What is the effect, downside or purpose, if any, of using buck converters in parallel? For example, to connect the Vout and Gnd together on two converters. Could this be used to increase the current capability? Assume that the converters are identical in design.
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No real benefit is achieved. The buck that regulates to a slightly higher output voltage wins and the other buck basically switches off. However, it's probably a bit worse with synchronous buck regulators because they have active high and low mosfets and can act as a shunt regulator.
This is likely to be a no win situation for either buck with one device trying to regulate downwards by shorting the output whilst the other will try and connect it's series pass transistor to the incoming supply rail far too often in order to fight the other device.
My advice is get chips that can work in master-slave mode or design a better buck regulator.
The purpose is generally to get higher power output capabilities. In general, it may be less expensive to design a single 20-30W converter and duplicate it than to design a single 100W converter.
To reduce loading on the input supply and improve noise characteristics, you can find multiphase buck controllers. These work by sending the same clock signal but phase-shifting between the multiple converters.
This can be done to: