May 2011A Programmable Logic Controller (PLC) is a ruggedized computer used for automation of electromechanical processes in industrial systems. A PLC differs from a general purpose computer in that it simultaneously processes many parallel I/Os replacing the mechanical relays, contacts, coils, timers and sequencers in industrial control systems. A PLC's inputs are connected to sensors that monitor temperature, motion, position and pressure and the outputs activate electric motors, actuators, hydraulics and relays. The key advantage that PLCs provide over mechanical systems is that they can be easily reprogrammed. This allows the PLC to be tailored to the particular process by the addition of plug-in modules into the chassis or rack.
The following is a real-world example of how one company is using the Lattice POWR607 to reduce costs and increase design flexibility in a PLC.
Hot swappable PLC modules can be inserted or pulled from a chassis with a powered backplane. PLC design complexity and power requirements are increasing as the industrial processes they monitor become more sophisticated. Although the modules require only a few amps during normal operation, plugging a board into a powered backplane can instantaneously draw over 50 amps while charging the card's input capacitance. The initial flow of current can cause the backplane voltage to drop below nominal, forcing the system into a brown-out condition or hard failure. To protect from excessive current, a hot swap controller consisting of an external power MOSFET and gate drive circuitry is required. The previous design used a Microchip Technology PIC-8 controller and separate hot swap switch to perform power management. The PIC-8 performed voltage monitoring, sequencing and reset functions. The PIC-8’s slow response to fault conditions sometimes resulted in on-board Flash corruption or runt packet transmission. The design was difficult to modify and proliferate to other modules, requiring changes to the processor’s assembly code and extensive regression testing. The cost of implementing changes and poor reliability made this method prohibitive for PLC designs.
The Power Manager II POWR607 device integrates the functionality of both the PIC-8 and hot swap controller with a flexible and easily upgradeable power management algorithm. The POWR607 is programmed to monitor and control all aspects of the hot swap application, sequencing, reset and watchdog. Changes to the algorithm can now be implemented without changing assembly code. It can readily be reprogrammed for different levels of power and in-rush current, depending on the module’s application. It adds the feature of safely controlling the current into the board during a hot swap event by monitoring both the current and the MOSFET voltage. This ensures that the safe operating area (SOA) of the input MOSFET power switch is not exceeded. Once the input power to the module is stable, the POWR607 sequences the power to various functions and enables ICs by releasing resets. A spare digital I/O is programmed as a watchdog timer to monitor system functionality.
PLC Power Management
This customer has found that an added benefit of the POWR607 CPLD architecture is faster system response. Reaction times are reduced to respond to faults in tens of microseconds instead of the milliseconds required for the previous sequentially monitored microprocessor based system. Quick response time is critical in power applications, allowing the system to respond to faults such as short circuits and shutdown before they can cause Flash corruption or catastrophic damage to the board.
The POWR607 is fully programmable with the ability to respond to faults by either shutting down the system or reinitializing the startup sequence. The POWR607, as well as other Power Manager II devices, is easy to program with the free PAC-Designer software. PAC-Designer helps reduce design risk by allowing system power management designs to be fully simulated before being further tested using the POWR607 Evaluation Board. Once a design is committed to hardware, any modifications can easily be updated by reprogramming the POWR607. In this example, the POWR607 simplified the design, reduced the cost and improved the reliability of a PLC module by replacing the hot swap controller, PIC-8, and discrete circuitry with an integrated solution.
Learn more about hot swap with Power Manager II devices and design resources such as development kits and application notes in the links below:
© Lattice Semiconductor Corporation 2012
