I'm thinking 15W 12V secondary, but I could be way off. I'm figuring ~2V drop on the regulator, and 1.7 for the bridge, giving 9.7V, and then just adding some headroom.
The biggest concern I have is with the power dissipation in the regulator. I'm pretty sure you can overheat the thing (depending on the heatsink) at less than 3 Amps when there's more than a couple of volts dropped across the regulator.
It's been a long time since I've calcuated ripple. The voltage regulator will knock-out the ripple, as long as the ripple doesn't exceed the point where the voltage regulator drops-out. So, I usually just use a capacitor that's 1000uF or more, one that's available, and one that will fit physically. (A higher voltage into the regulator will tend to minimize ripple problems, but more it's more power/heat in the regulator.)
QuoteI'm thinking 15W 12V secondary, but I could be way off. I'm figuring ~2V drop on the regulator, and 1.7 for the bridge, giving 9.7V, and then just adding some headroom.12VAC is plenty for a 6VDC supply. I think you're forgetting that the peak AC voltage is about 1.4 times the RMS. At 12VAC your capacitor will charge-up to about 17V minus the diode drops. (I'll usually use a 12V transformer for a 12V power supply.) And at no-load, the transformer will probably put-out more than 12VAC. If you can find a 9V transformer, that should be enough.
Interesting to note that TI recommends an input bypass of 0.1uF ceramic. Due to the discrepancy of capacitance from the values you're recommending, should I assume that the 'bypass' is serving a completely different function here? (I'm aware of bypass as used for, e.g. VCC in to an IC.)
I assume there aren't any particular gotchas for the bridge IC, as long as I pick one that'll handle the current. This one or any similar.
ETA: Finding more web sites repeating the same ripple calculation. Well, okay, I'll take it on faith. 24 mF gives me .7V ripple, if I'm not dropping a decimal point or something. And that just seems way off. .7 = 2/(120*.024)
How about a 7.5V supply and then your 6V regulator after that?http://www.mpja.com/75-Volt-Power-Supply-20A-150W-Switching-Hengfu/productinfo/16018%20PS/