Small-power motor drives such as fan motors, dishwashers, circulation pumps, and major applications are constrained by challenging form-factors and lower standby current. There are a large number of customers adopting smart power module solutions that reduce the mechanical heatsink needed plus the number of screws previously needed for the discrete implementation. Additionally, the modules integrate the power transistors and system added-value features such as bootstrap diode, under voltage lockout (UVLO) protection function, and thermal sensing function of HVIC. Imagine not only lessening the mechanical and manufacturing challenge — the integration makes it easier to obtain a lower noise layout.
Because these smart-power modules meet the needs and challenges for small-power motor drives, some customers want to extend the range of these smart-power modules by attaching a heatsink to increase the power rating of the drive. Lower-power applications usually do not use a heatsink, because a nearby chassis of the motor can act as a heatsink for heat dissipation. However, if designers try to drive higher power from the same power module, mounting a heatsink can be a relatively easy solution. Some smart-power module packages don’t have mounting holes for a heatsink, so it is not straightforward to attach one.
If you are interested to extend a 100W module to 150W using one of the heatsinking methods highlighted, keep reading. [DUT: FSB50550A, Operating Conditions: VDC = 300 V, VCC = 15 V, FSW = 5 kHz, Sinusoidal PWM, TAmb . = 54o C, THeatsink = 100o C, Heatsink Size (DxWxH): 40 x 24 x 15 mm; results vary with heatsink and thermal adhesive used.] We will discuss how to attach heatsinks to modules without mounting holes.
A. Thermal adhesive material
In order to expand power range of the smart-power module, a small heatsink is sufficient because its major applications is for low-power systems. Method A is to use higher thermal conductivity adhesive material to attach heatsink to 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 and the heatsink used.
B. Direct assemble with PCB by screw (1)
Method B is to use screws to attach smart-power module package to heatsink. This method is stable against external vibration, which may be common in motor applications. Users have to be careful not to bend PCB when assembling the heatsink to the package. If the bolt is tightened too hard, 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 a thermal conductivity pad or thermal grease.
D. Heatsink clip
Method D is to utilize the unique shape of the package. As shown in the green dotted line in figure 4, the SPM 5 package has a trench at the bottom center of the package. A clip can be designed so that the SPM 5 package can be attached to the heatsink by this clip.
In applying this method, important considerations are shape and material of clip. In order to hold the package tight and stable, the 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 5d is a sample drawing of clip. It’s just an example of possible structure. This method has manufacturing benefits while still offering the advantage to extend the power range of the module and the motor drive. Increase manufacturing throughput is gained by assembling the SPM 5 package to the clip-heatsink before wave-solder reflow in manufacturing.
Some 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 an application with severe vibration because the heatsink could be detached from the package. Customers should be verifying reliability through vibration test to apply method A or B. Also a thermal conductivity adhesive dispenser can be considered to improve productivity in the 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.
- SPM 5 products have various power rating lineups from 10W to 100W. For more information click here; and product search by clicking here.
- To analyze and simulate smart-power modules in motor drive applications click here.
- To view application notes about “Motion SPM® 5 Series Version 2 User’s Guide” click here.
- For “Motion SPM® 5 Series Thermal Performance Information by Contact Pressure” click here.
- For “PCB Design Guidance for SPM® ” click here.