OPA548: RθJA vs RθJC

Hi Team,

Any update regarding this?

Thanks!

Art

Hi Art,

Q: Just like to ask why RθJC is higher than RθJA for the TO-220 package. Has it something to do with the exposed metal?

The differences are contributed by the testing method defined in JEDEC, see p. 5 of the slua844b application report and figure 5. The thermal resistance figures in TO-220 package should not be used to compared to nominal Op Amp packages. The thermal information presented in section 6.4 should be used to compare comparable TO-220 package types. 

According to the recommendations of JEDEC, the RθJA parameter is not suitable for estimating junction temperature in actual applications. It is more suitable for comparing the thermal performance of different devices in the same package type, and it must be noted that the comparison is based on a similar PCB design. Never just compare the value. 

The testing method forces almost all of the dissipated heat through the single surface of the device (case top or bottom of package), so RθJC applies to the condition that the chip dissipated power conducts through the single surface of the device package (case top or bottom). This means that the RθJC parameter is generally suitable for the condition that the only heatsinking is attached at the top (or bottom) of the package where more than 90% of the heat is distributed from the top (or bottom), which is very similar to JEDEC test conditions. For a package with bottom thermal pad, the bottom metal pad is allowed to be soldered onto the PCB, but for the RθJC-top parameter, it must be ensured that the top is the main path of heat sinking.

https://www.ti.com/lit/an/slua844b/slua844b.pdf?ts=1608669915042&ref_url=https%253A%252F%252Fwww.google.com%252F

If you have additional questions, please let us know. 

Best,

Raymond

Thanks for looking into this Raymond!

Our customer do understand why there is a difference between the RθJC and RθJA, but how can they calculate if they need a heatsink? They were hoping that because the OP-AMP in their case doesn't dissipate that much energy, a heatsink would not be necessary. But all the formulas they find include a heatsink.

Art

Hi Art,

We need to know the total energy (in Watts) dissipated at the OPA548 (need to know the voltage supply rails and output source/sink current). The dominated heat dissipation in OPA548 is through the TO-220's bottom side (0.2C/W), which 90% of heat inside of the Op Amp is dissipated through the surface. If the junction temperature  calculation through the back (bottom) side is not insignificant, then it may be ok to eliminate a heat sink, please use the equation shown the red rectangular in Table 2. Quick Summary above. 

The safe operating area recommendation of the part is presented in Figure 40 of the datasheet. It will be better of mount the TO-220 Tab on a PCB, and use the surface of PCB to dissipate the heat, if it can be implemented in an application, see the sections of 10.1.2 Amplifier Mounting and 10.1.3 power dissipation. Since I do not have actual power dissipation figures in your application, I am just making some suggestions without using heat sink method.  

Best,

Raymond

Happy Holidays Raymond!

Thank you for your help. Below are the response from our customer.

"If i understood everything correctly. The OPA548 will dissipate 1.8 Watt, because it will be supplied with 24 volt and will have an output of 18 volt and 0,3 ampere. (24-18)*0,3=1,8 W. If I use the formula from table 2 "thermal resistance summary reason" I get a junction temperature of 40,16: 40+0,2*1,8=40,16. This would mean that i do not need a heatsink. Also if I look at figure 40 from the datasheet this shows that it is within the safe operating area. Am I correct?"

Again thank you for the help, I really appreciate it!

Art

Hi Art,

Happy New Year!

With 1.8W dissipated by OPA548, the junction temperature will likely run pretty hot. Here is why. 

It is assumed that Tcase temperture on the back is maintained at 40C. If Tcase is fairly constant, then your interpretation of Safe Operating Area (SOA) chart is correct. 

OPA548 running without heatsink at Pd = 1.8W, the Tcase temperature on the back is likely unable to maintain at 40C. If we use the Junction to case thermal resistance on top figure at 37.4C/W, 1.8W  inside of OPA548 will estimate that the junction temperature will operate at approx. 40C + 37.4C/W*1.8W = 107.32C, (assume Tamb = 40C).  

If we assume the Tj = 110C, the bottom case temperature will be like at Tb_tab = Tj - 0.2C/W*1.8W = 110-0.36=109.6C. If we divide Tb_case/1.8W= 61C/W. A standard TO-220 package without heatsink, the heatsink-to-ambient thermal resistance in air (under natural convection) is about 70C/W (3 leg TO-220 package typically). OPA548's TO-220 package has 7 pins with slightly larger heatsink & surface area (maybe thicker tab), 61C/W sounds reasonable for the package without use of heat sink. 

Based on the estimation, the OPA548's junction temperature is likely operating at pretty elevated in temperature. If the application has air flow over the TO-220 package, it is likely to be ok (will lower the Tj temperature). If the system is operating in nature convection in a closed enclosure, the Tj temperature may be running too hot and it is operating near the edge of SOA curve (I do not have the figures about the worst case scenario in the application). 

You do not need a large heatsink, but it will be helpful if the TO-220 tab can increase the heat dissipation in its surface area.  

If you have additional questions, please let me know. 

Best,

Raymond