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.

Capacitors draw large currents from the power source at start-up, which can lead to tripping of the power source due to overload. To limit the inrush current into capacitors, power switches implement constant current charging of capacitors at start-up. To charge the capacitors with inrush current, the output voltage is increased linearly with time. As an example, the TPS2660 device has a capacitor on the dVdT pin to control the output slew rate and limit the inrush current for output capacitor. Figure 2-1 provides the application circuit with the TPS2660 for charging capacitors with constant inrush current.

At power up, the output capacitor has zero voltage and there is power dissipation of (V_IN X I_INRUSH). As the capacitor gets charged, the voltage drop across the power device and the power dissipation decreases. For charging the output capacitor to V_IN voltage, an average power of (0.5 × V_IN X I_INRUSH])is dissipated in the power switch during the start-up. Figure 2-2 provides power dissipation for C_OUT of 1 mF and I_INRUSH of 115 mA.

For lower voltages and lower output capacitance, the capacitors can be charged with constant inrush current and constant output slew rate. But as the output capacitance and input voltage increases, the power dissipation in the power switch at power up increases and can lead to thermal shutdown and hiccup in start-up. Table 2-1 provides the power dissipated for a start-up time of 209 ms and constant output slew rate of 115 V/s.

With increased power dissipation at higher voltage and increased output capacitance, the power switch goes into thermal shutdown and leads to hiccups in start-up. Figure 2-3 shows the hiccups in start-up with output capacitance of 15 mF and V_IN of 24 V due to thermal shutdown of power switch.

See the TPS26600 Design Calculator to design with TPS2660x devices with clean start-up.