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Improving SSR Overcurrent Protection

Switched-mode power supplies contain a silicon-controlled rectifier an electronic current controller that can conduct extremely high currents. The SCR is typically 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 one of the most widely used and cost-effective 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 رله الکترونیکی 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 function together 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 no problem 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 perfectly sinusoidal, 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 parallel 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 power supply malfunction occurs, and the fault current is split into the multiple BJT paths. The parallel BJTs continue to switch the large inrush current with lower base current by providing symmetrical fault current.

However, the drawback of this parallel configuration is the requirement of a power monitoring device that senses the fault current.

OCP can also be achieved 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 efficiency. However, this type of module is larger in size.

In summary, the most commonly used OCP method is the most widely used in switched-mode power supplies due to its ease of implementation and low cost. It exploits the principle of electronic monitoring when the output is short-circuited to detect a system malfunction. Other methods of overcurrent protection are the parallel configuration of BJTs and the OCP protection system. However, they can result in distortion in the sinusoidal waveform and other reliability issues.