Model A and B Power Supply
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.
Power comes in from the LEFT side of the image, from a “MICRO USB TYPE B” jack, goes through “miniSMD” F3 (a fuse) and then has a D17 (Transient Voltage Protection Diode) across it as well as some capacitors (C2, C3 and C6). That voltage is the +5V0 USB bus voltage. Hurray!
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!)
In the top left you can see that +5V0 voltage going into a NCP1117-3.3 (3.3V regulator), and the output of that going into a LP2980-2V5 regulator and NCP1117-1V8 regulator.
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)
For more detail: Introducing the Raspberry Pi Model B+