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12V & 5V dual rail power supply

Former Member

Hello all,

 

I am just starting a new project that requires a split power supply of non-trivial currents starting from UK mains. Specifically, 12V 5A and 5V 2A The currents are absolute maximums and will generally be more like half those values and will some times be significantly less. I have been looking for an off the shelf supply that can do this but I can't find much. There are some options about but they all seem to be roughly equal split by power output and with minimum load requirements, e.g. 12V 4A plus 5V 8A with at least 1A load on the 5V for the 12V to be within regulation.

 

So the question is, does anyone know of some perfect supply from someone that I have missed? If not, which alternative is likely to be cheapest, most efficient and smallest out of two independent switch modes, a 12V switch mode followed by a 5V DC-DC or a 5V switch mode followed by a 12V DC-DC? The 12V and 5V supplies will need to share a common ground but I can't see that being an issue for the twin switch mode option.

 

One further issue is that I wish to switch the system on/off via a solid state relay. There are plenty of these about that can handle the power requirements easily, however a lot of them seem to have leakage currents in the milliamp range. In the past, I was hit by this just about managing to keep a power supply running. It was only producing a pulsing ~9V instead of the rated 15V but it defintely refused to die! So AC-DC supplies with switch off on under voltage protection would be preferable.

 

Thanks,

John.

  • 0

    Hi John,

     

    Buying a standard 12V/5A AC/DC power brick would be the easiest. The nice thing is that the safety is completely assured. There is are several items for sale at Farnell. The 5V is best made with a DC/DC converter (standard module or build yourself with a standard application, for example On-semiconductor NC1580). The 5V circuit will preload the 12V supply, but normally a minumum load for a SMPS isn't critical.

     

    Switching a SMPS with a solid-state relay is not the best thing to do. Most SSR's have a snubber circuit across with at capacitor, which will charge the bulk capacitor of the main power supply and it will run in a burst- or hiccup mode. The best way to switch-off the application on the DC-side (modern SMPS are very efficient) but if you want to switch off the main power supply you can buy one which and on/off signal input.

     

    There is a trick of making a snubberless SSR.

     

    I will describe it in text. Take a bridge rectifier. Take a 600V MOSFET. Connect the Drain to the (+) pole of the bridge recifier. Connect the Source to the (-) pole of the bridge rectifier. Connect 1 of the two AC inputs directly to the mains (N). Connect the other AC input to the 'power supply to switched' The other input terminal of the power supply is connected to the mains (L). You can make a very tiny capacitive power supply to charge the gate of the mosfet to switch on and off. Be aware that a switch connected to turn on the mosfet is live and dangerous. If you use a opto-isolator to switch the gate on and off, this is more safe.

     

    Best regards,

     

    Enrico Migchels

    Power Conversion Design Engineer

    Heliox B.V.

    Best - The Netherlands

    www.heliox.nl

  • 0

    John,

     

    This is a challenging requirement due to the high power on the 12V relative to the 5V. Also making it difficult is the requirement to remotely shut down the power supply for this wattage range.

     

    There are some power supplies out there that will have a main 5V rail along with a 12V secondary that will do the required power such as the XP Power JPS130PT31. But this model will have a 3A min load on the 5V output in order for the 12V to regulate. If you can use a 1A preload along with your 2A load this may not be a bad solution, but it does not have the remote on/off feature.

     

    If the dual outputs are needed from a power supply along with the remote on/off then you could go with the RCL175PD22 from XP Power as well. This would only have a 1.5A min load on the 5V.

     

    The other alternative which was mentioned was a single output 12V supply along with a 10W 5V dc/dc converter. For the remote on/off feature and a little head room to allow for efficiency in the DC/DC converter you could go with the ECM140US12-A power supply and a JCH1012S05. (Other dc/dc converters could certainly be used with different options and sizes, but this is a good representative option.)

     

    As with any power requirement there are likely other ways to solve the problem depending on other variables in your system (size, cost, temperature, performance, etc) and those could be discussed in more detail as needed.

     

    If you have any further questions please feel free to post here, or contact your Newark / Farnell representative, or contact XP Power directly.

     

    Thanks,

     

    -Kent Smith

    Applications Engineer

    XP Power

  • 0 in reply to ksmith

    Thanks for the info, people.

     

    I guess there is no off the shelf one box solution. Kent, all the supplies you mention are far too large for what I want. Dumping several watts simply to keep the supply ticking over is just too wasteful.

     

    It looks like the 12V switch mode followed by a 5V DC-DC is the best plan. Possibly a TDK Lamda 12V supply as it seems I might have just won one in the road test image. I'm guessing you mean an NCP1580, Enrico? I've found that on the OnSemi website but no-one seems to stock it. At least, both Farnell and the RS websites are denying all knowledge and DigiKey will only sell them in packs of 2500 or more! I'm currently thinking of using the NatSemi LM2676 (or LM2675 as the 5V load seems to peak at 0.9A). They look like simpler devices to use and are easily available.

    Re the snubber keeping the SMPS alive, I assume the result is the same as the SSR simply having a non-zero leakage current when off (and indeed might actually be the source of the leakage). However, quite a lot of SSRs are available without a snubber circuit built in. I am looking at using a Crydom CX240D5, it is 3V3 trigger capable, has a huge surge current capacity, very low leakage and no snubbing.

     

    Thanks,

    John.

  • 0 in reply to Former Member

    Hi John,

     

    About the DC/DC - converter controller. It is indeed the NCP1580. I used the chip in the past and it did not cause too much problems. There are serveral alternatives available.

     

    Using a ready-to-go snubberless SSR is the best way to go if cost is not your biggest problem. The RC is definite the cause of leakage. I stumbled upon this problem also once, so i'm a 'expert by experience' :-). Nowaday there are triac's which do not suffer from high dv/dt (that's why the RC is there), but they have some limitations in driving. If you buy a galvanic isolated black box with 4 terminals you don't have to know al the details.

     

    Lot's of succes.

     

    Best regards,

     

    Enrico Migchels

    Power conversion design engineer

    Heliox B.V.

    Best - The Netherlands

    www.heliox.nl