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Convert 24AC to 12VAC/DC 1.5A
connectTek:
I need to design a CCT that converts 24ac to 12 VAC/DC, at 1.5amps, must be efficient and very small, approx 30mm X 30mm x 20mm h.
Any ideas where to start?
Cheers
Ian.M:
As your application is so space constrained, start by getting 5+ years experience with switched mode PSU design and active power factor correction! Then you *MAY* be able to design a high frequency* buck converter that's good for 1.5A and is still small enough to leave space for a bridge rectifier and enough volume for large enough capacitors to ride through the intervals around the AC zero crossings where the input voltage is under 12V.
Try using LTspice to sim a 24V RMS AC supply feeding a bridge rectifier, reservoir capacitor and constant power load to get an idea of how much capacitance you need at the input of the buck converter to keep its input voltage high enough to ride through the zero crossings. Assume you can get 80% efficiency out of the buck converter, set the load power in the sim to 22.5 watts (12V*1.5A/0.8 ).
Edit: I've simmed it and the minimum cap is somewhere around 270uF, but its going to be a PITA finding one that's small enough for the rest of what you've got to cram in there, with enough ripple current rating and high enough temperature and reliability ratings. A larger (value) cap eases the buck converter design requirements by reducing its input voltage range, and a physically larger cap will run cooler and (like for like) be more reliable, so its a trade-off between volume for the cap and volume for the buck converter.
* Less fussy, easier to work with lower frequency buck converters need larger magnetics and output capacitors for the same load current.
Zero999:
It should be doable. I wouldn't bother designing the SMPS from scratch.
I'd go for a DC:DC converter module with a bridge rectifier and smoothing capacitor on the input. I did a quick simulation in LTSpcie, with a 20W constant power load, based on an efficiency of 90% for the DC:DC. Assuming the DC:DC converter works down to 16V, a 680µF capacitor will do: 470µF is adequate but we have to take into account ageing. Here's a link to one which is only 13mm*20mm and is rated to 50V.
https://uk.rs-online.com/web/p/aluminium-capacitors/7111481/
The DC:DC converter can also be tiny. Here's one which is only 17.5mm×11.5mm×9mm and rectifier diodes are quite small. Although it's only rated to 36V, so would be marginal for 24VAC input, assuming the voltage can be a bit higher than that.
https://docs.rs-online.com/fc3c/A700000006631878.pdf
https://www.mouser.co.uk/datasheet/2/348/rf201l2s-1017114.pdf
Ian.M:
Don't forget derating for temperature - that module would be right on the bleeding edge at 1.5A load current and high input voltage, and wouldn't even stand running at 70 deg C, so it would need *GOOD* cooling. Also, your choice of cap is inadequate as its only good for 770mA ripple current (@105 deg C) and even applying its temperature multiplier @70 deg C of 1.65, that only gets you 1.27A RMS, which is less than the simmed ripple current of 1.36A RMS. It will run hot, and the hotter it runs the worse its ripple current rating.
connectTek:
Thanks guys.
You've given me enough info to start.
Thank again
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