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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.

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    \$\begingroup\$ Both terms are pretty much interchangeable. One or other may make more sense in some situations. In some cases the continuation of current flow is the main aim (eg buck converter) so FWD would probably be the best term. In others the aim is to minimise voltage spike (such as on a relay coil) and catch diode may be a better term. But in fact both actions occur in both cases - the focus varies with application. ... \$\endgroup\$
    – Russell McMahon
    Jun 23, 2016 at 8:37
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    \$\begingroup\$ ... To those add "flyback diode" . I have been severely taken to task for using the tern "flyback diode" and "flyback converter" in situations where they seemed apposite (to me). Some argue that "flyback ..." should be limited to TV applications where the functionality is used in the "flyback oscillator" but arguably it's a chicken and egg situation. \$\endgroup\$
    – Russell McMahon
    Jun 23, 2016 at 8:37
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    \$\begingroup\$ @Russell I understood 'flyback' was so-called from CRT TVs because the spot 'flies back' to the start of the scan line? \$\endgroup\$
    – peterG
    Jun 23, 2016 at 8:54
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    \$\begingroup\$ @peterG Yes, as per my comment on Flyback oscillator that causes the spot to flyback. But the waveform and the spot do similar and while it's probably right that they had the screen action in mind when they named it, it was the already known sudden flyback of waveform that made them use it there. So ... :-). Hence my chicken and egg comment. \$\endgroup\$
    – Russell McMahon
    Jun 23, 2016 at 12:40
  • \$\begingroup\$ It's worth noting that the diode is not necessarily the "optimal" component, just a handy way to obtain the required functionality. You may for example want an active switch to reduce losses. That may influence your choice of words. \$\endgroup\$
    – pipe
    Jun 24, 2016 at 23:48

2 Answers 2

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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.

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The Art of Electronics

This question will likely receive multiple answers, each equally valid, so let me start by quoting the venerable Art of Electronics, Third edition: (emphasis mine)

[...] with the "catch diode" (or "freewheeling diode") now conducting to complete the circuit.

As far as I can see from a quick search through the relevant chapter in the physical book, it is only mentioned once, after which it is simply referred to as the diode.

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