78xx SMPS replacement - can it be used with the zener to GND? - Page 1
Forum section: electronics / projects

author: ilya
author_rank: Contributor
timestamp: May 23, 2022, 03:31:42 pm
content: Mean Well (and some others) make a convenient 78xx and 79xx chips replacement using SMPS (https://www.meanwell.com/webapp/product/search.aspx?prod=N78).Is it possible to use the zener to ground to increase the output voltage with those? I'm attaching an example of what I'm talking about.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2022, 03:45:21 pm
content: Sort of...but no.The same concept applies, that raising the GND voltage, also raises the output. And the minimum input needs to be higher by the same amount, of course.But because it's a buck converter, GND current is the difference between input and output currents: it will pull that node below 0V, if using a diode. You'd have to use either a pullup resistor into the diode, with enough current to swamp this out (terrifically inefficient, indeed as inefficient as an original 78xx -- can you see why?), or some arbitrary voltage source (which could be another SMPS able to source that current).So it's not a great idea, and better off getting an adjustable version so you can tweak it as needed, or just get the correct rated version.Tim

author: ilya
author_rank: Contributor
timestamp: May 23, 2022, 03:55:32 pm
content: Thanks, Tim. This is what I suspected.I'm looking for 18V-20V output SMPS modules, but can't find any suitable candidates unfortunately.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 23, 2022, 06:37:37 pm
content: Not like that, but normally the buck regulator relies on a feedback resistor network which can be modified.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2022, 06:50:48 pm
content: Some other component values may have to change as well, basically because the duty cycle is different so the change in current over a switching cycle is also different. Depends on type. At this point you're probably looking for either an IC to do it (add L, C, R, D and lay out as per application notes) or one of those shitty off the shelf modules (and hope the performance is okay for your purposes; they're very noisy).Tim

author: ilya
author_rank: Contributor
timestamp: May 23, 2022, 07:14:12 pm
content: Speaking about off-the-shelf SMPS modules. Are they really hopeless? Won't proper CM and DM filtering with an LDO afterwards suppress the noise enough for them to be useful in a LN high gain circuits?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2022, 07:49:47 pm
content: The eBay/Ali Express specials, have very little filtering.Brand-name modules -- including the 78xx substitutes, as it happens -- also have minimal filtering, as they must, given the package size.DC converters at least have the advantage that they're only three terminal, so you have no CM. DM filtering on each side is sufficient. Make sure the ground point of that filter is common to both input and output (single point, or as much so as possible), and you avoid DM-CM mode conversion.Isolators, offline AC-DC, etc. converters have the added complication of having no choice but to mind the CM. Most of which are fairly optimistic in their performance -- typical test conditions are with an isolated resistor on a heatsink, so, very little "reaction mass" to push CM noise up the mains leads. They typically don't pass, in real situations with outputs grounded, without an extra line filter or so. But they're also usually not too far off, at least, so that adding a line filter (often "for free" as an e.g. IEC power cord entry module) is usually good enough.LDOs, obviously do nothing at all about CM, by definition; and don't do much to noise at high frequencies (typical LDOs have minimum PSRR in the 3-10dB range, usually falling in the 10kHz-1MHz band). It can help -- but it matters what, and how much you need.Tim