DMG2305UX MOSFET Q3 – Dead Pi B+ (V1.2)

In this continuing article to repair a dead Raspberry Pi B+ (V1.2), we focus our attention on the MOSFET Q3 on the board. The circuit uses a p-channel MOSFET to provide protection against reverse polarity, and control the current to the Pi circuitry. You might be wondering whether it was necessary, when a simple rectifier diode could work to protect polarity reversals. Although a simple diode would work in most projects, where voltage levels are not critical, it would not be ideal for the Raspberry Pi, because a diode poses a considerable voltage drop of approximately 0.8 V. In addition, you have to consider the power loss caused by the diode, which is the amount of current drawn by the Pi multiplied by the forward voltage of the diode. Hence, the use of a MOSFET is standard and kosher practice in electronics, because they do not pose a significant voltage drop when conducting in saturation mode. However, this is the Rolls Royce of circuits and you would expect something like this from Cambridge.

In these types of applications, it is standard practice to connect the positive input voltage to the drain gate, which can make it appear that the connection is backwards. If you are a beginner, then you will be glad to know that power MOSFETS usually conduct equally well in either direction. However, in this particular orientation, we make use of the internal parasitic diode by pointing it in the correct direction so that it conducts when the polarity is correct. This means that when the MOSFET is not conducting there is still current flow through this diode with a voltage drop of 0.7 V.

The MOSFET conducts fully with the presence of a negative voltage at the gate junction during which time the source and drain junctions have negligible resistance between them, and, almost all the voltage transmits to the Raspberry Pi circuitry.

Reversing the input voltage causes the gate junction to become positive, and therefore the MOSFET stops conducting. In addition, the reverse bias posed by the internal parasitic diode also blocks the current, and therefore reverse voltage cannot pass through to the Pi circuitry. MOSFETs tend to behave like almost perfect switches, in that when they conduct, there is negligible resistance between the drain and source, and when they are open, there is almost infinite resistance.

Diagnosis

As you can imagine, the DMG2305UX MOSFET (Q3), and the DMMT5401 matched transistors, play a crucial role in controlling the current to the Raspberry Pi circuitry. If either of these components fail then there will not be any power reaching the processor and your raspberry pi will not boot and remain dead.

The first component I might check at this stage is the DMG2305UX MOSFET. Therefore, the following diagrams and pin-outs should help if you are trying to repair your Pi B+. The pinout orientation on the diagram is the same as that on the board so that you can follow the logic easily.

When power MOSFETs fail they tend to go short between all the terminals and therefore if you measure +5 V at the gate junction G (pin 1), then as Sherlock Holmes would say that something is afoot! Alternatively, as I say sometimes, it appears hinkey!

If you do not find +5 V at the drain junction D (pin 3), then you need to check the fuse F1, and make sure there is voltage entering through the input socket J1. However, if there is +5 V at the drain junction, but 0 V or an abnormally low voltage at the source junction S (pin 2), then again, it could be the MOSFET that may have failed by going open. If you live on Mars where the supply of replacement components might be low, you might simply by-pass the MOSFET by joining the source and drain together. However, as the man says, you break it, you bought it, and all warranties are null and void.

If the MOSFET turns out to be fine, then you may have to focus your attention on the component DMMT5401, which is U14. This is in the following pages of this multi-part article.

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