I usually work with 12VDC or less, but I needed to get a quick ~350mA@5VDC supply out of a 24VAC transformer available to me. "No problem," I think: "I'll bridge rectify, filter a bit, and feed it to this Recom 7805-like switching regulator I have in my parts box."
So I do that, and test it by feeding the rectifier both ways from my 12V testing battery, everything looks good. Hooking it up to the 24VAC transformer, though, releases the magic smoke.
Turns out, you gotta figure you'll see ~80Vp-p when you're working with a 24VAC transformer (24 x 1.414 = 34, but unloaded you might see 28VAC or 30VAC). Rectify and you might still be looking at 40V. Recom is only good to 28V, so ... lesson learned.
Next thought: I can use a trusty 33063. I have some of those, and I know how to work them at lower voltages. But Vmax is 40V, and I know that's close to what's lurking in the rectified transformer.
So my question: what do people who work with AC in the range from ~25-75 do here?
It seems like I could use a 5-10V linear regulator to drop 40V down to a comfortable range for the 33063. If I use an external transistor to feed 40V to the inductor, I should be OK, and the linear regulator only has to provide ~5mA to run the buck controller.
Is there a smarter approach, or is this something an actual competent designer might do?
Answer
Output power: 5V x 0.35A = 1.75W. Estimated efficiency is %85, so the required input power is 1.75 / 0.85 = ~2W. At 38VDC, input current is 2W / 38V = ~55mA. After rectification and filtering, if you get 40VDC then you can place a resistor with a value of RS = (40-38)/0.055 = 36R (Place 39R). Power dissipation of this serial resistor is about PR = 39 x 0.055² = 0.11W. So you can put a 39R/0.25W resistor. You can also make a simple RC low-pass filter by putting a 220uF elco after this resistor to filter-out some ripple.
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