Power Supply

One of the most exciting updates/upgrades of the new Model B+ is a fancy new power supply. The power supply of a computer is terribly boring sounding, but its really important. A good power supply makes everything hum along cleanly. A bad power supply causes hiccups, crashes, 'bricked' boards, SD card failures, USB failures...you name it!

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.
Lets look at the 5V supply schematic first
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
Not-so-great:
  • 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!)
Let's also look at the 3.3V, 2.5V and 1.8V power supplies
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)

Model B+ Power Supply

To make the B+ more reliable and actually reduce the current draw, the power supply is completely redesigned.
Lets look at the power supply input first
There's still the microUSB jack on the left, and the 1A fuse has been upgraded to a 2A fuse. There's also a DMG2305UX P-Channel MOSFET. This acts as a polarity protection switch but is much lower 'drop-out' than a diode. It has only 52mΩ resistance so @ 2A its about 0.1V voltage drop. Most diodes would be at least 0.5V.

Watch this great video about this technique here:
To the right is a protection TVS diode (D5 part #SMBJ5) which protects from over-voltages.
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:
Instead of heat-spewing LDO (low dropout) regulators, we now have a dual buck converters. These are high efficiency converters that can take 5V down to 3.3V or 1.8V without as much heat loss. They're more expensive than LDO's but not terribly so!

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...
10 results, sort by price...
And look at the least expensive option (only 40 cents!) marking details...

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

There's one last piece that looks new, there's now a new part between the 5V input and 5V output called AP2331
This is a hot-swap protector for the 5V supply. While it's not clear what H5V goes to, it seems likely that since the description says:
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.
Last updated on 2015-05-04 at 04.27.56 PM Published on 2014-07-14 at 02.13.10 AM