Motion SPM® 5 products have various power rating lineup from 10W to 100W, and major applications are small power motor drives such as small power fan motor, dishwasher, and circulation pump. These products include bootstrap diode, under voltage lockout (UVLO) protection function and thermal sensing function of HVIC. Motion SPM®5 products have lower stand-by current which are desired for small power motor drive applications. There are already a large number of customers adopting Motion SPM®5 products into their products thanks to these nice features but some customers want to attach heatsink to increase the power rating of the drive. Low power applications usually do not use heatsink for heat dissipation even though chassis of the motor sometimes is used as heatsink. For example, a 100W module can be extended to 150W using heatsink. (DUT: FSB50550A, Operating Conditions: VDC = 300 V, VCC = 15 V, FSW = 5 kHz, Sinusoidal PWM, TAmb. = 54℃,THeatsink = 100℃, Heatsink Size (D*W*H): 40 * 24 * 15 mm. where results vary with heatsink and thermal adhesive used.)
[Figure 1-a] Motor Operation Test without Heatsink (FSB50550A, TC=103℃ @ PIN=100W)
However, if they try to drive higher power from the same power module, mounting heatsink can be a relatively easy solution. Due to the fact that SPM 5 packages don’t have mounting hole for heatsink, it’s not easy to attach heatsink to SPM 5 package. We will discuss how to attach heatsink on SPM 5 package in this blog.
A. Thermal Adhesive Material
In order to expand power range of the SPM® 5 module, small size heatsink is sufficient because their major applications are for low power systems. Method A is to use higher thermal conductivity adhesive material to attach heatsink to the package and, therefore, is much easier than other methods. Representative thermal conductivity adhesive is Loctite 384, thermal conductivity is 0.717 [W/m·K]. It is important to note that thermal adhesive alone doesn’t provide the heat transfer to dissipate the heat. The thermal adhesive makes heat transfer easier between the module to the heatsink used.
B. Direct Assemble with PCB by Screw (1)
Method B is to use screws to attach SPM®5 package to heatsink. This method is stable against external vibration, which maybe common in motor applications. Users have to be careful not to bend PCB when assembling heatsink to the package. If bolt is tightened too strong, it can cause cold solder or solder crack of components soldered around mounting holes.
C. Direct Assemble with PCB by Screw (2)
Method C is an improved version of method B. The key is the shape of heatsink to prevent PCB bending. This method can reduce possibility of cold solder owing to PCB bending, but needs more PCB space. Gap between heatsink and SPM® 5 package should be filled by thermal conductivity pad or thermal grease.
D. Heatsink Clip
Method D is to utilize the unique shape of package. As shown in green dotted line in figure 4, SPM®5 package has a trench at the bottom center of the package. A clip can be designed so that the package can be assembled to heatsink by this clip.
In applying this method, important things are shape and material of clip. In order to hold the package tight and stable, clip has to have tension over certain level. And width and thickness of clip should not exceed the size of trench in SPM 5 package. [Figure 6-d] is an example drawing of clip. It’s just an example of possible structure. Method D has merit on mass production comparing to other methods. Method D can lead to increase productivity (throughput) because user can assemble SPM to heatsink before PCB assembly process.
Some of heatsink attachment methods described here have been adopted by some users. In order to apply these methods in real applications, however, we have to consider many things based on different operating conditions in each application. For example, customers may not apply method A in application with severe vibration because heatsink could be detached from the package. Customers should be verifying reliability through vibration test to apply method A or B. Also thermal conductivity adhesive dispenser can be considered to improve productivity in case of method A. The methods described in this blog are just to demonstrate basic concepts, and customers have to validate the effectiveness and reliability before applying these methods in real production.
To find out more information, SPM® 5 products have various power rating lineup from 10W to 100W: click https://www.fairchildsemi.com/products/power-management/motor-control/ and product search by clicking https://www.fairchildsemi.com/products/power-management/motor-control/motion-spm-smart-power-modules/
To analyze and simulate smart power modules in motor drive applications: https://www.fairchildsemi.com/design/design-tools/motion-control-design-tool/
To view application notes about :
“Motion SPM®5 Series Version 2 User’s Guide” https://www.fairchildsemi.com/application-notes/AN/AN-9080.pdf
“Motion SPM®5 Series Thermal Performance Information by Contact Pressure https://www.fairchildsemi.com/application-notes/AN/AN-9082.pdf
“PCB Design Guidance for SPM®”