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I need a power supply for a device that contains a bluetooth chip and antenna and a relay. The input voltage ranges from 12AC or 12 DC to 24AC or 24DC, because it need to be powered from diferent sources in different situations. The output voltage needs to be 3.3V and the output current around 200mA for the bluetooth consumption peaks and the relay actuation.

Because it needs to be powered form AC and DC, i was thinking the input cam be a bridge rectifier and a bulk capacitor.

The next stage is where i need help. First of all, i'm learning and i'd like to learn how a real product engineer would design a real solution for this problem. I thought two different solutions:

  1. The most simple solution: Regulator. The maximun input voltage for a regulator for this desing is with 24v AC, 34V DC after rectifier. So a regulator with max input > 34V and 3.3V output will do the job. But it can get really hot because at 200mA it disipates 6W!

  2. Buck converter. Much more efficient, better solution than the regulator. But i was worried about how it affects (noise of the switching frecuency) the RF circuit (bluetooth) or EMC certification.

Witch one is the best solution?, is another one better?, how a real product engineer would solve it?

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  1. Most engineers would now use a Flyback Converter in most similar cases.
    It's similar to a buck/boost converter, except it uses a flyback transformer in place of the single inductor.
    While it adds a bit of complexity, and loses a bit of efficiency versus the buck/boost, it adds electrical isolation, which is often required by either regulations, or design/safety specs.

  2. While an analog Vreg (regulator) could perform this job, they are usually only used when absolute stability of the output voltage is paramount, and for large voltage differentials are often employed after some form of SMPS converter (buck, boost, buck/boost, or flyback topology) as a "smoothing" for the output.

  3. Depending on the exact stability required by your components (relays can throw a LOT of noise into a circuit when switching on/off their control coils), you would have to make decisions as to how many & what size of storage/decoupling capacitors will be adequate for smoothing the output from an SMPS, and if a Vreg would be necessary (or even beneficial) as a "final smoothing stage," should you choose to use the SMPS topology.

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  • \$\begingroup\$ No, most engineers would not use a flyback converter except if they needed isolation. Since this is obviously not powered from mains voltage, there is no reason to assume he needs any form of isolation. \$\endgroup\$ – metacollin Apr 12 '17 at 22:05

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