Offline Linear regulator vs Offline SMPS? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: February 04, 2022, 07:55:13 pm
content: A customer came today with a 350W (69V, 5A) Linear regulator power supply. VAC input was 212-265VAC. 50Hz mains transformer----rectifier-----smoothing capacitors----linear regulator---output.They want us to make something similar because all the parts are obsolete.They say a linear regulator must be used because the application requires a low noise power supply.The thing is, the linear regulator based power supply they presented, had no mains input filter, and not a single common mode choke at input or output (or anywhere). So any mains borne noise could simply propagate right through the transformer's enormous inter-winding capacitance, and go on to the output, and on to the load. And since 50Hz transformers comprise very high levels of primary-to-secondary capacitance, there is likely to be huge passage of common mode noise from input to output, and on to the load.The power supply also comprised considerable internal wiring, which is basically going to act as loop antennae, ready to pick up noise from the surroundings, and send it to the load. Also, it goes without saying that it had supply and load wiring, so again…more antennae, shovelling noise into the load.Also, since it was using a Txfmr/linear regulator, it comprised a mains bridge rectifier, and as you know, these have considerable “switching noise”. A bridge rectifier is essentially a switching power supply...and in a 50Hz transformer based regulator with smoothing caps, the switched currents are significantly higher than in an SMPS....so its more noisy than an SMPS.So would you agree, that this linear regulator is a waste of time? An SMPS with common mode input and output filters would be far better? Even if you use only a linear regulator, there are likely to be SMPS’s in the near vicinity which will couple noise into the linear regulator wiring…..and also, other SMPS’s will be likely connected to the same phase, and so noise from them will come down the mains wiring and into the linear regulator, and go right through it, and on to the load.An offline linear regulator based power supply is basically an extremely noisy PSU, that shovels loads of common mode noise from the mains, through its enormous transformer interwinding capacitance, and onto the load. No application requiring low noise, shoud use a 50Hz Txfmr/linear regulator, would you agree?Also the high interwinding capacitance will spew heaps of noise through the interwinding capacitance and onto the load.Can we agree, that “electrical noise” is not a good enough reason to use a linear regulator instead of an SMPS?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 04, 2022, 08:15:53 pm
content: There's no such thing as an (unconditionally) noisy SMPS, only insufficient filtering and mitigation.An understanding of the application (what is the nature of the load) is required to know what actual noise response is required (maximum amplitude vs. frequency). Many people think things should be "low noise" but don't actually have a clue what that means, quantitatively.Linear supplies can indeed be noisy, though it's usually limited to diode recovery: mains frequency and harmonics, rarely over some kHz (with poor diodes, some MHz). This is still much easier to regulate out, and less prone to CM interference, than SMPS sources. Conversely, the faster an SMPS runs, the easier it is to passively filter, in terms of physical size; layout however is quite critical, and all the filtering is for naught if poor layout and shielding are used.Linear supplies typically have pitiful power factor, if that should be an issue. And, needless to say, dissipates a fuckton of heat, compared with an efficient SMPS. Consequently, the SMPS can be much smaller, in addition to the smaller transformer, and to some extent, smaller capacitors.For my part, I've designed SMPS with noise under 100uV in 100MHz+ BW; basic design is any other converter, with good layout, and a few extra stages of filtering. Little if any shielding was used. Performance better than that, I'm pretty sure, would demand shield cans over the PCB, or a metal enclosure with pass-thru filters.If it can't be replaced by an off-the-shelf supply (which, a pair of 36V's in series should be fine?), it's probably better to stick with the linear supply and maintain its design. It also doesn't sound like much production quantity, so a custom SMPS might indeed be negative value.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: February 04, 2022, 11:15:36 pm
content: Thanks, do you agree that even with a 50/60Hz Transformer based Linear reg PSU, the "low noise" labelling is nonsense if they are not also using common mode chokes at its input and output?

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: February 04, 2022, 11:31:22 pm
content: Quote from: Faringdon on February 04, 2022, 11:15:36 pmThanks, I do not agree before knowing more about the current supply and the way the output is filtered. Does the transformer have inner shields?What are the noise requirements? (perhaps what the customer is using actually isn't "low noise" at all or not something really hard to achieve)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 04, 2022, 11:47:59 pm
content: Mains is not usually very noisy (worst case can be), nor are transformers particularly good at coupling that noise: EPC and LL tend to attenuate high frequencies, serving as a poor filter by itself. Greatly improved with a shield, as mentioned.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: February 08, 2022, 07:11:45 am
content: Thanks, these are good points. The point about this product is its a 300W SMPS that nearly rips your muscles off when you try and lift it...and all because the "customer needs low noise"...the thing is, the output wire terminals are just plain screw terminals.....not proper shielded connector terminals......so the output wiring is going to be picking up all the usual noise... and propagating it to the load.Also, being a 50Hz txfmr/linear reg thing, its peak currents are very high...not good for noise. If they want low noise, but arent worried about the output noise as dicussed (so there "Noise concerns" cant be that serious)....they could just do with a plain LLC converter, with decent input and output filters, and shielding?I must admit ive been to lots of places where they are using linear regulators because of "low noise needed", and it just ends up being more like "we cant be bothered to change because the customer doesnt understand it anyway".

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: February 08, 2022, 02:44:13 pm
content: As with my previous reply, measure the current linear supply and come back with those results so we can really help you.(also, post pictures, we really dig those)