In the last decade or so, significant advances have been made in the design of industrial power supplies and dc-dc converters, from the materials and device levels to size and weight reduction, thermal management, and package design. However, one often-overlooked category is protection of circuits and systems provided by the power supply and accompanying circuit breakers. These advances have contributed greatly to reliability and system availability while maintaining safety as well.
One of the most far-reaching is selective fuse breaking (SFB) or selective shutdown, which when enabled in both power supply and thermomagnetic, as well as other types of circuit breakers, provides significant benefits. There are two types of trip mechanisms in these thermomagnetic breakers—temperature-sensitive and magnetic—the former having a response delay and the latter almost instantaneous.
The temperature-sensing element of the circuit breaker consists of a bimetal strip with a heating coil. When current exceeds a threshold, the protective device generates heat in the coil, which causes it to bend and actuate the switch, shutting off power. The temperature-sensitive circuit is even effective when current is temporarily greater than nominal, such as when overload currents are shut down.
The magnetic trip mechanism consists of a solenoid coil and a plunger or pivoted armature. When current exceeds a specific threshold, a magnetic field is created in the coil, which attracts the armature to it and interrupts the circuit. Response time of this type is much faster than its counterpart, typically 3 to 5 ms, allowing it to respond to short-circuits and excessive overload currents.