Switch Mode Power Supply

[Cam]

Hi all,

I'm struggling to find an off the shelf linear supply that will fit inside a 1u case and supply enough current.. I know I could design/build one, but I don't really know enough about it! (ie, why one 5V supply can do 1A, and abother 3A - is it just component values, trace sizes, or something else, and if so where do I start!). A fight for another day I think, I don't want to get into it!

So... I thought I might give a switch mode supply a shot. I used an expensive "audio specific" supply a few years ago for a +/-25V project, and once I got the earthing good it was to all intents and purposes silent.  It was only working with line level signals however, and not through circuitry that added much/any gain. It's great, but it cost £150! 

I'm looking at supplies like this one http://uk.rs-online.com/web/p/embedded-switch-mode-power-supplies-smps/3192916/ . It has all the rails I need, but I've no idea how much noise it would throw out. Anyone have much experience with them? It won't be powering any super-sensitive audio (a few mic pres and some op amp buffering of line level signals being the important(ish) things).. Do they throw off much RF in general, and screw with signals that way? Or would I expect a cheapish supply like this to make an acoustic buzz in the room? How much muck ends up heading down the rails - is it just unusable?

Finally, these supplies need a minimum current draw of 500mA to work - I won't be drawing more than 200mA at idle - what ways would people go about loading a supply like this, so I was always drawing (say) 300-400mA?

It'd be great to hear peoples thoughts. Terrible direction?

C

 

[Tubetec]

Hi Cam,
Switchmodes tend to opperate at in the killohertz range ,maybe 50 khz ,so extra filtering is normally required to keep these frequencies out of your audio path. They certainly can work well ,but usually the cheaper ones are rubbish, the other thing is they dont tend to have the lifespan of the old fashioned linear supplies due to components being run close to the limits. Ive also come across very strange things happening ,say in the case of stomp boxes ,where several switchmodes are used to power  different  pedals within the same signal path, interactions between multiple supplies can result in very bad noise performance .Another factor which you rightly pointed out is the minimum supply current needed for correct opperation. The quality of the unit you choose will make a big difference ,but for audio work its worth considering adding extra filtering on the rails,this need only be an inductor  in series and capacitor to ground ,but again placement and sheilding can be the difference between good and bad,and downright ugly.There are a few linear regulator boards kits available on ebay which might be worth a try,if you need it all to fit in a 1U rackspace a toroid or R core power transformer will be the way to go . Generally running a power supply at around 50% capacity is a good rule of thumb, that will ensure a long lifespan . For me the old fashioned linear supply is best, it still works out cheaper than an off the shelf hi end Smpsu,and you can rest assured it will give many many years trouble free service.

 

[Bo Deadly]

Use LED drivers like this (just picked this one at random but I use Mean Well like this):

http://uk.rs-online.com/web/p/led-drivers/6663607/

As long as they have a transformer output then you can "stack" them to get both positive and negative voltages (although the protection circuitry might not work for the negative voltage).

They do generate HF noise but if you use a large common mode choke, noise will be greatly attenuated. With some regulation it will be very quiet (and more powerful and more reliable and not burst into flames or get really hot compared to a linear supply).

The only catch is it cannot be physically close to any sensitive circuitry like high impedance inputs or inductors and the like or that will pickup the switching noise and no amount of filtering will help. I have a separate external supply and then use a custom CPC connector cable to deliver various unregulated voltages.

It's the wave of the future, ... wave of the future, ... wave of the future, ... wave of the

 

[Tubetec]

Hmmm the led driver thing looks interesting,small money too .
I did once use a circuit from a  disposable camera flash unit to create a few hundred volts for the plate supply of an Ecc83 from a 1.5 volt cell and a mobile phone charger to supply the heaters,Rc smoothing silenced the buzz saw waveforms from the flash unit effectively ,while I added a choke/cap to the out of the mobile phone charger ,It wasnt pretty but it did work.
I have in the past been termed a Luddite for my persistance with the old way of doing things ,but each to their own as the man said.
cheers again.

 

[ruffrecords]

Use LED drivers like this (just picked this one at random but I use Mean Well like this):

http://uk.rs-online.com/web/p/led-drivers/6663607/

The one you picked is constant current but there are constant voltage versions. I use them for dc heater supplies for big tube mixer projects. They work surprisingly well with a little extra filtering.

Cheers

Ian

 

[metalb00b00]

Hi all,

I'm struggling to find an off the shelf linear supply that will fit inside a 1u case and supply enough current.. I know I could design/build one, but I don't really know enough about it! (ie, why one 5V supply can do 1A, and abother 3A - is it just component values, trace sizes, or something else, and if so where do I start!). A fight for another day I think, I don't want to get into it!

So... I thought I might give a switch mode supply a shot. I used an expensive "audio specific" supply a few years ago for a +/-25V project, and once I got the earthing good it was to all intents and purposes silent.  It was only working with line level signals however, and not through circuitry that added much/any gain. It's great, but it cost £150! 

I'm looking at supplies like this one http://uk.rs-online.com/web/p/embedded-switch-mode-power-supplies-smps/3192916/ . It has all the rails I need, but I've no idea how much noise it would throw out. Anyone have much experience with them? It won't be powering any super-sensitive audio (a few mic pres and some op amp buffering of line level signals being the important(ish) things).. Do they throw off much RF in general, and screw with signals that way? Or would I expect a cheapish supply like this to make an acoustic buzz in the room? How much muck ends up heading down the rails - is it just unusable?

