Power management for Programmable Logic Controllers

9th August 2012

ES Admin

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The complexity of a power engineer’s task in contemporary electronic design is made even more difficult by the range of components used and, in particular, the individual power supply requirements each of these components requires. By Jeff Hooker.

Whether designing a single board solution or a much larger system, the power requirements for accuracy of supply and sequencing are paramount. For many backplane applications, it is not always possible to turn off the system when removing or inserting a board back into the system; this is particularly true where there is a need to eliminate system down time. Programmable Logic Based Controllers, or PLCs, rely on repair and configuration changes while minimising assembly line down time.

Cards that can be inserted and removed from powered sockets without disrupting the system are referred to as hot-swap cards. Hot-swap cards are designed so that the card remains undamaged and does not disrupt the host system (brown out) when plugged into or removed from a socket.

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To ensure reliable operation, the input current surge from the backplane into the hot-swappable board must be controlled during insertion. The initial inrush of current comes from charging the board’s hold-up capacitor on the front end of the power supply. This capacitor stores the charge that allows the board’s power supplies to operate for short periods if the board is removed from the system or the input supply is interrupted. A MOSFET is typically used as the switch on the board to initially isolate the backplane power from the card’s input capacitance and inductance. After the card is inserted, the MOSFET is slowly enabled, limiting the flow of current into and out of the board. Without the MOSFET to isolate the holdup capacitor, the input current flowing into the card can be very large. A board that may have a normal operating current of two amps may instantaneously draw over 100 amps as it initially charges the input capacitance. This momentary, excessive current can affect the reliability of the system by stressing components and causing brown outs (drooping backplane voltage) if the system cannot maintain minimum voltage levels during the current surge.

If this abstract has piqued your interest, read the full article online in the August issue of Electronic Specifier Design, by clicking here.