Motors, Generators, Transformers and other expensive equipment on the grid are just one power fault away from damage or failure. Relays and circuit breakers can make the difference between reliable protection and expensive repairs on equipment sitting further down steam,
Molded case circuit breakers (MCCBs), air circuit breakers (ACBs), and overload relay protection are examples of protection devices that commonly provide the safeguards your designs need. These devices offer rapid detection and safe interruption during faults. To be able to rapidly detect faults, these protection devices themselves need to be powered. Often, that power is derived from the grid itself. Commonly there is an AC to DC conversion followed with the DC voltage being dropped to levels that can power the electronics. The type of power solutions vary from high wattage solutions that use flyback PWM controllers to low wattage solutions that use self-powered solutions. The protection devices could alternatively be powered through an auxiliary DC input.
Dual powered means input power can be derived from either A) rectified current inputs and/or B) from auxiliary DC input voltage. This block diagram illustrates the input power options on the input side (left side).
Design Block Diagram
Texas Instruments (TI) has a reference design to speed up your implementation. A self or dual-powered supply is often used in the design MCCBs or ACBs with an electric trip unit circuit. In this reference design the rectified input charges the capacitor to generate the output voltage. The regulated DC output voltage is set by a Zener Diode and a MOSFET shunt regulator. The output voltage minus the Zener voltage is compared against a set voltage by the comparator to regulate the output DC voltage. A DC-DC converter is used to generate voltage levels required for the protection device. To implement the task, TI leverages its LM293 dual differential comparator IC, the LM5017 constant on-time synchronous buck regulator IC, the TPS7A6533 low-drop-put regulator, and their extensive line of microcontrollers.
To achieve operation over a wide range of current input, a shunt regulator comprised of a MOSFET and comparator are incorporated into the design. The use of an LM5017 in a non-isolated output configuration allows the clamping voltage to increase as the device input is rated up to 100V. The combined circuit of Zener diode, comparator, and MOSFET works as a shunt regulator and regulatesthe output DC voltage. The MOSFET-based shunt regulator is controlled by either the comparator or by pulse width modulator (PWM) output from the microcontroller. The microcontroller senses theoutput voltage. Based on the set regulation voltage, the microcontroller (Tiva TM4C123) regulates the output voltage by controlling the PWM output to the MOSFET.
An auxiliary DC input voltage also can be applied to generate the required power supply along with the
self-powered current inputs. The shunt regulation is bypassed when auxiliary voltage is applied.
MCCB breakers with electronic trip units (ETUs) would be a target use for this type of self/dual power supply. Benefits of self-powered protection relays include insensitivity to voltage drops from faults, and they are not dependent on UPS. The microcontroller-based system also offers high reliability and repetitive accuracy.
TI has provided all of the usual details; schematics, bill of materials, and board design files to implement the design. These schematics show the use of the LM293 (dual differential comparator), LM5017 (constant on-time synchronous buck regulator), and TPS7A6533-QI (low-dropout regulator) ICs.
LM293 and LM5017 ICs Schematic Detail
TPS76533-Q1 Schematic Detail
This reference design provides the foundation for MCCB, ACB, and protection relays builds. Grab the documents and make them fit your application. The added documentation from TI will ensure your product gets to market before everyone else.
Texas Instruments has sponsored promotion of their industrial communications solutions on ENGINEERING.com. They have no editorial input to this post - all opinions are mine. Bruce Schreiner