480VAC to 120VAC Power Supply

[andersendr]

So I've been looking around at different ways to step down 480VAC to 120VAC.  The general practice and what I have used in industry is to use a transformer.  I am wondering if there are any other sort of commercially available techniques?  Basically I don't want to give up that much real estate inside of my panel for the transformer and I don't want to mount it on the outside of the panel.  There are plenty of 480VAC to 24VDC switch mode power supply's on the market so I would think there should be something similar for 120VAC.

[theatrus]

A transformer.

Unless your power needs are tiny, the significant complexity of an AC-DC-AC converter are not warranted.

[SeanB]

You will have to use a plain transformer. Otherwise convert using a switchmode down to 24VDC as most industrial logic uses it, and use a second one to get your 5V and 3V3 rails.

Why do you need 120VAC anyway.

[AlfBaz]

What sort of AC output do you need, does it have to be a nice sinusoid? If not how about a late firing triac or back to back thyristors?

[T3sl4co1l]

Using bidirectional switches, you can build a synchronous buck converter that works for positive or negative voltage or current.  A bypass cap on the high side, and a filter choke and cap on the low side (plus whatever EMI filtering..), is all that's needed besides the switches.

The output characteristic depends on the control method.  You could go for fixed ratio (fixed PWM), which will behave very much like a transformer, where regulation is proportional to losses.  Some active feedback could provide a slow servo, achieving AC voltage stabilization (like a variac with servo control).  Or a fast feedback loop could even provide limited power factor correction: especially if a four switch ("flying inductor") design is used to work against a filtered DC rail, in which case harmonics and reactive power can be drawn from the line, and compensated out.  DC current can also be drawn from the DC rail, or delivered to it, in which case the power is simply synchronously delivered into the grid.

But yeah, you use a transformer.  I don't know if this product even exists (the "switching variac" as it were).  There's absolutely nothing preventing its existence -- and it would be smaller, lighter, and could possibly be made more efficient as well -- but it will also be significantly more susceptible to transients (source and load), overheating (due to ambient or self heating), and general reliability concerns (i.e., it's not made of dumb wire and iron).  If it does exist, it could even be cheaper than the transformer (part of why linear power supplies aren't used anymore -- switchers, when produced in vast quantity, are very cheap), but it seems likely that the quantity is small (if at all), and the price high as a result.

Tim

[SeanB]

I have seen small switchers that are made on a PCB designed to fit the footprint of the popular small 9VAC mains transformer, designed to be a drop in replacement giving a 9-12VDC output to the load. Of course not as reliable ( being made down to a cost and poorly at that, but they do at least to some extent work) and cheaper than the copper in the mains windings. I would also be very wary of any isolation claims.