Electronics > Projects, Designs, and Technical Stuff
Reliable Low Cost Method to Convert 24VAC to 24VDC
james_s:
Why do you want a diode before the regulator? The dissipation will be exactly the same, you'll just have more parts getting hot. The *only* time you need a LDO is when the input voltage is too close to the desired output voltage to cover the dropout voltage of the regulator. Any linear regulator will dissipate exactly the same amount of heat when dropping the same voltage to deliver the same current.
I rarely use linear regulators anymore, small switching regulators are cheap and trivially easy to use these days. So long as you use reasonable quality capacitors they will be very reliable.
TimNJ:
Have you looked at any papers on D2PAK dissipation vs ambient vs copper area?
https://www.st.com/resource/en/application_note/cd00004438.pdf
By the looks of it, 2W seems like a more reasonable upper limit for dissipation on a D2PAK, if the ambient is quite high, 60C for example.
GeorgeIoak:
My thought of placing the diode before the regulator is exactly as you stated, to spread the heat between 2 devices rather than having the regulator dissipate all of the heat. The lower the temperature of parts the better the chances of the part not dying too soon.
I've done many switching regulator designs so that doesn't scare me off but the total cost of a switcher (counting both parts and assembly) is going to be much higher. When you're building thousands of units $0.50 (or more) can add up quickly.
@TimNJ, thanks for that app note, I hadn't seen that one and it looks useful. I think anyone's attic could get up to 60C so that's what I have to design for
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