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I saw this schematic for the RPi power supply with USB-C. I do not understand how the D1 diode acts as over-voltage protection. Can someone explain?

enter image description here

The image is from the RPi documentation.

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    \$\begingroup\$ Do you know how Zenner diode work? \$\endgroup\$
    – internet
    Commented Jun 15 at 14:38
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    \$\begingroup\$ To be clear, it’s only for very short overvoltage transients. \$\endgroup\$
    – Ste Kulov
    Commented Jun 15 at 16:14

2 Answers 2

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That is a TVS diode - Transient Voltage Suppression diode.

It tolerates the rated 5V, but somewhere between 6.4 to 7.0V it starts to conduct and can conduct and clamp transients up to 65A to a safe level.

And plugging and unplugging USB cables can produce transients, because a cable is basically an inductor with current flowing in it, so sudden scratchy connection in the connector can produce large transients.

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Think of D1 as a Zener diode. In this application, it is a 7-volt Zener diode. Under normal operating conditions, it is effectively an open circuit with 5 volts across it. At 7 volts, the Zener turns on and starts to conduct and hold or clamp the voltage at 7 to 9 volts or die trying.

Littelfuse makes the SMBJ5.0A, and calls them TVS Diodes. They have 1 pico second turn-on times and can shunt up to 600 watts for 10 microseconds—passive and inexpensive circuit protection.

Although not as effective, several diodes wired in series could do the same. For example, 10 diodes wired in series would turn on around 7 volts and clamp to the point they burn up.

TVS diodes are available in various form factors, voltages, and power ratings up to 10's of Kilowatts. If you need more, add them in parallel.

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