Finally, these supplies need a minimum current draw of 500mA to work - I won't be drawing more than 200mA at idle - what ways would people go about loading a supply like this, so I was always drawing (say) 300-400mA?

It'd be great to hear peoples thoughts. Terrible direction?

C

what is your DC voltage supply requirements?
if +/-15V, I recommend Mean Well PD-2515.
fits inside 1U, has enough juice per rail (800mA), pretty reliable, cheap ($16/ea), and quiet.

 

[Cam]

what is your DC voltage supply requirements?
if +/-15V, I recommend Mean Well PD-2515.
fits inside 1U, has enough juice per rail (800mA), pretty reliable, cheap ($16/ea), and quiet.

I need a 2.5 - 3 Amp +5V supply, and a small (500mA) +/-15V supply.  The +/- supply is the only one I need to be relatively clean - the 5V is on powering relays and LEDs.

Thanks all so far - interesting reading!

The supply will be in the same case as my audio circuitry btw - I hate external supplies!

EDIT - I'm also thinking - just using a couple of fairly hefty 5V bulbs instead of LEDs to light a few things would bump the current up enough to keep the supply happy... But I'd really rather not have bulbs anywhere near it! It feels like a bodge!

 

[metalb00b00]

I need a 2.5 - 3 Amp +5V supply, and a small (500mA) +/-15V supply.  The +/- supply is the only one I need to be relatively clean - the 5V is on powering relays and LEDs.

Thanks all so far - interesting reading!

The supply will be in the same case as my audio circuitry btw - I hate external supplies!

EDIT - I'm also thinking - just using a couple of fairly hefty 5V bulbs instead of LEDs to light a few things would bump the current up enough to keep the supply happy... But I'd really rather not have bulbs anywhere near it! It feels like a bodge!

if your 1U casing is deep enough, you can fit two SMPS inside.

Mean Well RS-15-5 (+5V 3A)
Mean Well PD-2515 (+/-15V 0.8A)

 

[Cam]

if your 1U casing is deep enough, you can fit two SMPS inside.

Mean Well RS-15-5 (+5V 3A)
Mean Well PD-2515 (+/-15V 0.8A)

Have you experience of these/this company? I like everything about this idea (plus they do a 48V one for any phantom I might want in the future!)

And more importantly, how do the "0V" outputs of two or more switch-mode power supplies play with each other? On my PCBs the 0Vs eventually all tie back to one point... Do they mind being tied together? (I don't see why they would care, but I know very little about them!).

 

[Bo Deadly]

And more importantly, how do the "0V" outputs of two or more switch-mode power supplies play with each other? On my PCBs the 0Vs eventually all tie back to one point... Do they mind being tied together? (I don't see why they would care, but I know very little about them!).

It depends. Look at the block diagram in the datasheet. For example, in the Mean Well LPC series I mentioned previously (i use constant current but constant voltage should be ok too), the block diagram looks like this:

If the block diagram has a transformer (and not just an inductor which doesn't provide isolation) and opto-isolators then the outputs are floating and you can tie negative terminals together as well as "stack" voltages by connecting the negative of one to the postive of another to make both negative voltages as well as and higher voltages.

But again, the over voltage protection and similar features may not work as expected if +V is tied to ground and -V is used to make a negative voltage. But I have never had an issue. In fact I think in the case of LPC some protection is working because I actually reversed an op amp supply once and not only did the supply survive but the op amp did as well. With a linear supply it is not unheard of for an op amp to literally explode when reversing supply voltages.

 

[JohnRoberts]

But again, the over voltage protection and similar features may not work as expected if +V is tied to ground and -V is used to make a negative voltage. But I have never had an issue. In fact I think in the case of LPC some protection is working because I actually reversed an op amp supply once and not only did the supply survive but the op amp did as well. With a linear supply it is not unheard of for an op amp to literally explode when reversing supply voltages.

That is a matter of current limiting, not switching vs linear power supplies.

Many  (all?) op amps connected across power backwards look like a short (actually looks like a diode drop). Without current limiting it will draw the full supply output current. For several amps that will make up to a few watts so could release the magic smoke from the tiny package. 

BTW, you should probably discard that op amp... it may have suffered some subtle damage.

JR

 

[sahib]

Hi all,

I'm struggling to find an off the shelf linear supply that will fit inside a 1u case and supply enough current..

C

Edit:

I should have read your later posts.

I have plenty of boards that will do your job.  In the below picture the one on the top is a triple one but I also have a dual version of it. I have tested it at 600mA @16V with 15.5VAC input and the heatsinks only reach to about 37 degrees C.

The one below is a switch mode 5.5V @ 5A, based on MC33167 left over from my old company. But for this it would be probably easier for you to use a switch mode as it would take probably the same space, by the time you add the transformer but not as magnetically offensive as a transformer.

 

[metalb00b00]

Have you experience of these/this company? I like everything about this idea (plus they do a 48V one for any phantom I might want in the future!)

And more importantly, how do the "0V" outputs of two or more switch-mode power supplies play with each other? On my PCBs the 0Vs eventually all tie back to one point... Do they mind being tied together? (I don't see why they would care, but I know very little about them!).

Your relay and LEDs ground can go to 0V (it's marked V-) that's connected to the +5V SMPS.
Your audio ground connect to 0V (again, marked V-) of the other SMPS.

For phantom, buy off the shelf $5 DC step up converter kit (XL6009 for an example) to step up that +5V (or +15V) to +48V.  Make sure the converter kit output electrolytic capacitor is rated 50V because some are only rated 35V.