Protecting Power Supplies from Over-Current Conditions

To ensure a lifetime of safe and reliable operation, power supplies are fitted with various protection mechanisms. Some of these limit the current delivered under short-circuit failure conditions, ensuring that the power supply’s components don’t overheat or break down, allowing the unit to return to normal operation once the fault has been removed. However, some use cases and loads, under certain conditions, can trigger these protection mechanisms. This can lead to confusion as to the source of a fault. To understand whether it is the power supply or the equipment it is powering that is at fault, it is essential to understand the protection mechanisms used and their limitations.

What is a short circuit?

Most engineers have experienced the fallout of a short circuit at some point. The bare end of a wire touches a grounded casing, and – pop – a component’s magic smoke is released. But, when designing a power supply, it is necessary to define what a short circuit is to design appropriate protection circuitry. A short circuit is typically defined as a connection with a resistance below 1 Ω or a shunt resistance that results in the output voltage dropping below 100 mV. In a single output power supply, the short circuit would be between +VOUT and -VOUT.

What protection does fold-back current limiting offer?

An alternative protection mechanism is fold-back current limiting. This reduces the current and voltage once the current draw exceeds a predefined value. As more current is drawn, both current and voltage are reduced further. Because this mechanism is not universally defined, power supply designers are free to implement it as best suits the application. When fault finding such equipment, it can cause confusion as the voltage and current appear to be outside the data sheet specification.

Suppose the application regularly draws more than the specified maximum current, but the levels are not high enough to be considered a short circuit. In such cases, the power supply risks permanent damage or failure. Under true short circuit conditions, fold-back current limiting provides better output transistor protection for the power supply than the constant-current limit approach.