Dear Dr. FAE. It would be really great if you would provide MOSFETs with lower on resistance. Resistance causes heat and this heat represents wasted energy. Why do you hate the environment?
- - Angry in Augusta
Dear Dr. Fred. I am switching your FETs at a high frequency and the switching loss is the dominant loss factor. I don’t understand why you don’t work harder in making parts that switch with less of a gate charge requirement and thus, less wasted energy. Why do you hate the environment and waste so much power?
- - Irked in Indianapolis
Dear Angry and Irked. To your great fortune, the barista made me a most-lovely double-latte with an elaborate heart drawn in the perfect centimeter of creamy foam. Therefore, I vow not to allow anything to aggravate me today. My day will be blissful. Bad traffic on the commute? No problem. Slow pedestrians in the crosswalk? Does not bother me. Annoying questions from the public? I laugh.
We often get questions like this. And really, are there any more unappreciated than the hard-working geniuses who labor in the labs and fabs to give you better MOSFETs year-after-year? We recently announced our first sub-milliohm 30V N-channel (FDMS7650) MOSFET in the Power56 package, and you can believe the sparkling apple juice flowed freely in our engineering departments when this part was released.
So, here’s the deal my friends. You think you’re buying one MOSFET, but the reality is-you’re buying many, perhaps millions. Perhaps many millions. That’s how we reduce the on resistance: by cramming more and more parallel transistors on the die. Our figure of merit is resistance per square centimeter of die area and we attack and reduce this number every year.
Figure 1 Internal FET Construction
I know what you’re thinking…with all of these transistors built with trench technology to maximize the drain and source surface areas, it must be a real challenge to keep the capacitance low to minimize gate charge requirements. Yes, that is a challenge. Here’s the basic equation for capacitance:
Figure 2 Emath, Fundamental Capacitance Calculation
This formula reminds us that capacitance increases with “plate” area and when the distance between the plates is reduced. What this means: as we push resistance lower by adding parallel transistors, the capacitance will generally increase. Of course, we do what we can…note the K (dielectric constant)…we’d benefit if that number could be reduced. Also, we’d benefit if pi was increased, hence our appeal to international standards bodies to increase pi to a bigger, easier-to-remember number like 5 (that’s what we call sophisticated engineering humor, friends).
We deliver value to the market in two ways, we either give you the same on resistance with a smaller die at a lower cost or you get lower on resistance at the same price. How often do people call us up and thank us for this? You’re right, not very often.
We’re conscious of the gate charge requirement and do everything we can to control it (via charge balance) and make it small. As the letters above illustrate, it’s tough to please all customers, but we’ll keep trying.
In answer to the latter questions-as a child I had a beautiful Turkish Angora cat named Emir Percival Silkstone. He was killed by the environment. That’s why. Now, if you don’t mind, I shall return to my gorgeous, aromatic latte.