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How to control latching relays in smart power meters - an alternative use for brush motor driver ICs
Magnetic latching relays are key components in today’s smart power meters, where they are used to facilitate the remote disconnection and reconnection of power to the consumer premises. A key benefit of such relays is their low power consumption as they maintain last contact position after the control power has been removed. Turning the relay on and off simply requires a momentary application of a trigger pulse current to the relay coil. A pulse in one direction will set the relay to the first position, and a pulse in the other direction to the second position.
In general, drive circuits for latching relays are more complex than those for other relays. Typically, a half-bridge circuit is needed if dual supply voltages are available, and a full-bridge will be required if just one supply is available. Discrete circuits may be developed for the control of latching relays, but it is increasingly common to employ dedicated driver ICs. Whichever approach an engineer chooses, key design criteria are likely to include minimum circuit size, high-efficiency operation and the implementation of appropriate protection functionality.
Because the requirements for controlling latching relays – bridge circuitry, low-power operation, the need for protection, compact design – are similar to the drive requirements of brushed motors, it is also possible to deploy dedicated motor control driver (MCD) ICs in latching relay control circuitry. What’s more, because many smart meters have two latching relays, and because some motor drive ICs combine dual control with high levels of integrated protection functionality, this approach can help to simplify circuit design and reduce component count.
Take, for example, Toshiba’s TC78H610FNG. Supplied in a compact SSOP 16 package measuring just 5.0mm x 4.4mm, this direct driver IC incorporates two MOSFET-based H-bridges. As a result, it is ideally suited to driving the two single-coil latching relays found in modern smart meters. Electric current direction is easily controlled by the application of appropriate input signals to the IC.
Furthermore, because the IC integrates high levels of protection – it has built-in circuitry for over current detection (ISD), thermal shutdown (TSD) and under voltage lockout (UVLO) - the need for additional protection circuitry and components is drastically reduced.
The diagram below illustrates how this MCD can be incorporated into a smart meter’s latching relay circuit, with signals PHA_A and PHA_B being used to control the current to the relays.
To find out more about Toshiba’s comprehensive family of motor control driver ICs, click here.