Thermal Management
Guidance for thermal analysis, design, and verification for Salient Motion's controller products
This guide is provided to you to help support your application's thermal and performance needs. It provides some background information regarding electronics thermal management, useful tips and tricks, and additional resources. For product-specific thermal guidance and information, jump to our Thermal Application Guides.
Thermal Management Overview
For electric motor applications demanding high power and efficiency, thermal management is one of, if not the most, critical design factors to consider. Poor rejection of heat from motors and controllers can lead to many negative performance impacts, including reduced power output (due to temperature throttling) system reliability and lifetime, overall lower power conversion efficiency, and of course, operator safety hazards.
Salient Motion's products offer state-of-the-art motor control efficiency with our field-oriented-control. Higher efficiencies lead to less waste heat, allowing for higher power-dense, space, and weight efficient current delivery. However, we cannot avoid the laws of thermodynamics (as much as we wish we could), therefore thermal management is something we spend much of our design efforts on.
Terms to Know:
Thermal Resistance,
Rth[C/W]: The measurement of a component or device's ability to reject heat. Typically seen when referring to PCB components (Rth,junction-case and Rth,junction-board), and heat sinks or other heat rejection components (Rth,case-ambient).Thermal Time Constant,
tau[s]: The response rate of a component or device's temperature due to its thermal mass and thermal resistance.Duty Cycle [ON%]: The percentage of MOSFET "On" time. This is dependent on your battery voltage, motor's rated voltage, and motor's rated speed. For 100% duty cycle, the controller's output voltage must match the rated voltage of the motor to achieve its rated speed.
Conduction Losses: Waste heat due to the internal resistance of the circuity of the controller, primarily from high current through PCB traces and components.
Switching Losses: Waste heat due to the losses due to the imperfect MOSFET switching times.
Ambient Operating Temperature,
Ta[C]: Temperature of the ambient conditions local to the device. This can be higher than the outside air temperature when devices are packaged inside vehicle bays, electronics enclosures, or other internal locations.Junction Temperature,
Tj[C]: Temperature of a component or device's junction; the hottest location where most of the waste heat is generated.Natural (Free) Convection: Removal of heat to the local ambient environment in still or nearly still air.
Forced Convection: Removal of heat to the local ambient environment with moderate to high velocity air or coolant. Often either through vehicle motion, or fan/pump.
Heat Transfer Fundamentals
To understand thermal management, a little background on how heat flows and what we can do to control it will set you up for quick and successful application integration.
Thermal Resistance Network
The best analogy to understand heat transfer for electronics is the circuit analogy. Consider a typical electronic device in a basic enclosure, containing several heat-dissipating components. Assuming steady state conditions (temperatures and power output are constant) and focusing on just one component for now, we can represent the heat and temperature characteristics of the device:
With this network model, we can answer the following:
1) Given the known heat dissipation Q and the worst-case ambient temperature, what is the minimum thermal resistance Rth,total needed in order to ensure Tj is less than its max rated temperature?
2) To achieve < Rth,total, how much conductive heat transfer and convective heat transfer is needed?
Thermal Resistance
Thermal Resistance is a commonly used term used in electronics and hardware design. The reason it is so useful is it describes so much with so little:
How many degrees will the component's temperature increase given the heat dissipated? [Rth = C/W]
How much heat can the component dissipate before hitting its maximum rated temperature with an off-the-shelf heat sink of known thermal resistance? [Q = (Tj-Tamb)/Rth]
How much conductive thermal resistance does the design have with the selected enclosure hardware and materials? [Rth = L/(kAc), L = length, k = thermal conductivity, Ac = cross-sectional area]
How much convective thermal resistance is achievable with a given heat sink total finned area, and air/coolant flow rate? [Rth = 1/(hAs), h = heat transfer coefficient, As = surface area]
Taking advantage of this powerful term, we can come to a thermal management solution very quickly with just a few simple design trades. One common example is asking if the design should use a passive cooled solution (free convection), or an active cooled solution (forced convection).
Selecting a Thermal Solution
Selecting and designing a thermal solution can be tricky across many applications and industries. If you need help, reach out to us and we can provide additional guidance.
This section provides a walk-through guide on how to begin down selecting a thermal management architecture and ensuring design success on the first iteration.
When selecting a cooling solution for your controller, we can categorize possible design architectures into three main classifications:
Passive cooling systems
Air-cooled active systems
Liquid-cooled active systems
Note: there are numerous other classifications and types of thermal management solutions that exist, but for simplicity, we will only consider these three main categories. If you are problem requires something more unique or specific, reach out to us and we would be happy to discuss other solutions.
Passive Cooling vs. Active Cooling
In many applications across industries, passive cooling solutions are more than sufficient for controllers, particularly in applications consisting of lower system duty cycles such as servos and actuators. However, for speed and torque control applications demanding high duty cycles or continuous operation, passive may not be sufficient or optimal for your application.
Determining whether or not an active solution, such as adding a temperature-controlled fan, will depend on what is most critical for the application. If space and weight are critical, and low voltage DC power is available, adding a small electronics fan can trade aluminum heat sink mass for greater power output. Alternatively, if there is mass budget available for a larger heat sink or thermal solution, reducing the need for a fan or pump can simplify a system architecture, and reduce a potential failure mode.
Air Cooling vs. Liquid Cooling
In applications where higher amperages are needed, air cooling may not be sufficient or optimal, especially for applications with space and weight constraints.
An example here depicts some early testing results of a Cyclone implementing both a forced air and forced liquid cooling solutions. Above 200A, Air-cooling solutions begin to result in temperature rises > 20C for a large 6lb heat sink with a high powered fan attached to it. The liquid solution, consisting of a basic coolant loop with off-the-shelf components, held < 20C temperature rises up to 250A, and could reach currents even higher for applications needing more.
Thermal Interfacing
In electronics with higher heat dissipation, it is necessary to create strong conduction paths out of critical components. This is commonly solved with the use of thermal interface products. Some of these products include gap filler pads, thermal pastes and epoxies, thermal tape, and more.
Selecting a Thermal Interface Product
Choosing the correct thermal interface product when attaching a thermal solution or mounting to a chassis will involve several design considerations to think about, but with the right solution can greatly increase your application's overall performance.
For our motor controller products, we recommend one of the following:
Thermal Gap Filler Pads (silicon or non-silicone based options).
Liquid Thermal Gap Filler (silicone or non-silicone based options).
Thermal Greases (lower viscosity liquid gap fillers, typically non-silicone)
Thermal Epoxies (1 or 2-part options, higher strength, permanent installation)
By default we recommend thermal gap filler. Gap filler materials comes in a wide range of strengths, viscosities, and thermal impedances, giving you much more available product options. They are easy to apply and have a wide tolerance for compression and designed gap tolerances.
Vortex Thermal Interface Design
We offer Vortex as a stand-alone PCB option to allow our customers flexibility on weight and space when integrating into their applications. Because of this, we provide specific interface feature specifications so customers can ensure optimal thermal performance. Jump to Vortex Thermal Interface Design.
Verifying your Thermal Solution
If you have questions with validating your design, reach out to us.
Thermal Application Guides
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