that would be analagous to how 220 VAC is provided from separate 120 VAC line 180 degs out of phase.
that’s not how DCC is generated
there’s not necessarily a bi-polar power supply: +14V, -14V and 0V, with the 0V common connected to one rail and the other rail alternately connected to +14V and - 14V. this could be done
more conventionally, a single 14V DC supply with two outputs, 14V and 0V, are alternately connected to opposite rails using an h-bridge. replace motor terminal with rails
the result is the following DCC waveform with the reference on one rail, not the power supply 0V common, and the other rail alternately +/-14V. ~14V AC, Vpk is 14V, Vpk-to-pk is 28V,
I’m not sure of the internal topology of Digitrax boosters, as the schematics are proprietary, but they do generate a common the measures half of Vpeak (not Vp-p) between it and either rail output, despite using an H-bridge output driver like most all others.
I notice there is no such common on the PowerCab - perhaps why to expand it, you don’t add a booster, you replace the built in booster with another, which DOES have a case common to attach even more boosters.
Actually, the mystery may be revealed by the Hans DeLoof DIY booster. 15VAC is supplied to power it, same as with most DCC boosters. V+ and V- rails to drive the H bridge are generated by a simple two diodes (and some filter caps) so that gives you DC at +15 or so and -15 or so relative to the other AC input terminal. I’ve not taken apart my DB150 - others have said the ealier Digitrax boosters (that could run on AC or DC input) have a full wave rectifier right behind the input terminals - maybe to drive the processor, but probably not to generate the power rails for the H bridge.
because of the higher power (10A) output of a booster, it would be simpler to use an AC power supply and two discrete high power mosfets to alternately connect +/- V to one rail rather than use an h-bridge. (I don’t know of any single chip 10A h-bridges). the power supply 0V common is connected to the other rail and is the common connected to all boosters
two half-wave rectifiers produce +/- V. the DCC signal from the command station can be applied to one mosfet and its inverse to the other.
the complication is short circuit protection that requires a processor which may also disable the outputs if there is no DCC signal
It’s a low power design, 3A max. The third schematic is the simplest one - no Loconet, just a basic booster similar in function to ones like the Tam Valley units. It does indeed use a micro to cut power on shorts or on loss of input signal.
It uses an LM675T power amp as the driver - somewhere I have a couple of these, as I think I was going to build this or a similar one that used the same amp as the driver.
