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Switched-mode power supplies contain a silicon-controlled rectifier an advanced semiconductor switch that can conduct high currents. The SCR is typically realized using a pair of bipolar junction transistors in conjunction with Zener diodes. Overcurrent conditions such as short circuits are a serious concern, as they can cause the semiconductor devices to overheat and suffer thermal stress, and ultimately fail.

Polarity-reversing overcurrent protection is a prevalent and affordable methods used for protecting an SCR from overcurrent conditions. This is achieved by reversing the polarity of the SCR that exists between the input and output of the power supply. To realize polarity-reversing overcurrent protection, two SCRs are required. The SCRs are turned on and off by a technique called Zener diode voltage regulator.

Zener voltage regulator consists of a Zener diode and a BJT that work in conjunction to turn on and off the SCRs when the output of the power supply is in a fault state and reaches the threshold of the Zener diode voltage regulator. In terms of turn-off timing, there is little issue because the BJT in conjunction with the Zener diode quickly switches off a very short time period. However, to ensure that the output of the power supply is free from electromagnetic interference, the turn-off time of the SCR can affect this condition. To avoid the negative impact, a push-pull topology is used.

Another overcurrent protection technique used in switched-mode power supplies is the parallel configuration of high power bipolar junction transistors that operate together. This configuration provides reliable protection by reducing the base current flow. This is achieved when a short-circuit fault occurs, and the fault current is split into the multiple BJT paths. The parallel BJTs continue to handle fault currents with lower base current by providing symmetrical fault current.

However, the drawback of this dual-path topology is the requirement of a current transformer that senses the fault current.

OCP is possible using a separate OCP protection module. The protection module has a power detection device or a current transformer that measures the inrush current in one part of the power supply, then shuts down the system. These modules can be more affordable compared to Zener diode and BJT configurations and have higher reliability. However, this type of module is bulky.

In summary, the polarity-reversing overcurrent protection method is the most widely used in switched-mode power supplies due to its simplicity and cost-effectiveness. It exploits the principle of electronic monitoring when the output is short-circuited to detect a fault condition. Other methods of overcurrent protection are the dual BJT topology and رله الکترونیکی the OCP module. However, they can result in system instability and other performance problems.