In the world of electrical engineering, it’s important to realize that numbers can lie. This is especially true when looking at MOSFETs.
The first page of a datasheet is notorious for this because manufacturers often cherry pick numbers at specific test parameters to stack up well against the competition. The published parameters help us understand the basics of the part we're looking at, but rarely do they actually help us make a final decision on which part we need to use. To understand the parts better and make the best choice when it comes to the system you are creating it is well worth it to dive deeper into the specifics of the datasheet.
When sorting through the parametric data you might end up with 5 to 10 different parts that all seem to meet the needs of your system. All the parts seem equal in capabilities, so how do you make the right purchasing decision? This is where the world of the Safe Operating Area (SOA) becomes important. All of the parts you are looking at may claim to have the same voltage and amperage characteristics but when you dive deeper into the datasheet you may notice significant differences in their performance. I picked the SOA characteristic specifically for this article but there is a lot more you can find in the datasheet to help you choose the right part—like how Rds(on) can change over temp or how hard you need to drive the part for a specific set of current and voltage outputs.
What is the safe operating area? The SOA is a region defined by voltage and amperage telling you how long a part can survive operating under certain conditions without damaging itself. The SOA is based on a combination of factors from the type of substrate to how well the specific package can dissipate built up heat. For many MOSFETs it is incredibly rare to find one that will survive the maximum voltage and amperage rating listed on the datasheet at the same time for anything longer than a microsecond. Even a microsecond is often being generous. Usually a part will fail quickly at the top end of its operating range, where it’s only useful in bursts. There is also a burst or peak current at which a part can survive but this is distinct from an operating burst as it pertains to the SOA.
I want to look specifically at two different parts today. Both of these parts are 650 volt 32 amp MOSFETs. According to their datasheets, these parts should both be adequate for the same system. If we look at the SOA graphs for the same package type and at the same 100 volt rating, we can see how long they will survive if we put 10 amps through them. For the part on the left (STI40N65M2 from STMicroelectronics), if you follow the graph, you will see it is designed to survive for a little less than one millisecond. If you look at the part on the right (IPP60R099P6XKSA1 from Infineon), under the exact same conditions, you will see that it can survive 10 milliseconds. Both of these parts have the same specifications on the first page of their datasheets, but you can see that the part on the right will survive edge-case scenarios much better that the part on the left. It also happens to be $0.10 cheaper. It pays to delve into the statistics.
Part choices matter, so don’t be afraid to dive deeper into the datasheet to make sure you get the right one for your system. Going beyond the first page can make all the difference.