I'm looking to convert 6v AC (.5A) to 5v DC for use in charging a phone via USB. idealy a relatively inexpensive pre existing solution would be perfect, but I've been unable to find anything like that online so I'm curious how I might go about building my own. Would this be a PWM circuit? a transformer? what is the easiest and most cost effective solution.
You need two basic blocks, one to convert the AC voltage to DC and another to step down and regulate the resulting DC voltage.
To rectify the AC to a DC you need a full bridge with a smoothing capacitor
In the output of this you'll get about 6v x 1.414 ~ 8.44v - 1.4v = 7v (the 1.4v drop is caused by the bridge diodes), then you only need to connect a regulator to step this down to 5v.
One option is a linear regulator like LM7805 (note that it needs about 7-7.5v in the input so it may be a bad choice). The schematic in this case looks like the following
If the input voltage is not high enough then you should use low dropout regulator like LM1117 that needs just 6.2v input for 5v output.
Both the mentioned models are linear regulator, what this means is that they vary their series resistance in accordance with the load in order to keep a constant output voltage. Because of that working principle they dissipate the excess power as heat, that power is the product of input/output voltage difference and load current.
A more efficient option is a switching regulator that has a different principle and operates as a fast switch that changes the duty cycle in order to keep the output voltage constant. An example of such a regulator is LM2596. The specific device needs 7v min for 5v output and the circuit looks like
You can use either solution (as long as the input of the regulator is withing specs).
6 v AC can be rectified to about 7.1 volts dc. Add a smoothing capacitor then a low drop out voltage regulator and this should be fine, even if the ac voltage dropped 10%. Depending on current drawn, you may need a small heatsink on the regulator.
You need to check the phone charging current to prove it works on paper but, the above will work for moderate charging currents up to 250mA. This is an indication so you can check you are in the right ballpark for the phone.