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Switched-mode power supplies contain a silicon-controlled rectifier an electronic current controller that can conduct high currents. The SCRs are normally realized using a pair of bipolar junction transistors to act like a switch. 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 an electronic control system.

Zener voltage regulator consists of a Zener diode and a BJT that work in conjunction to turn on and اس اس آر 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 turns off the SCR within 100 nanoseconds. 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 requirement. To avoid the negative impact, a multiple-stage control system is used.

Another overcurrent protection technique used in switched-mode power supplies is the split branch configuration of high power bipolar junction transistors to reduce currents efficiently. This configuration provides robust safeguarding by reducing the base current flow. This is achieved when a short-circuit fault occurs, and the fault current is split into the parallel-connected BJTs. The parallel BJTs continue to switch the large inrush current with lower base current by providing fault current reduction.

However, the drawback of this dual-path topology is the requirement of a power monitoring device 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 turns off the power supply. These modules can have lower cost 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 reverse polarity when the output is short-circuited to detect a fault condition. Other methods of overcurrent protection are the dual BJT topology and the OCP module. However, they can result in distortion in the sinusoidal waveform and other reliability issues.