time for the system to reach thermal equilibrium, and therefore presents the simplest calculations.

For those applications where the load is not continuously on, the heat sink rating may be reduced (larger numerical value) to some extent, depending on various conditions. The extent to which the heat sink rating may be reduced is based upon the relationship between the thermal time constant of the solid state relay/heat sink and the SSR's on/off times for the application. The thermal time constant is the amount of time that it takes the solid state relay/heat sink in the application to reach a stable temperature if operated at 100% duty cycle.

Generally, if the on/off times are less than the thermal time constant, the heat sink's rating may be reduced to some extent. The best way to determine the thermal time constant is to measure it in the application or to simulate it in a lab using the selected solid state relay/heat sink with an embedded thermal couple. The process is simple in principle: monitor the SSR's output semiconductor die temperature until the rate of change of temperature decreases to less than 1 degree C per minute.

However, if actual or simulated tests are impractical or impossible, then some approximations can be made. Typically, if an SSR is properly chosen and correctly matched to its application including ambient temperature, air flow, etc., it will stabilize (to within 1 degree C per minute rate of change) in approximately 30 minutes.

Therefore, any application is considered to be at 100 % duty cycle for the purposes of heat sink selection if the solid state relays on-time is 30 minutes or more, regardless of the off-time. For those cases where the on-time is less than 30 minutes, the following approximation can be used to calculate the effect on the heat sink rating:

SSR/heat sink thermal time constant (minutes) (Ttc) divided into the total on-time during the time constant period (Ton), times the heat sink thermal rating, or: (Ton / Ttc)Rs-a = revised Rs-a


EXAMPLE:
Time Constant (Ttc) = <30 Minutes
Duty-Cycle = 50% (1 minute on / 1 minute off)
Rs-a at 100% Duty-Cycle = 2.0°C/W

revised Rs-a = (Ton / Ttc)Rs-a
revised Rs-a = (1 minute / 2 minutes)2.0°C/W
revised Rs-a = 0.5 x 2.0°C/W
revised Rs-a = 1.0°C/W

The die temperature within the solid state relay may now be calulated as if the heat sink had a thermal impedance of 1.0°C/W, rather than it's specified 2.0°C/W rating. Conversely, the heat sink in the application may be substituted with one that is smaller, less efficient, and less expensive. To determine an appropriate rating for an alternate heat sink for this application, simply reverse the Ton and Ttc values.

revised Rs-a = (Ttc / Ton)Rs-a
revised Rs-a = (2 minute / 1 minutes)2.0°C/W
revised Rs-a = 2 x 2.0°C/W
revised Rs-a = 4.0°C/W

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Crouzet Solid State Relays Effect of Forced Air
The normal ratings published for extruded heat sinks are based upon free convection airflow over the heat sink's entire surface. It is assumed that the heat sinks are mounted in a position favorable to the promotion of free convection airflow.

The effect of forced airflow on a heat sink's normal thermal rating can be significant. The airflow may be from a fan, air duct, or compressed air. The volume of airflow and its temperature relative to the SSR/heat sink will determine the actual effect on the heat sink rating.

In certain cases, the cost of facilitating air flow with a smaller heat sink may be a net savings verses the cost of a larger heat sink providing the same thermal performance without forced airflow. In all cases, the air should be clean and free of debris, particles, oil or water, etc. Contaminated air will coat the thermally conductive surfaces of the heat sink and diminish its efficiency, effectively raising its thermal impedance and the temperature of the SSR mounted on it.

Each heat sink will react differently to the effect of forced airflow. If possible, the heat sink manufacturer should be contacted to determine the change in the heat sink's thermal rating based upon airflow. You will need to estimate the impact of forced airflow in those cases where this is not possible or the information is not available.
  1. Determine the velocity of airflow over the heat sink in linear feet per minute (LFM) through measurement or calculation. If the fan is rated in cubic feet per minute (CFM), then convert CFM to LFM by dividing the CFM rating by the open surface area of the fan in square feet. Then derate by 30% to account for any back-pressure effects, which reduce volume/velocity. (LFM = CFM / area)
  2. Multiply the heat sink impedance (Rs-a by the following correction factor:

    100LFM = .757
    200LFM = .536
    300LFM = .439
    400LFM = .378
    500LFM = .338
    600LFM = .309
    700LFM = .286
    800LFM = .268
    900LFM = .252
    1000LFM = .239
Therefore, a 2.0°C/W heat sink with only 300LFM of forced airflow would have a thermal efficiency of 0.88°C/W, which more than doubles it's effective rating.

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Crouzet Solid State Relays Summary
The selection of a solid state relay and heat sink for a given application as discussed in this document contains a number of variables and considerations for both the SSR and heat sink. Regarding solid state relays, two parameters in particular are of significance when comparing and selecting a relay: thermal impedance from the output semiconductor to the base plate (Rjb), and forward voltage drop (Vf).

The solid state relay's thermal impedance, which is measured from the output semiconductor to the base plate, determines the temperature rise above the base plate for every watt of power dissipated in the output. The lower the thermal impedance, the less the difference in the die temperature and the base plate temperature. Better quality relays will have lower thermal impedances, and as a result, greater reliability because of lower operating temperatures and thermal excursions.

Lower thermal impedances are possible through the use of more thermally efficient materials, such as copper verses aluminum, solder verses epoxy, thinner ceramic made possible by direct bonding, etc. These materials, while more expensive than lesser performing materials, can significantly improve the solid state relays's thermal performance.

