I need to design a power supply for my home automation products. I need +12V DC (1A) and +5V DC (500mA) respectively for two different products. One option is to use a circuit with coil based transformer; second option is to use an SMPS based circuit. Size, durability and cost are the key constraints of my product. Please suggest which option should I go with?

  • 2
    \$\begingroup\$ as they say, pick any two \$\endgroup\$ – Jasen Jan 3 '16 at 7:10
  • \$\begingroup\$ Is your device double insulated or earthed, and does it have any user-touchable parts? And could you rank your constraints please? \$\endgroup\$ – ThreePhaseEel Jan 3 '16 at 8:06
  • \$\begingroup\$ If you are looking at production you seriously need to look at where you intend to market. There are in the present day certain places where chargers and wall warts need to meet certain efficiency requirements before they can be sold. \$\endgroup\$ – Michael Karas Jan 3 '16 at 12:49

First I'd suggest you to double check your power requirements. You are consuming 12+2.5 = 14.5 watts of power which seems too much for a home automation product. If you are just running some relays, micro-controllers and sensors on that power, you are probably doing it wrong. Again, you might be running some extra load on it and it might be correct.

Now coming to your hardware selection, there are a few questions that you need to answer first:

1) Are there any physical constraints on size of the power supply?

2) Are you going for a bulk production?

3) Do you have some knowledge about electronics and are you willing to put some time in R&D?

If your answers are mostly "NO", then simplest thing would be to go for a transformer based power supply. You can choose a center tap transformer or two different transformers - One for 12V and other one for 5V. Add two linear regulators 7812 and 7805 to make things more robust and you are done. Obvious cons will be high cost and bigger physical size but it will get you up and running very fast. Alternatively you can just buy two power adapters.

However if your answers are mostly "YES"(which is the case actually), then I'd suggest you to go for switching power supplies. Why? Low cost, smaller size and higher efficiency. In the cost of one normal transformer, you will have complete power supply designed if you choose your part vendors wisely. If all major companies are giving you an SMPS based chargers, there must be a good reason to it. If you have decided to go for this, these are the points you need to research on:

1) Selection of an offline switcher IC. You need to choose an IC suitable for your power requirements. For ex - Viper22a is one of the commonly available ICs and it can give you around 20 watts of power if your input voltage is in the range 195V to 265V.

2) Selection/design of the ferrite core transformer - This is the toughest part in my opinion unless you have a good knowledge about magnetics. A cost saving idea would be to go for multiple secondary windings - dual to be precise. One for 12V and the other one for 5V. You need to select a suitable transformer core for the power requirements. You can follow app notes for this purpose.

3) Once these two key components have been selected, you need to pay some attention to noise. What's the max ripple noise you can accept? If you aren't performing any precise adc measurements, 5-10% of output voltage is usually acceptable. For analog stuff, you need rock solid power supply or you will end up introducing some measurement error. Once the acceptable noise has been agreed upon, you need to design a suitable filter for the same. In some cases, just a capacitor is enough. In other cases, an LC filter might be required.

Note: There is yet another cheaper version - Non isolated power supply where you use an inductor rather than a flyback transformer. However you are always at a risk of shock if someone touches the circuit and usually not recommended.

  • \$\begingroup\$ Thanks Whiskeyjack; for the elaborate reply. a) I need to drive at max 8-10 relays (controlling house hold devices). Each EMR (electro magnetic relay) normally requires 40-50mA at +12V; thus requiring 400-500mA. (b) The size that I am targeting is around 14-15cm in length, 3-4cm in width, 2-3cm in depth. I thin coil transformer shall consume a lot of space. (c) Another thing that I am evaluating is Relays; where to use EMRs or SSDs (solid state relays) as the later are small in size. \$\endgroup\$ – d.sehrawat Jan 3 '16 at 14:52
  • \$\begingroup\$ Triacs can be used to control low power loads(tubelight, fan, bulbs etc up to 1A). It will save you a lot of power on the DC side. You can use relays for high power loads. You can also go for 5V relays and avoid 12V line altogether. Use a single 5V line to power everything. \$\endgroup\$ – Whiskeyjack Jan 3 '16 at 15:28
  • \$\begingroup\$ Exactly. That's what I want to do; get rid of 12V DC altogether. Also, Solid state relays (SSDs) are based on Traics and I believe they can be used for even high power loads as well (with very little current on DC side). So I think a) Using SMPS based power circuit to generate +5V DC (100/200mA) and b) using SSD based relays should be fine for me. Right? \$\endgroup\$ – d.sehrawat Jan 4 '16 at 4:03
  • \$\begingroup\$ Triacs can be used for high currents if you provide proper heat sink which is another pain due to size constraints. Without heat sink, they will carry 1A or so without getting much heated. Refer to this answer which I posted few days back: Mosfet heating calculation \$\endgroup\$ – Whiskeyjack Jan 4 '16 at 4:48

You need durability - go with the plain regular transformer. This way you do not need the high voltage capacitors (expensive 400V ones) and switching mode transformers - both of them point of failure with high percentage in the low cost "lines".

SMPS is OK for higher power needs and where weight/dimensions are constraints. Cheap Chinese ones generate a lot of noise at the power lines - you may need filters.Inrush current can also be a problem with cheap SMPS.SMPS is also more likely to fail when there are power line transients.

P.S. Just a recommendation - you may use 230V/24V transformer and step down (buck) converters/stabilizers (LM2596 based for example) to not waste energy in heating :) - hybrid, but rock solid solution.

  • \$\begingroup\$ Thanks for the reply. But most of the related content on internet prefers SMPS for durability. The size that I am targeting is around 14-15cm in length, 3-4cm in width, 2-3cm in depth. I thin coil transformer shall consume a lot of space. \$\endgroup\$ – d.sehrawat Jan 3 '16 at 15:03

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