Why do we care about avalanche rating for power MOSFETs in circuits that exhibit inductive kick (flyback converters, motor drives, etc.)?
I understand the basic destructive mechanisms behind avalanche breakdown. As described here, the primary mechanisms are triggering the internal NPN into snapback, and plain old overheating until the semiconductor becomes intrinsic.
I also understand that avalanche breakdown can be avoided entirely by employing freewheeling diodes, active and passive snubbers, or simply placing a Zener diode in parallel with the MOSFET.
Please help me understand: With all of the choices for absorbing the inductive kick energy, why should MOSFETs need to be "avalanche rated?" After all, with proper engineering, shouldn't it be possible simply to avoid pushing the MOSFET into breakdown? Not to mention, many snubbing techniques produce substantially less heat because they return some of the energy back to the power source.
Given that some MOSFETs are 100% tested for their avalanche rating, it seems like some people really care for it. The MOSFET manufacturers wouldn't take on the added responsibility if they didn't have to.
Another way of asking the question is: How much is avalanche rating worth? Is it unavoidably necessary in some circuits, or is it just a convenience? Did avalanche rating ever save you in a design?
Best Answer
First of all please note that there is no industry standard for doing avalanche rating, and that often avalanche rated FETs are tested under rather unrealistic conditions (e.g. single pulse only whereas using them in repetitive will cause self destruction). However characterizing and hardening it is sometimes not much harder, in fact there are some transistors that are not rated at all but known for having usable avalanche behaviour.
For the classic scenario of driving something inductive, I can imagine multiple reasons why you use such a transistor:
As such it doesn't often seem to be a base of a good design, but there is a market for it, and so people cater this market.