Huawei ETS2000 Series Maintenance Manual page 19

Figure1-20
Second-time power voltage-limiting module
6. Short circuit protection and overvoltage protection module
Q302, Q320, Q321, and D381 constitute the short circuit protection and
overvoltage protection module. If short circuit happens to VREG_PHONE, Q321
will be through to trigger a circuit constituted by Q302 and Q320. As a result, the
DT voltage of U302 will be improved to 5V, the U302 switch output will be closed,
and Q301 will be protected. If a voltage feedback loop fault causes the output
voltage to be improved, D381 will be through to trigger a circuit constituted by
Q302 and Q320. Then the DT voltage of U302 will be improved to 5V, the U302
switch output, and the power supply will be closed. As a result, the high voltage
will be prevented from blowing the back level circuit. See Figure 1-22 for the
circuit.
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