Dissipate Heat in Your Design

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Motor control driver datasheets specify a maximum current for use, but taking this value for granted is a mistake that can have significant impacts on many aspects of your design.

In fact, the available current will always be lower than what is listed. The specified current is the current that bare die can withstand, without taking into account thermal issues. In this article, I’ll discuss both sides of the thermal issues you might need to factor in to your design, first by identifying components that generate heat and then by offering ways to dissipate this heat energy.

Temperature on the board has 3 sources:

1.) The RMS current going through the driver (this is the value specified in the datasheet.
2.) Switching losses.
3.) Temperature from other components transferring heat through copper layers.

Thermal dissipation is most affected by three factors:

1.) The area, thickness and layout of copper on the board.
2.) The package of the driver.
3.) The external temperature and environment (like airflow).

Common specifications for die drivers on the market have a maximum RMS current of 3 Amperes where the standard switch on resistance of a transistor is 300mOhm at 25 degrees Celsius. As the current goes through an H-bridge, the resistance of the bridge is in fact 600mOhm. The power to dissipate from this source is R * IRMS2. Based on these numbers, that is around 5.4W to dissipate.

A design with low power dissipation characteristics has a temperature increasing of 50oC/W, and an optimized design would allow a dissipation of around 15oC/W. Given this, a MOSFET’s RDson will increase roughly from 25oC to 125oC, and since drivers have a thermal shutdown at around 130oC—this means that even in the best case scenario where the temperature would increase only to 81oC  (5.4 * 15oC) , the driver will still reach thermal shutdown. The challenge with power dissipation is that it’s a positive feedback system that triggers a runaway heat problem.

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STMicroelectronics Motor Controller and Driver ICs View

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STMicroelectronics Motor Controller and Driver ICs View

Realistically, the power that is easiest to dissipate is between to 1 and 2W. Above this level, even a good design won’t meet the requirements and will need extra help. For instance, in the oil industry, where pumps use huge currents under extreme heat in deserts, the system is sealed in air conditioning because there’s no way to dissipate that much heat otherwise.

Currents in motors are controlled through pulse-width modulation (PWM) which means the transistor must turn on and off. Drain source capacitance (Coss) is present in all MOSFETs. In each cycle, the energy stored in Coss is dissipated in the MOSFET. The lower the RDson, the bigger the capacitance. In low power applications (< 50W), switching losses can represent half of the energy to dissipate.

Once the trade-off between RDson and switching losses is complete and the estimated energy to dissipate is calculated, designers must find the best way to dissipate the energy.

Junction-to-ambient thermal resistance is the most commonly used parameter to evaluate power dissipation. Several factors affect this thermal resistance but the main ones are PCB design, chip and pad size, external ambient temperature, and airflow. 

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STMicroelectronics Power Management Development Boards and Kits View

For instance, a powerSO36 package mounted on a 4 layer FR4 PCB with a dissipating 2-oz thickness copper surface of 40cm2 on each layer and 22 via holes below the IC has a junction-to-ambient thermal resistance down to 12oC/W. The HTSSOP28 package under the same conditions has a thermal resistance of around 25oC/W. With a 7x7 QFN, the thermal resistance is around 30oC/W. On a 2 layer board, with a very small area of copper to dissipate, the thermal resistance can reach up to 50oC/W, allowing only small currents.

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Allegro MicroSystems Motor Controller and Driver ICs View

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Allegro MicroSystems Motor Controller and Driver ICs View

Finally, as current is applied to a motor, the optimum frequency is often between 20kHz and 40kHz thanks to PWM. When below 20kHz, PWM enters the audio range which can result in a lot of ripples in the current. A higher frequency can decrease these ripples, but when switching above 50kHz, switching losses can be so large that only these losses will trigger the application for thermal shutdown.

Power dissipation and the heat it generates are always going to be a factor in your design. By understanding where this heat comes from and how to best dissipate it, you can ensure that your build stays up and running, and away from thermal shutdown.  

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