28v to 12v conversion

[pranam77]

Betwixt had it right. If you are going from 28V down to 12V with 1A output, then the regulator has to dissipate 16W. That's a huge amount of power to dissipate, especially from a TO220 package..

Yes i too agree with this. But the OP needs to clarify again that wether his gadget (never mentioned anywhere till now) draws 1A constantly or the gadget draws 200-500ma and he is saying 1A to be on the safer side with a margin? Whereas the dropping and the heating issue is concerned, he may use a LM317K with T03 package with a suitable heat sink. But still i would like the OP to clarify the above points and too let us know from where the 28 volts is derived?
Cheers

 

[joeygemma]

The 28 volts are coming from an aircraft and the gadget would be a USB device (iPod, smartphone, etc.). I believe those devices are quite capable of drawing 500mA, but I know they charge quicker while drawing 1A.

The LM2575 seems better than the LM7805.

Could I just step it down several times with multiple linear regulators?

 

[betwixt]

Dropping through several regulators will work but the overall power dissipation is exactly the same. You are still dropping the same voltage at the same current so the same number of Watts is being dissipated as heat. It will almost certainly be bigger and more expensive though.

The LM2575 and a whole host of similar devices is the way to go. They are a type 'switching' regulator rather than 'linear' regulator. Basically, instead of converting the excess power as heat, they rapidly switch it on and off. By rapidly I mean typically 20 thousand times per second. If you imagine that each of these cycles has an off period and an on period, by changing the ratio of on to off the AVERAGE power can be altered. Always off would give no output, always on would give the same out as went in. Following the switch stage there is a filter to even out ( = take the average) of the switched pulses so it looks like a steady DC voltage again.
They are very efficient because the switching element is always fully off or fully on. When off, the voltage is high but no current flows so the power dissipated is zero (V x I = W so anything times zero is zero) when on, the voltage is zero and the current is high so W is still zero. Of course they are not 100% efficient but they are far better than linear types.

Brian.

 

[Kripton2035]

hi,
please stop thinking about linear regulator as soon as you are dealing with more than 1 A or 5V to lower...
except if you live in a cold country and want to heat your house a little with it ...
the LM2575 family is really easy to set up, and does the job perfectly ...
regards,

 

[FvM]

LM2575 is from Simple Switcher chips first generation and has low switching frequency and only moderate efficiency. I prefer second generation LM267x types. LM2672 or LM2675 can easily supply 1A without a heatsink. Inductor size can be also smaller due to a doubled switching frequency. Newer types have an even higher switching frequency, but are more demanding in terms of circuit layout.

 

[alaajh]

thank u this good