Electronics > Beginners
Weird switching topologies you don't see anymore
T3sl4co1l:
Here's a buck converter with what looks like an unusual flyback connection:
https://patents.google.com/patent/US3959716A/en?oq=7782002
Probably they opted for this, to get a common [AC] ground for the switch -- easier to drive, no bootstrap, and less level shifting needed. Switching supplies were new back then, transformers were cheaper than transistors (well, maybe not quite that cheap), and so they weren't afraid of using coupled inductors. We tend to avoid coupled inductors nowadays, just because of commercial availability, really.
Analysis:
We can guess the phasing of the coupled inductor (who needs to read the text?). If the diode conducts while the switch is on, that would be bad (large peak currents). Also, if it were phased so that this happened, it would only conduct if Vin > 2Vo (assuming a 1:1 turns ratio), which is weird and useless.
We conclude they are phased the same direction (dot on the same side). The diode is then doing catch (flyback) duty. The inductor voltages during turn-on, off and idle can be written out assuming one or the other (switch or diode) is shorted, or both open.
If we swap the switch to the Vin side, and the diode to the GND side, we find it's essentially a centertapped inductor, except it's not a center tap at all but a double end tap -- in other words, a short circuit. (Actually a common mode choke, but not a very useful one as one end is shorted.) We can remove one winding, shorting nodes A and B together, and we find we have an ordinary buck topology.
I hope this analysis has been educational. Any other oddball-but-not examples?
Ooh, flyback transformers are a good one, come to think of it. Anyone want to do the "B+ Boost" from ye olde television? ;D
Tim
rsjsouza:
Quite interesting; thanks for highlighting this.
Check the 1983 design review of a 250W power supply by Unitrode. It has a coupled inductor feedback network.
TI still makes this document available for download at: http://www.ti.com/ww/en/power-training/login.shtml (you have to login)
chris_leyson:
Kind of reminds me of the fly-buck converter which can be a useful topology if you need an additional low power auxiliary output voltage. The only drawback is that the auxiliary output is not short circuit proof so needs additional current limiting hardware, a short on the aux output effectively shorts out the main buck inductor. EDIT: To be fair I think the fly-buck is a relatively recent topology.
David Hess:
Tektronix used an odd design for a line voltage buck converter in their 22xx series oscilloscopes.
The second two examples look suspiciously like the boosted current buck converter configuration which uses a center tapped inductor but without the 2:1 step-down ratio.
planet12:
--- Quote from: chris_leyson on July 23, 2019, 09:51:23 pm ---To be fair I think the fly-buck is a relatively recent topology.
--- End quote ---
Depends what we're meaning by "recent" - I've seen this technique in old MC34063 app notes as a way of generating an extra rail.
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