Output forward voltage drop is significant because it determines the power dissipated in the output semiconductor. Lower voltage drops produce less power, thus lower temperatures and thermal excursions. A reduction in the forward voltage drop can be achieved by using SCR die with a larger surface area. The greater the conduction area of the die, the less current “crowding” occurs, which effective reduces the voltage drop and power dissipation. Also, the larger contact area between the die and insulator improves thermal conduction and reduces overall temperature rise.

The forward voltage drop of a solid state relay may also be lowered by reducing the thickness of the SCR die. Thinner die have a lower voltage drop, which results in less heat generated in the SCR assembly. Conversely, thinner die also have a lower breakdown voltage. However, the output of most Crouzet's solid state relays come equipped with internal transient protection and are designed to attenuate electrical spikes well below the rated breakdown voltage. In this case, it is not necessary to have SCR die rated to 1,600V if the transient protection will begin conducting at 1,100 volts.



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Crouzet Solid State Relays Glossary of Terms
• Ahs - Total exposed radiating surface area (in2) of a panel
• Airflow - The amount of air, measured in cubic feet per minute (CFM) or linear feet per minute (LFM), that flows over the exposed surface area of a heat sink
• °C/W - Degrees Centigrade per watt of power being dissipated. Used to define the thermal impedance of a device or system
• Duty-Cycle - The ratio of on-time to total operation time of a solid state relay.
• Iload - Load current through the output of a solid state relay, measured in amps rms
• Pd - Power dissipation in a on-state semiconductor, measured in Watts
• Rjb - The thermal impedance between the SCR die's junction temperature and the base plate of a solid state relay
• Rs-a - The thermal impedance of a heat sink or panel
• Rti - The thermal impedance of the interface material used between the base plate of a solid state relay and the heat sink
• solid state relay - A relay where the load switching function is performed by a semiconductor
• Self Heating - The condition wherein the power dissipation of a solid state relay increases the ambient temperature, thus further increasing the amount of heat being generated
• Ta - The ambient temperature around a solid state relay within the application during normal use
• Tbp - The base plate temperature of a solid state relay
• Tbpmax - The maximum allowable base plate temperature of a solid state relay
• Tj - The junction temperature of the SCR die in the output of a solid state relay
• Tjmax - The maximum allowable junction temperature of a solid state relay
• Thermal Conductivity - The measure of a materials ability to conduct thermal energy. The reciprocal of thermal impedance.
• Thermal Excursions - Changes in temperature over the minimum to maximum range of a device, measured in °C per unit of time
• Thermal Fatigue - The failure of a mechanical joint due to the stresses created by repeated changes in temperature in conjunction with the thermal coefficients of expansion of the connected materials
• Thermal Interface Material - A thermally conductive material placed between the base plate of a solid state relay and a heat sink, which fills air voids in the surfaces and helps transfer heat away from the base plate.
• Vf - The forward-voltage drop across the output of a solid state relay.

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SSR / Heat Sink Structure
Required Information
Heat Sink Selection - Example 1
Heat Sink Selection - Example 2
Example Cautions
Effect of Duty-Cycle
Effect of Forced Air
Summary
Glossary of Terms
Knowledge Base Index
Knowledge Base Home
SSR Applications - Heating
SSR Applications - Motor Loads
SSR Applications - Lighting
EMC Compliance
Optocouplers & Noise Immunity
Transient Protection - TVS vs. MOV's
Snubber Networks
Heat Sinks & Thermal Management
solid state relay power dissipation
solid state relay / heat sink assemblies

GN SERIES SOLID STATE RELAYS
AC Output Solid State Relays
Ratings from 10 to 125 Amps
DBC Substrate
Built-In Transient Protection
LED Input Status Indicator
COOLTECH DIN-RAIL SOLID STATE RELAYS
No Heat Sink Calculations Required
Epoxy-Free Design
DIN-Rail or Panel Mount
DBC Substrate
Built-In Transient Protection
LED Input Status Indicator
Relay or Contactor Configuration
DUAL OUTPUT SOLID STATE RELAYS
True 40A Per Channel Relays
240Vac or 600Vac Outputs
Built-In Transient Protection
4-15Vdc or 17-32Vdc Inputs
Optional Keyed/Locking Input Connector
SOLID STATE RELAY ASSEMBLIES
DIN Rail Mount Heat Sink Assemblies
No Heat Sink Calculations Required
Standard Ratings up to 45A at 660Vac
Internal Transient Protection
LED Input Status Indicator
RHP HYBRID SOLID STATE RELAY
20A/240Vac Output (resistive)
Compact 17.5mm Housing
No Heat-Sink Required
>5M Operations at Full Load
UL/cUL Listed
THREE-PHASE SOLID STATE RELAYS
25A, & 50A Three-Phase Outputs
25A & 50A Motor-Reversing Outputs
DBC Substrate
Built-In Transient Protection
Interlock Circuit (Motor-Reversing)
LED Input Status Indicator
DC OUTPUT SOLID STATE RELAYS
10A, 15A, & 30A FET Outputs
10A / 60Vdc Transistor Output
Low On-State Resistance (FET Relays)
IP20 or IP00 Housing
LED Input Status Indicator
PC MOUNT SOLID STATE RELAYS
Up-To 25A at 480Vac Output
SCR or Triac Output
SIP or Flat-Pack
4Kv Optical Isolation
UL Recognized / CE Complaint
SIMM INTERFACE RELAYS
6.2mm DIN Mount IP20 Housing
Transistor, Triac, or EMR Outputs
6 Amp Form C Output Relay (EMR Output)
LED Status Indicator
HEAT SINKS & ASSEMBLIES
DIN or Panel Mount
Standard Heat Sinks up-to 0.5°C/W
One or Multiple Relays
Custom Assemblies