Effective Overload Protocols

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 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 degrade performance, and ultimately fail.

Polarity-reversing overcurrent protection is a prevalent 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 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 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 little issue because the BJT in conjunction with the Zener diode quickly switches off 100 nanoseconds. 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 that operate together. This configuration provides reliable protection by reducing the base current flow. This is achieved when a power supply malfunction 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 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 small transformer 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 be more affordable compared to Zener diode and BJT configurations and have higher reliability. 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 dual BJT topology and the OCP protection system. However, they can result in system instability and other reliability issues.