The power supply is what takes the microUSB port voltage and creates the 5V USB, 3.3V, 2.5V and 1.8V core voltages. The 3.3/2.5/1.8 are for the processor and Ethernet.
There's a couple good things about this design:
- It's really inexpensive, allows the Pi Model B to be simple and low cost
- There's a TVS to protect against overvoltage/negative voltages (within a volt or two, its not a huge TVS, it wont protect against 120V or 220V mains!)
- There's a fuse to protect against over-current of about 1A
- If the voltage coming into the Pi microUSB port is NOT 5V, say 4V - the Pi 5V power pin wont be 5V, it will be 4V which is too low! There's no warning or 'repair' circuitry to fix the low voltage
- This can happen easily with a poor quality USB port that provides only say 4.5V or 4.75V coupled with a poor quality USB cable with very thin wires. The wires are so thin, that they act like resistors and there's a 'voltage drop'
- If the voltage is noisy or fluctuates, this can also be really annoying for the Pi or any USB devices plugged into it
- If you plug something into a USB port on the Pi, the sudden current draw will cause a brownout on the 5.0V line, resetting all the other USB devices (and possibly the Pi!)
We're using the 5V power supply to generate the 3.3V supply, it does that by essentially 'eating' the 5-3.3 = 1.7V difference and dissipating the power difference in heat. This is why the big chunky 3.3V regulator gets kinda hot (but don't worry, it does not get so hot it is damaging, its just burning off that extra voltage difference in heat)
To make the B+ more reliable and actually reduce the current draw, the power supply is completely redesigned.
Watch this great video about this technique here:
So not a lot has changed here (other than putting in a protection FET)
There is a PNP-matched-pair action going on around the polarity FET, but its 3AM and I'm not 100% sure what it's for so I'll wait till I get some rest before doing any analysis.
Let's look at the 3.3V & 1.8V supplies:
The input to the dual buck is 5V (VIN1 and VIN2) - there's no part number marked here for some reason but it has 12 pins, is a DFN-shaped part (I deal with DFN's all day so I can spot them), and has the marking code C2=CGU0G. with some searching around for a 12-DFN dual buck with 1.8V and 3.3V fixed outputs...
Looks like its an RT8020AGQW 1A-max dual step-down converter with high 1.5MHz frequency. Nice chip! Its up-to-95% efficient, so less current is drawn to run the 3.3V power rail
The AP2331 is single channel current-limited integrated high-side power switches optimized for hot-swap applications. The devices have fast short-circuit response time for improved overall system robustness and provide a complete protection solution for application subject to heavy capacitive loads and the prospect of short circuit. It offers reverse-current blocking, over-current, over-temperature and short-circuit protection, as well as controlled rise time and undervoltage lockout functionality.that it is used to keep the USB hot-swap power draws from resetting the main 5V power supply. Hopefully this means you can plug-unplug USB devices like WiFi adapters without them resetting the Pi.