Project files: Filtered IRM power supplies (part 1)

As promised a while ago, here are my designs for the “filtered” power supplies based on IRM AC/DC modules. These are excellent for adding compact and powerful single and dual supplies which still have a reasonably good performance to any small preamp/headamp amplifier.

More MeanWell IRM PSUs…

As regular readers will know I have been using the Mean Well IRM-series AC/DC PSU modules quite a lot. There are many positives to these modules (size, cost, standby consumption etc.) and only a few downsides, mostly the limited output voltage options and the rather noisy output. The noise doesn’t matter so much when used for auxiliary supplies, but for things like preamps, headphone amps etc. I’m sure it’s possible to do better.

However, because the IRMs are a switching design with a relatively high switching frequency (app. 100kHz), cleaning up the output on the IRMs should be relatively easy with just a passive “Pi”-filter (CRC or CLC). Because of the high ripple/noise frequency even a low series resistance/inductance and a little bit of extra capacitance should have a dramatic effect. Some time ago I started making a single-rail PSU based on an IRM-module for a clean and compact 5V supply for my “Music Box” project, but I realised that actually these would be applicable in many projects so I ended up with several different versions (single/dual, different power levels, integrated/separate filtering etc.)

As I’ve just received what I think is the last variant, an integrated dual version with space for two 15/20W IRMs, now seems to be a good time to collect everything and release the project files so they will be up as soon as I am back from my summer holiday in a week or so. Until then, I hope you enjoy the summer 🙂

Improving a Mean Well IRM PSU…

If you’ve been here before, you might have noticed that I have been using Mean Wells IRM-xx series PSU modules quite a bit. They are small, cheap, easy to use and available from many of the parts sources I normally buy from. Being cheap switching supplies they have quite a bit of ripple and noise which on the face of it is a problem. However, in practice many supporting applications aren’t too fussy about the quality of power and for those that are, alternative PSU arrangements can usually be found.

But what if the noise from the IRMs could be removed? One application where that would be useful (and where several people other than me have tried it already), is for Kevin Gilmores various discrete amplifiers (Dynalo, CFP etc.). They are normally powered by 16-24V DC and require a few hundred milliamps per board, so it’s pretty ideal for an 24V IRM with some additional filtering and regulation. Here I’ve made a single channel PSU intended for a 15/20W IRM-module, so output currents are in the region of 0.8-1.5A or so.

Most integrated voltage regulators have low rejection of HF-noise, but as the IRMs have a high switching frequency a passive filter seems an ideal way of damping the noise before it gets to the regulator. I can’t find any spec to state how much capacitance the IRMs will tolerate so I’ve stayed on the conservative side, but even so a small passive CRC/CLC filter is very effective at 100kHz so it should be fine.

Instead of the “usual” LT108x LDO voltage regulator I’ve gone for an LM2941 instead. This has an even lower drop-out voltage which will help if the starting point is a 15V module. The downside is a max. output voltage of 20V – 22V would have been better (and while we are at it, can we get 9V and 18 versions of the IRM20 please Mean Well :D). Actually, I’m going to try to do an LT10xx-based version as well, but for now the LM2941 works fine. For this test example I’ve set the output voltage to 19.5V and I get 19.4V even under load so that is perfect. Next step: Build two more boards for my first “real” use-case for these 🙂

Another mains controller…

I’ve designed and built a few control boards for switching on mains (e.g. this and this), because it tends to be a thing that many of my projects need. Good (and good looking!) mains switches are hard to come by, especially for higher currents, so it makes sense to use a lower-voltage switch combined with a relay or an SSR for this duty. An obvious downside to the relay-based approach is that a standby voltage is needed to control the relay, but as described in a previous post there are now several types of switching AC-DC converters able to do that job very cheaply and reliably.

However, more often than not I have found that I prefer to keep the standby PSU separate and so this addition to the control-board portfolio was delberately made smaller and to fit my usual 2”x2” format to make it stackable with my softstart-board. For anything with a large transformer in it, this is a combination that is very useful.

Another addition is an external trigger input (isolated with an optocoupler) which I don’t often use to be honest, but which I could see some potential in anyway. To make this feature a bit more versatile I have opted for the “deluxe-version”, by feeding the optocupler from a constant-current source made from an LM317L. This should mean that it’s not just the usual “12V-trigger” input, but actually it would work with any voltage between app. 3-30V and draw less than 20mA from the triggering device.

“In flight” (or at least on the way) are boards for a matching standby PSU based on the Mean Well IRM power modules – when everything is here and tested I’ll publish some files and more pictures 🙂

Project files: IRM Switching PSUs

What is it?
Since I first discovered the IRM-series of compact switching supplies from Mean Well I’ve grown quite fond of them. They are compact, cheap and very easy to implement so they are perfect for everywhere an “aux-voltage” is required to power non-critical circuitry. Through the different applications I’ve found for these I have managed to build up a full series of boards suitable for the IRMs.

While some of the boards can be (and are intended to be) used for “serious” stuff (to be shown later on), a very obvious application for most of these boards are as AUX-supplies for powering relays, displays, logic circuitry etc. where a bit more or a bit less ripple and noise are of no consequence, but where the compact size and low standby consumption is a real plus.

There are four board versions, suitable for the IRM modules in all versions from 3-30W output power (the 30W board is missing from the pictures as I couldn’t find the prototype when they were taken – sorry! 😀 ).

How big are the boards?

• The 3W board measures 1.8” x 1.5” (app. 46 x 38 mm.)
• The 5/10W board measures 1.2” x 2.65” (app. 31 x 67 mm.)
• The 15/20W board measures 1.25” x 2.95” (app. 32 x 75 mm.)
• The 30W board measures 1.6” x 3.6” (app. 41 x 92 mm.)

What is the status of the boards?
All of the board files are version 1.0 or higher. Some tweaks have been done after the initial protoypes for a few of them, mostly because of errors/issues with the IRM module footprints.

Does it use any special/expensive/hard-to-find parts?
No, none. Several places to get the IRM-modules them selves (Mouser, Reichelt, TME etc.) and everything else on the boards is more or less optional 😀

Anything else I need to know?

• The modules have worse specs for ripple and noise than most linear regulators, but obviously the switching frequency is quite high (66-100 kHz depending on model), which means that passive filtering like an LC or a CRC (“pi”) filter would be an ideal way of reducing the output noise. I have a couple of examples for that which I might show later.
• I haven’t been able to find a spec for how much capacitance the modules will tolerate on the output, but it probably should not be overdone.
• Remember that obviously one side of the board carries mains voltage, so take the necessary precautions when working with them.

Downloads:
Download design files here

Related information:
Note: Always read the “intro post” for additional important information about my designs.