PLCs benefit from a DC/DC point of load power solution that supports the needs of advanced analog and digital integrated circuits, offers high efficiency with good thermal performance, and reduces the overall component count and cost. Point of load strategies can vary, but PLCs are usually provided a 24-VDC input supply from the power supply or occasionally a 12-VDC input. However, the line voltage is susceptible to input voltage transients that originate from motors or relays, causing excessive voltage spike which can damage the system. Voltage spikes also come from power transmission wires routed longer distances introducing parasitic inductance loops causing problems to the DC/DC converters. It is good design practice to account for unpredictable voltage spikes by choosing a DC/DC converter that withstands an additional 50% voltage rating, or 36 V, from a 24-V rail if no other line voltage conditioning exists in the system.

In almost all cases, 5-V and 3.3-V rails are used as secondary regulation rails from 24-V or 12-V source to power low-voltage sub-systems. Since newer microcontrollers, FPGAs, memory ICs, clocks, and AFEs operate with lower voltages, it is difficult to regulate a 1-V rail with a 24-V input while switching at a higher frequency, such as 1 MHz or above, to maintain a small form factor. As shown in Equation 1, to regulate 1 V from a 24-V input (4.2% duty cycle), the minimum controllable on time of the DC/DC converter must be lower than 40 ns when switching at 1 MHz to avoid noisy pulse-skipping.

Line transients can come from motors and relays in the system, and can cause an excessive voltage spike on the input voltage line. Voltage spikes can also come from power rails or signal transmission lines that are routed longer distances causing problems to the DC/DC converters or interface circuits. Because PLCs are employed on factory floors that may have motors or other inductive loads and loops, they are susceptible to line transient spikes. Figure 2-1 shows an example of a line voltage transient which may have a short duration, but can severely damage circuits inside PLCs without proper protection.

Figure 2-1 Example of a Fast Line Voltage Transient That Can Damage PLC Circuits

A protection circuit, or clamping circuit shown in Figure 2-2 would typically be used for protecting the load from voltage spikes. The diode D2 is used to set the clamp voltage and a pass FET is used to allow the current to flow to the load protected. Unfortunately, these circuits take up space and require more additional circuitry. As semiconductor process technology advances, suppliers are able to offer higher input voltage converters to integrate components and save space. It is true that a 28-V converter rated at 3 A is a less expensive than a 60-V, 3-A converter with the same MOSFET resistance. But the reliability and space savings of a higher rated converter is worth the small added price. Instead of relying on voltage protection circuits, non-isolated synchronous buck converters with integrated FETs are available with ratings up to 100 V to protect downstream circuits.

Figure 2-2 Discrete Voltage Clamping Circuit