1 Introduction

The Programmable Logic Controllers (PLCs) are widely used for automation in industries. The PLC collects data from sensors, analyzes this data using CPU, and controls the industrial process through actuators. The PLC CPU needs energy storage to provide a backup for storing critical information in case of loss of power. The energy storage is either provided by a battery or a large capacitor. A large capacitor is preferred over a battery for energy storage due to its lower cost. Figure 1-1 provides a block diagram for the PLC CPU.

The 24-V field power is the primary source of power for the PLC CPU. An eFUSE device with integrated MOSFETs and protection features can be used at the power input of PLC CPU to protect from surges and faults on a field power bus. The energy storage capacitor is used to power the DC/DC converter during an event of failure of power on a field power bus. A capacitor of value typically more than 1 mF is used to provide power to PLC CPU during failure. This capacitor draws large current during start-up and can cause the eFUSE to go into shutdown due to overload or due to excessive thermal dissipation. Another similar application of energy storage requiring large capacitance is for motor and servo drives. TI Design Compact, Efficient, 24-V Input Auxiliary Power Supply Reference Design for Servo Drives provides the complete design procedure and test results for power supply in servo drives.

In PLC systems, there is a 24-V power bus which provides power to modules in the PLC system. This power bus is powered from DIN power supply. The number of modules connected on this power bus varies with architecture of PLC systems. The output capacitance seen by DIN power supply is unknown. Typically, a DIN power supply of less than 250 W is designed for a maximum capacitive load of 10 mF. Figure 1-2 provides a block diagram of DIN power supply.