Best way to power a microcontroller, SMPS vs Linear Regulator
1) Buck converter is:
- more expensive than a linear regulator
- takes typically more room on the PCB
- is typically more difficult to design (sometimes, just a bit more, sometimes way way more)
- is more noisy (the amount of noise depends on a lot of factors, though)
But it is indeed much more power efficient, especially if there is a big difference in the input voltage vs output voltage, which is your case here. The buck will output almost the same power than it takes at the input (efficiencies are typically ~80-90%), whereas the linear regulator will take as much current as input as it needs to supply (which means that efficiency is Vout/Vin, something like ~27-44% in your case, which is very bad).
2) Yes, this is actually the only reason why linear regulator can be a bad choice: the efficency (and your computation of dissipated heat is fine). Now, having that much power dissipated leads to two big problems:
You will probably need a heatsink (check the datasheet of the linear regulator: at anything more than 1W, you need to check carefully even in TO-220 package. When using smaller packages, it is often not possible at all). So this negates the "more room on PCB" inconvenient of buck regulators.
If you run on batteries, it means much less runtime. Sometimes, you can't afford it (do the computation).
3) Most likely not, if you use standard integrated solutions to do the stepping down. Those are made to provide power to IC chips, and the datasheet/application notes of the step-down controller/regulator you'll choose should give you some information about the amount of noise you'll get. But for digital operation, supply noise isn't typically that much of a problem.
4) Given the huge difference in input/output voltages, the current you need, and the fact you'll partly run on batteries, it seems a logical choice to go for a buck. But you need to double-check all that yourself. Maybe in your case it is acceptable to have a huge TO-220 dissipating 1.3W in your enclosure, and the runtime you need isn't that high.
If you go for a buck, here is what I can suggest:
- One solution is to go for a complete module. You then don't need to design anything. Check mouser/digikey, they have DC-DC converters that you can just solder to your PCB like a linear regulator. If you're an aliexpress/ebay kind of guy, you'll probably find plenty of cheap things there also.
- You can design your own (scary, I know... wow, an inductor!). What I can suggest in this case is to have a look at the tools provided by the various manufacturers (e.g. TI webench, but Linear technology has one too, ...). You just supply them with your requirements (input/output voltage, current, ...) and it spits out a lot of possible designs (you'll see some of them are actually very simple) with different chips from their catalog. All the passive component values are already calculated for you, and they even suggest you part numbers for the inductor, etc... So you just have to check the recommended PCB layout from the documentation to design your board, buy the parts, solder, and it should work.
1) SMPS is more effective in converting energy, but is more noisy because of the switching. The linear regulator wastes power proportional to the difference in input and output voltage, but operates with low noise.
2) Depends on whether or not you can dissipate 1.3W - only the designer (you) can know that. 1.3W can be a lot of power for a small IC, so you might need a heat-sink.
3) Different switching frequencies make noise on different frequency-bands. Only the designer (you) can know if that will be a problem. You should follow a reference design for the specific MCU to ensure the input voltage ripple is acceptably low.
4) Depends on the how the trade-offs are weighted for the specific application. One cannot be objectively better than the other. It is almost always a trade-off in engineering.
- What is the difference in using buck converter vs linear linear regulator
Very minimalistic explanation:
SMPS
A SMPS (switch mode power supply, e.g. Buck) basically compares the output voltage to a given reference. If the output voltage is above the reference, the regulator basically cuts connection between input and output. If the output voltage is below the reference, input and output are connected. Output capacitance and inductance are used to store energy on the output side and smoothen the output voltage.
benefits: Efficiency and therefore power dissipation (--> heat) because the switches are either closed (no current -> no power dissipation) or open (lowest resistance state -> minimal power dissipation).
downsides: additional parts (usually a voltage devider, inductance, capacitance and maybe a ferrite bead for noise suppresion) and increased price (device itself and additional parts).
Linear
Unlike an SMPS a linear regulator doesnt use a transistor as a switch (on/off) but in linear mode (any state between on and off is allowed as well). This leads to increased power dissipation, as you can imagine the transistor as a regulated resistor that is being adjusted for a voltage drop of Vin-Vout.
benefits: cheap; easy; less/no noise due to no switching, might need only a capacitance downsides: efficiency, especially on high load;
- Would linear regulator (small packages) be a bad idea because it would heat up a lot because there is a big difference in voltage (12-3.3=8.7, 8.7*0.15=1.3W) ?
I would answer this with yes. If you have a look here and consider values like the ones on chapter 6.4 in e.g. this datasheet you will see that the thermal resistance easily goes over 100°C/W (meaning: a temperatue rise of 100 °C for 1W power dissipation). I think having this in a small case wont work, even with a (small, because small package) heat sink and lots of copper area on your PCB determined for cooling (so you won't be able to benefit from the small package at all).
As a rule of thumb I usually use a linear regulator if i need either very low currents (just a few mA at max), very small voltage drop (1..2V) and/or super clean supply voltage for an ADC or other analog parts. Means in most cases i prefer to use SMPS. These require more parts usually (more caps, resistors, inductance) so its a more expensive and 'complicated' solution.
- Would the frequency of switching, or output voltage ripple (noise) be a big influance on the normal operation of a MCU?
If you design a SMPS based on the devices data sheet there are usually calculations given for the expected ripple noise. These are usually within 1% of the output Voltage what is no problem for digital systems. I have created an excel sheet ot help dimension caps etc, but I dont know how to add an attachment here ...
Also you would probably want to add a 10..100nF cap to each supply input of the MCU and keep the traces from Cap to MCU short to minimize ripple seen by the power pins.
- Conclusion, what is the best way to power it with the input voltages between 6V and 12V?
As you need a big voltage step, more than a few mA and didnt mention any special requirements regarding noise (for analog stuff) I'd go with a SMPS.