Switch-Mode Hybrid

[Jon86]

Now just tell me if I'm being stupid here, but how about a Hybrid Switchmode/Linear PSU?
I know switchmode PSUs don't have a very stable/smooth output voltage, so why not use a linear regulator on the output to fix this?

For example:
7V 2A Switchmode PSU (Tiny and efficient) feeding into a simple linear voltage regulator, max heat dissipation of around 4W, producing a smooth 5V at 2A.
Are there any problems with this? Has it been done? Am I missing something? 

[Bored@Work]

Yes it has been done. Yes, it has been discussed here. You didn't ask this, but Dave even made a few videos about it, as part of his power supply design videos.

Sounds like an opportunity to use the new URL link shortener to point to the series http://llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogo.ch/16d

[Jon86]

Ah, I see. I didn't watch all of Dave's vids on the power supply design...
Thanks anyway, sorry for bringing it up again.

[c4757p]

... URL link shortener ...http://llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogo.ch/16d

  Bookmarked.

[peter.mitchell]

new URL link shortener to point to the series http://llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogo.ch/16d

BRAVO!

[LaurenceW]

One GOTCHA to watch for is that 78xx (and maybe many other) simple linear regualtors have really quite poor high frequency regulation/attenuation.

In other words, high frequency noise at the input can go straight through 'em like a damn good curry. So you'll need some sort of filter inductor between the SMPS and linear regs, and input caps around the linear reg, too. I don't think the components need to be huge - a cut-off freqnecy of a few KHz will more than cover it, I would think.

[Frant]

One GOTCHA to watch for is that 78xx (and maybe many other) simple linear regualtors have really quite poor high frequency regulation/attenuation.

In other words, high frequency noise at the input can go straight through 'em like a damn good curry. So you'll need some sort of filter inductor between the SMPS and linear regs, and input caps around the linear reg, too. I don't think the components need to be huge - a cut-off freqnecy of a few KHz will more than cover it, I would think.

There are actually not so many voltage regulator ICs with well documented frequency characteristics (i.e. with a ripple rejection vs. frequency diagram up to at least 1 MHz given in the datasheet). Of those well documented only a few are really good at it. I found these: TPS7A4700, TPS796xx, TL1963A, LT1129.

Regarding the mentioned passive filtering, one must exercise caution not to make the SMPS feedback loop unstable.