In many industrial and factory automation applications, there are multiple power rails that power various subsystems within the application. Some of these power rails include 24 / 48 VAC AC sources with a known operating frequency that requires a full-bridge rectifier and capacitors to convert its signal to a DC voltage where it can be monitored by a voltage supervisor. One drawback with the described conversion is the response time of the DC voltage when the AC power rail experiences a change of operating frequency or voltage amplitude. Due to the output filter of the full-bridge rectifier, the detection in the change of voltage or operating frequency may require several AC cycles before the voltage supervisor outputs a fault condition. The direct monitoring of the AC source by using a “Resistive-Drop” supply topology circuit provides the user a fast transient fault detection. In this design example, the TPS37A is being highlighted with the ability to offer a unique “window operating” solution by monitoring the output of the AC source for over or undervoltage operation.

Figure 10-4 Sensing an AC Signal for Power Fault Detection