UPDATE: Ok, two good answers, I've chosen the one with more votes as the "right" answer, leaving it to the wisdom of crowds to choose between two fine choices. For anyone who cares, I think I'll settle on the term "catch" diode, mostly because it has fewer syllables and I have a deep abiding respect for Horowitz & Hill. But I'll also adopt the metaphorical view that the switch set the current in motion when open and releases it ("throws" the current) when it closes, and the diode then "catches" it and delivers it to the output 🙂
Thanks very much to both respondents and those who commented for informing my choice 🙂
I've come to the point in my learning about switch-mode power supplies where I find myself referring to the "main" switching diode quite a bit. I'd like to use the most correct name consistently going forward because I'm forming the habit of what I'll be calling it in documentation and conversations like this, etc.
I had pretty much picked up the term "freewheeling" diode, which made some sense to me because the inductor could be thought of as analogous to a physical flywheel that stored energy in it's momentum, and would "freewheel" once energy was no longer added to it (or protest violently on attempts to stop it suddenly 🙂
However, just lately, I see the term "catch" diode used, as in this TI application note AN-1229.
Is it just one of those terms that never really normalized and everybody chooses between a half-dozen alternatives? Or perhaps is it a "catch" diode in one topology and a "freewheeling" diode in another?
I do note that the term "freewheeling" diode also appears in the context of avoiding turn-off spiking from an inductive load, where one is just looking to dissipate the energy stored in an inductor rather than carefully apply it to a useful load.
Best Answer
Either term gets used and understood in either case. But I feel there is a subtle distinction, that gives away how much the person using it understands about the circuit.
I think SMPS's tend to use freewheel diodes, where the energy storage in the inductor is an intentional part of their operation. The diode lets the inductor, or at least the circuit current, keep spinning. Catch doesn't seem enough to describe it.
Transistors that switch relays tend to use catch diodes, where the inductor energy storage is a nuisance. Nearly got zapped there, phew, good catch! Freewheel diode gets used here as well, though it seems to be a bit over the top, like a sledgehammer to crack a nut.