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I need help on choosing type of regulator base below design requirements

Design Spec:

Input Voltage = 12V

Output Voltage = 5V

max Current = 3A

Power (12-5)*3 = 21W

EX: Without heat sink 60C/W (lm1084 ) So: 21W*60C/w = 1260C which is out of questions

So what is best option regulator with budget of under $3 (for Qty 1) and must be surface mount

Main concern is the heat generated by the regulator

Shahreza
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1 Answers1

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How to select your DC-DC... To go from 12V to 5V you need a buck converter, so let's check that in the search engine. Now, for the other criteria...

  • Ready made or DIY?

That's the first question to ask, especially if you're not familiar with designing DC-DC converters. There are a lot of small board-mount modules available, for example this one. Search for "PoL (point of load) DC-DC" or "board mount" etc. You'll need a non-isolated buck. A ready-made module can be a very good idea for a small production run when you don't want to waste time on designing a DC-DC.

  • Synchronous or asynchronous rectification?

From 12V to 5V, duty cycle will be 5/12 = 0.42 thus a converter with asynchronous rectification will have a diode conducting 42% of the time, wasting 3A * about 0.5V * 0.42 = 0.625 Watts. A synchronous rectification converter will be a bit more efficient, it might waste about 0.3W instead in the bottom FET (wet finger in the wind calculation with 40 mOhms FET and switching losses equal to conduction losses). Since the difference between synchronous and asynchronous is less than 1.5% efficiency in your case, if asynchronous is cheaper, no problem, just use that. Synchronous becomes very useful at very low output voltages like 1.2V though.

  • Internal switch vs external?

An external FET means you can pick any FET you want, but it is an extra part and extra cost. However, an IC with an internal MOSFET will cost more than an IC without it. If uses synchronous rectification, then it is 2 FETs, or a single package dual FET. So you need to add the price of extra parts if you use a chip without internal FET(s).

  • Frequency

Higher frequency shrinks the inductor and the input/output caps, and reduces core losses. It also increases switching losses.

  • Other features

Like efficiency at low load, losses at very low load or no-load, "power good" output, ENABLE input, input voltage range (do you want it to go down to 5V?), how much noise/ripple you can tolerate, EMI, etc.

Knowing this, just input your requirements into webench and check if it generates designs you like. You can set it to optimize for cost, footprint, or efficiency. Of course, it wants you to buy TI chips, but nothing stops you from using it as inspiration and using another similar chip from another supplier.

The first suggestion is this chip which looks pretty good. Note if your "3A max" load spends lots of time drawing 3A, it may be beneficial to select a chip with peak efficiency at 3A, and that would be a chip capable of a bit more current, perhaps a 4A chip. The 3A chip's efficiency peaks at 1-2A.

bobflux
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  • Thanks for detail answer. My question is does the buck regulator generate as much heat as linear regulator. Since this is my main problem is the heat since thermal junction of linear regulator is too high – Shahreza Aug 01 '19 at 20:39
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    Switching converter will generate much less heat, if it has efficiency of 90%, at output power of 15W it will dissipate about 1.6W instead of 21W for linear regulator. But if you ask this question it means you don't have experience with switching DC-DC converter and you should consider buy a ready-made unit (only costs a few $). Only design one if you want to learn how to do it. – bobflux Aug 01 '19 at 20:51
  • That's exactly what I am trying to do build board for internal use. Thanks for the help – Shahreza Aug 01 '19 at 21:04
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    So it is low quantity, right? If you spend several days learning how to implement a DC-DC, it can be more expensive than just buying a ready-made module. If you want to design your own and learn, then sure, it is interesting, and a good goal too... but keep in mind what you want to do: do you want a power supply, or do you want to learn how to design one? – bobflux Aug 01 '19 at 22:48
  • I would like to learn first and then build protype – Shahreza Aug 02 '19 at 02:10
  • OK! In this case, webench is a good starting point, and it is a good idea to study the manufacturer's evaluation board for your chip and copy its layout. Datasheet and eval board manual usually contain plenty of useful advice for layout and parts selection. – bobflux Aug 02 '19 at 08:47
  • I have question I thought when you calculate switching regulator power you use below formula is that correct (Vin-Vout)Imax (1- Efficiency) so (12-5)(1-0.9) = 2.1W Then Thermal dissipation is equal to 2.1W * 95.9 = 201.39 C but how come on webbench is only saying 60C – Shahreza Aug 02 '19 at 16:27
  • Losses don't occur just in the chip, they are split between inductor and MOSFETs inside the chip, plus a little bit of extra losses in capacitors. Inductor is cooled by air and the PCB, as the output pin of the inductor can be connected to a large copper area. Chip is cooled by PCB, pins like GND and VIN can be connected to copper planes. Check "junction to board thermal resistance" in datasheet. – bobflux Aug 02 '19 at 21:01