Friday, July 18, 2014

Pi Power user guide

This page is intended to be the end-user documentation for Pi Power. I'll revise this page if/when anything changes. The current version of the Pi Power board is v0.3. Here's the history:
  • v0.1 - initial version.
  • v0.2 - MOSFET changed from SOT-23-3 footprint to TSOP-6. The actual part is now a Si3443CDV for improved thermal performance.
  • v0.3 - Added auxiliary power input pads on the bottom of the board.
The Pi Power is a Raspberry Pi power supply. It is rated to take in anywhere from 6 to 14 volts DC and output up to 2 amps at 5 volts. The power input jack is a standard 2.1mm barrel connector, tip positive.

Pi Power is compatible with all presently released models of the Raspberry Pi. If you're using a model A, then you can't take advantage of the added power for USB peripherals without modifying your Pi to remove the USB polyfuses. If you're using a model B+, then you may (or may not) want to elevate the extra 14 pins of the GPIO header with a 14 pin stacking header. You may want to do this so that you can plug 40 pin GPIO accessories in above Pi Power. You may not want to do this if you want to plug in 26 pin IDC (ribbon cable) connectors, since they collide with pins 27 and 28.

Pi Power's output power is regulated to within 1% of 4.968 volts. Ripple is 25 mV P-P when the supply voltage is greater than 9 volts, and rises to up to 45 mV P-P at lower voltages. Pi Power's efficiency is roughly inversely proportional to the input voltage. At 6 volts, its efficiency is in the low 90% range. At 9 volts, it's in the high 80% range, at 12 volts, the low 80% range, and at 14 volts it's in the mid 70s.

The sweet spot for Pi Power is at a load of 750-1000 mA and a power supply voltage of 9 volts. If you were going to buy a "wall wart" power supply for Pi Power, my own recommendation would be a 8-10 watt 9 volt supply (or 9 volts at 800-1000 mA). I myself use a 12 volt, 1200 mA supply. A model B Pi with a keyboard, wifi module and camera can be expected to draw up to around 325 mA @ 12 volts. A beefier supply is just margin for future expansion. This will, of course, vary considerably from one Pi and peripheral set to another. Model B+ Pis will probably draw less by themselves because of their improved power supply infrastructure (but to make up for that, they offer 4 rather than 2 USB ports).

The output specification of "up to" 2A comes with some minor caveats. If you really want all of that 2A of output, you'll find that the MOSFET will start to get warm. This will limit the upper bound of the maximum ambient temperature to some extent. At 2A with a 12 volt supply, you can expect the MOSFET to rise up from an ambient temperature of 25°C to almost 60°C. Temperature is proportional also to the input voltage. If you really want to push the envelope, you should try and run with as low an input voltage as you can. None of this paragraph really applies to normal Raspberry Pis with reasonable and modest USB and/or GPIO peripherals. If you're not exceeding 1500 mA of draw or temperatures that are dangerous to humans, you shouldn't have anything to worry about.

If barrel connectors aren't your thing, then you can use the two auxiliary input pads on the bottom of the board. They're intended to take a 2 pin .1" right-angle SIP header. The pins won't go all the way into the hole because of the barrel connector on the other side. You might consider trimming the pins shorter so they'll fit. While soldering it, you will want to connect a plug to the header to insure the pins remain parallel to the board. Alternatively, of course, you could just solder two wires there instead. Be sure to take note of the "+" sign. The TVS diode will present a short circuit to a reversed polarity input to protect Pi Power (and your Raspberry Pi).

If you want to alter the output voltage of Pi Power, you can replace R1 and R2. The output voltage is 1.242 * (R1+R2) / R2. If you prefer the voltage slightly high instead of slightly low, you could use R1=68k and R2=22k instead (5.08 volts).

Don't try to exceed 14 volts input voltage. As the voltage passes 14 volts, the switching frequency increases to the point where the capacitance of the MOSFET causes it to overheat. Furthermore, at 17 volts, the TVS diode will break down and start conducting, which may cause it to start heating up (which would be bad). Also, the breakdown voltage of the MOSFET is 20 volts and the voltage rating of the input filter cap is 25 volts. At some point under 6 volts, the output voltage will begin to sag. That likely would cause either your Pi or its peripherals to begin malfunctioning.

Don't connect anything to your Raspberry Pi's power connector while powering it with Pi Power, or connect Pi Power at the same time as other power supplies. Attempting to power your Raspberry Pi with two power supplies will not work properly - the two supplies will fight with each other to regulate the voltage, leading to unpredictable results that may damage one or the other supply, or your Raspberry Pi.

Before you begin

When you begin using your Pi Power, please take a moment to test it before you connect it to your Raspberry Pi. With a suitable DC input voltage between 6 and 14 volts, you should see 4.968 volts +/- 1% on the output. If you don't, then do not connect Pi Power to your Raspberry Pi until you resolve the problem. Pi Power injects power directly to the Raspberry Pi's 5 volt power bus, and if it's not working properly (for whatever reason), you could damage or destroy your Raspberry Pi.

You should also periodically check D17 on the Raspberry Pi (this is D5 on the model B+). This is a TVS protection diode located on the bottom of the board near the corner with the microUSB power jack (on the B+ it is on the top near the jack). If you feel it with your finger, you should not be able to detect any elevated temperature. If D17 is hot - regardless of whether you're using Pi Power or anything else - disconnect power immediately! D17 getting hot means that your 5 volt power supply voltage is too high and D17 is partially short-circuiting to reduce it. It can't do that for too long, however, before it releases the magic smoke. If D17 dies because the voltage is too high, then its death will very shortly precede the death of much more critical parts of your Raspberry Pi - very probably bricking it beyond repair.

Note that supplying 5 volts on the GPIO header bypasses the input fuse on the model A and B and the reverse polarity protection MOSFET on the model B+ (though Pi Power does have its own reverse polarity TVS diode on its input). This is why you need to check D17/D5 for heating. Normally, if those diodes were conducting, the excess current would open that fuse.


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