Achieving a Smarter, Safer Grid With Bidirectional Switching Circuits
This article describes how latching relays, solenoids and brushed DC motors can be used in smart metering and EV charging systems to improve the safety and efficiency of the power grid.
Many governments are investing in updating their grid infrastructure to keep up with increasing demand and technological advancements, including improvements for protection, convenience, and efficiency with systems such as smart metering or hybrid electric vehicle (HEV)/electric vehicle (EV) charging.
Three solutions help service this expanding market: latching relays, solenoids, and brushed-DC motors. Integrated H-bridges drive all three solutions, with bidirectional load control for the implementation of shutoff functions, relay/valve control, and residual current detection. These integrated drivers reduce board space, bill-of-materials costs, and power consumption compared to traditional discrete H-bridge implementations.
Latching Relays in Meters and HEV/EV Charging Systems
Latching relays, also called bistable relays, safely turn the flow of electricity on and off in systems that connect to the power grid. These relays have two stable positions – on and off or open and closed – and will hold those positions without needing continuous power. A monostable relay, in contrast, would return to its home position when the power is off. A latching relay can remain stable while unpowered because it does not need to be energized during a holding state.
There are several ways to drive a latching relay. One solution is to use an intergraded H-bridge circuit to put the relay into an on or off state, as shown in Figure 1.
Figure 1. Dual-coil latching relay (a) and a single-coil latching relay drive circuit (b) driven by an integrated H-bridge driver. Image used courtesy of Texas Instruments
See parts 1 and 2 of the Texas Instruments (TI) technical article series, “Using Smart Relay Drivers for Smart Meters,” for more information.
TI’s DRV8220 integrates the H-bridge; logic control; an automatic low-power sleep mode for power savings; and protection features such as overcurrent, undervoltage, and thermal shutdown into a single integrated circuit. Compared to a discrete implementation, the DRV8220DRL can reduce board space by as much as 93%, as shown in Figure 2.
Figure 2. Printed circuit board layout example of a discrete H-bridge implementation (a) vs. the DRV8220DRL (b). Image used courtesy of Texas Instruments
Solenoids and Brushed-DC Motors in Circuit Breakers and HEV/EV Charging Systems
Brushed-DC motors and solenoids are two other types of drivers common in grid infrastructure systems. Similar to smart locks in building automation, EV charging cable locks use brushed-DC motors to lock charging cables into place, providing a solid charging connection and protecting the cable from theft when unattended. Because the charging cable lock mechanism moves between locked and unlocked states, the motor must have bidirectional control, similar to the bistable relays discussed in the previous section of this article.
Another application of brushed-DC motor drivers is disconnecting the power supply when faults, such as short circuits or overloads, occur in systems like water and gas meters. Many circuit breakers have an automated function when these faults occur to activate a solenoid or brushed-DC motor to automatically disconnect the contacts, effectively severing the flow of current.
Circuit breakers in water and gas meters have two common requirements that H-bridge motor drivers can help address:
- Moving the solenoid or brushed-DC motor in two directions.
- Knowing when the locking mechanism and breaker contacts are fully open and closed.
TI’s integrated H-bridge motor drivers, such as the DRV8231A, also increase power efficiency through low-power-consumption sleep modes and reduce board space by integrating load protection and diagnostics features. One feature is integrated current sensing through TI’s current proportional to current (IPROPI) pin. The advantage of the IPROPI pin, in addition to reducing board space, is that it outperforms traditional external shunt resistor sensing by providing current information even during the off-time slow decay recirculating period, enabling other advanced driving techniques such as sensorless stall detection. The IPROPI pin works by sourcing current in the windings and sends a proportional analog current signal back to a controller to detect stall conditions or the end of travel, as shown in Figure 3.
Figure 3. Example of a current waveform and load state using IPROPI current-sense feedback for stall detection. Image used courtesy of Texas Instruments
Brushed-DC Motors in Residual Current Detection
HEV/EV charging, solar, and wind power systems are among the emerging grid infrastructure applications connecting power-generating circuits to the grid. These applications require leakage detection to ensure DC energy does not go back into the AC grid and to protect people from ground faults that could cause injury.
One method of detecting this residual current is to implement magnetic-based sensing, using square-wave excitation to drive a fluxgate in and out of saturation symmetrically. When leakage current faults occur, the symmetry in this waveform is unbalanced and shifted, as shown in Figure 4. When the sum of the current is approximately 0 A, there is no leakage current detected. When the sum of the current is above or below 0 A, leakage current is present, and the pulse-width modulation waveform shifts.
Figure 4. Magnetic-based residual current detection using H-bridge excitation, displaying no leakage current nor leakage current events. Image used courtesy of Texas Instruments
In a standard industrial 12-V system, using an integrated H-bridge motor driver such as the DRV8220 to drive the fluxgate sensor minimizes board space, integrates protection, and allows for coil excitation and the implementation of a residual current detection circuit.
Key Takeaways of Integrated H-bridge Motor Drivers
Integrated H-bridge motor drivers enable safety and diagnostic features in many grid infrastructure applications while optimizing board space, increasing power efficiency, and enabling design flexibility. TI motor drivers can drive brushed-DC motors, solenoids, and relays while providing more than an H-bridge solution. TI motor drivers benefit any grid infrastructure design with the added benefits of integrated current sensing, current limiting, stall detection, under-voltage lockout, and over-temperature protection.
For more information about H-bridge solutions in grid infrastructure systems, check out TI’s H-bridge motor drivers.
- Read these application briefs:
- See the TI E2E design support forums FAQ, “[FAQ] How to Detect Motor Stall Using Current Sensing.”
- Download the white paper, “Remote Shut Off in Flow Meter is Instrumental for Smart Utility Deployment.”