First of all, I have to make reference to a great datasheet I’ve found on a particular Maxim switched-mode power supply that was extremely informative for an computer-grade electronics design novice like me.
20200308/https://datasheets.maximintegrated.com/en/ds/MAX1771.pdf
Again, I reiterate, because this is important! Second, I have to draw attention to the key realizations that got me unstuck on the decisions that had to be made for a suitable switched-mode power supply for my Raspberry Pi projects.
There are two classes of integrated circuits for switched-mode power supplies: switching regulators and switching controllers. Switching regulators have the main switching MOSFET builtin, whereas switching controllers use an external MOSFET. This allows switching controllers to supply more current than switching regulators, but it comes at the expense of requiring more complex circuit design.
So, armed with that knowledge, I went searching for a switching controller rather than a switching regulator. And indeed, I found a chip that could supply the 1+ amp of power I needed for my Raspberry Pi projects, in through hole form factor to boot. Yeah, it sure would be nicer if I could use those surface-mount components, I would have a much greater selection of components at my disposal, but for now, I must stick with the more limited selections in through-hole mounting. But, nonetheless, the point is this: if you only need 500 mA or less of current, you have a pretty wide selection of switching regulators available, but if you need more than 500 mA of current, you’ll find there to be a wider selection of switching controllers to be available for that particular purpose.
Now, reading into the Maxim datasheet, I learned something new to keep in mind when selecting resistors: equivalent series resistance (ESR). What is that? Unlike the ideal mathematical model of capacitors, real capacitors are not exclusively capacitive. Rather, they can be modeled as if there is a resistor in series with the capacitor. But please note, this isn’t merely an ordinary resistor, it is one that has an “AC” resistance. That is, it is only resistant at certain frequencies. If no other frequency is specified, it is the self-resonant frequency of the capacitor. At that point is where the resistance is strongest, and it fades away above and below that point.
Cheap aluminum electrolytic capacitors (the kind you get in electronics kits for novices) have a relatively high equivalent series resistance. To get a lower ESR, use a aluminum polymer capacitor or a tantalum capacitor.
Specifically, in the case of switched-mode power supplies, you need a capacitor with a low ESR because it will be subject to high ripple currents. The ESR is responsible for all the energy that gets dissipated as heat from the capacitors.
20200308/https://en.wikipedia.org/wiki/Equivalent_series_resistance
Another word about heat. Switched-mode power supplies are never 100% efficient… they’re at least 80% efficient if not better. At 80% efficient, 5 volts, 1 amp, 5 watts… that means 1 watt of heat. Yeah, now that’s a significant amount of heat to waste for a hobby project board since it’s enough to feel warm to he touch.
So, the BIG LESSON learned… with switched-mode power supplies, when the power draw is low, its really easy to integrate the whole module on the same board as the rest of your electronics, and of course it’s easier to integrate the MOSFET switch in the PMIC IC itself. But, for high power draw, you need to use an external MOSFET. And not only that, you’re going to have more waste heat that you need to dissipate away. This means its more likely that the switched-mode power supply itself will be located on a different board away from your main logic circuits, where it is easier to dissipate the heat, and a cable will connect the two together.
Matter of fact, that’s a good reason behind the design of wall warts in small electronic boxes like network switches, home routers, set-top boxes, and so on. It allows you to easily have a smaller box without active cooling, the extra surface area of the wall wart provides adequate passive cooling of the switched-mode power supply. Otherwise, you’d need a larger box to provide all needed passive cooling, or use active cooling and that would generate more noise.