The programmability and data retention of these programming instructions sometimes require a backup power supply to store enough energy to properly shut down the PLC during sudden input voltage drops. This power loss often occurs in PLC cards that are hot-swappable to rack-mount systems.
To provide proper power outages, energy must be stored on the PLC card. The simplest method is to obtain the input voltage and directly drive a capacitor bank. However, this requires a larger capacitor bank, and if the input voltage is boosted by storing energy at a higher voltage through capacitors, then an even larger capacitor bank is needed. The system must then also support operation at this higher stored voltage, complicating the system design and increasing its cost.
The PMP9761 TI Designs reference design offers a "two-in-one" solution to this dilemma. It uses a buck-boost converter to bidirectionally boost the input voltage of the storage capacitor and decrease the system capacitor voltage. The system power supply remains unchanged; only the capacitor bank and buck-boost converter are added to provide backup functionality. This simple and straightforward implementation provides basic functionality for some PLCs.
This reference design describes a backup power supply circuit that addresses transient protection against power outages by using a buck-boost converter and a backup capacitor. The implementation is based on a fully integrated TPS63060 buck-boost converter circuit, resulting in a small overall solution size. This configuration has been tested and is fully documented with test reports and operating instructions.
The wide range of available spare capacitor voltages (2.5-12V) results in smaller spare capacitors and solution size.
• There were no faults during the transition from main power to backup power.
• The type and size of the backup capacitor are available to meet backup power needs.
• Up to 10 watts of output power
• Small and efficient single frequency converter solutions
• Supports pulse loads connected to weak power supplies
The TPS63060 device provides a power solution for products powered by 3 to 6 alkaline, nickel-cadmium (NiCd), or nickel-metal hydride (NiMH) batteries, or single or dual lithium-ion or lithium-polymer batteries. When using a dual lithium-ion or lithium-polymer battery, the output current can rise to 2A and discharge the battery voltage to 5V or lower. This buck-boost converter is based on a fixed-frequency, pulse-width modulation (PWM) controller using synchronous rectification for maximum efficiency. Under low load current conditions, the converter enters a power-saving mode to maintain high efficiency over a wide load current range. The power-saving mode can be disabled, forcing the converter to operate at a fixed switching frequency. The maximum average current within the switch is limited to 2.25A (typical). The output voltage can be programmed using an external resistor divider or internally fixed on-chip. The converter can be disabled to significantly reduce battery consumption. During shutdown, the load is disconnected from the battery. This device is packaged in a 10-pin SON PowerPAD™ 3mm x 3mm package (DSC).
The same architecture is useful for any system that requires proper shutdown, even when its input power supply voltage is removed, and for systems that need undervoltage handling via the input bus. Such mission-critical systems might include medical devices, sensor and monitoring systems in a factory, or fail-safe user safety systems such as conveyor belt disconnect devices. In these systems, reliable operation is required during input power transients, or the system must enter a known safe state when its power is suddenly removed. Storing energy in a backup power system is one way to achieve both of these objectives.
