Introduction

The advent of smart electricity meters brought standardized advanced metering infrastructure (AMI) to facilitate basic requirements for installation, metering, maintenance, privacy, and security. As shown in Figure 1-1, the AMI has five communication ports P₀-P₄, of which ports P₁ and P₃ are critical.

Port P1 (also called user port) provides interface with the end consumer and is designed for in-house communication to provide real-time data for further analysis via Other Services Module (OSM) such as in-home displays. P1 is a read-only interface. Port P3 supports two-way communication and acts as interface to an external communications module.

The ports P₁ and P₃ includes both power and data lines, and should be designed in such a way that they do not interfere with the metrology of the electricity meter under any external faults. These two ports should comply to safety protection requirements as per IEC 60747-5-5, and Dutch P1 Companion standards. eFuses in Smart Electricity Meters Application Brief highlights how the protection functions of TI eFuse and power MUX devices safeguard Port P₃ of the electricity meter from an external communication module malfunction or faults. This article addresses the protection solution for the user port P₁.

Port P1 Protection Requirements

The port P₁ provides galvanically isolated power to OSM on +5 V rail with maximum continuous current I_{L_CONT} of 250 mA. The protection requirements on +5 V power supply line at port P₁ side are

• Overload protection should trigger for currents above I_{L_CONT} + 10 mA (for example, 260 mA) but within I_{L_MAX} <=30 0mA
• Short-circuit protection
• Overvoltage protection from external overvoltage > 5.9 V, caused by or a failure at OSM side or by an incorrect connection. Similarly, OSM device should be protected from an overvoltage caused by a failure at metering system side, and should limit the voltages to less than 15 V.
• For sustained overload or short-circuit faults, current should fold back to maximum of 50 mA to reduce system operating temperatures and to avoid any harmful effects. The fold back current profile is illustrated in Figure 1-2

Implementing all these protection features could result in a large discrete circuit. In addition, realizing the foldback current limiting can be tricky to implement as it needs a means to detect overload and then adjust the current limit reference. All these discretes occupies significant board space and could not be possible to accommodate in a space constraint smart meter.

Solution with TPS2662 eFuse

To meet all of the protection requirements for port P1, an eFuse such as the TPS2662, can be used. It offers a current limit which can be adjustable from 25 mA to 880 mA, reverse current blocking and programmable overvoltage protection. Figure 1-3 shows the application schematic circuit of TPS2662 eFuse positioned on the +5 V power supply rail path for port P₁ protection.

• The overvoltage protection (set at OVP pin of TPS2662) helps to protect OSM against overvoltages from the metering system side
• The integrated reverse current blocking feature of TPS2662 helps to block the external overvoltages from port P₁ (for example, OSM side) reaching +5 V power supply
• The overload current limit I_OL can be set by the current limit resistor R_ILIM at the ILIM pin using the expression I_OL = 6.636 / R_ILIM. This also sets short-circuit protection threshold at 1.6x I_OL

To implement the fold back current limit function, different ILIM resistors can be used to fold back the current to a lower level. External mosfets can be used to switch these different resistors as shown in Figure 1-3 and described in the following:

• In steady-state operation, the FLT output of TPS2662 stays HIGH. This sets current limit value corresponding to the current limit resistance of R₅ || R₆ for example, 260 mA.
• During fault, the FLT asserts LOW and the current limit gets reduced to 25 mA corresponding to current limit resistor R₅.
• Current limit resistor R₇ helps to set higher current during startup to support load startup load.

Test Results

Figure 1-4 and Figure 1-5 shows the fold back current limiting behavior of TPS2662 for overload and short-circuit events. As shown, the current is limited to 25 mA for sustained faults at the port P₁.

At the startup, FLT stays LOW till the internal FET of TPS2662 is fully enhanced. This keeps the current limit at fold back value of 25 mA (set by R₅). So, any startup load current >25 mA makes the system fail to startup as shown in Figure 1-6. To overcome the startup concern, the current limit during startup is enhanced momentarily for a brief period of around 50 ms using a control signal from the system microcontroller (MCU). As illustrated in Figure 1-3, the switch Q₂ is controlled to increase the current limit during startup to a value corresponding to the resistance R₅ || R₇. The corresponding startup waveform with typical startup load (100 μF || 50 Ω) of other services module (OSM) is shown in Figure 1-7.

Conclusion

The port P₁ in smart meter acts as interface with the end consumer and hence demands stringent protection to avoid influencing the metering system as well as protecting the OSM (Other Service Module) devices such as in-house displays. The full suite of protection features from TPS2662 eFuse along with the fold back current limiting approach presented in this article makes it a suitable solution for the protection of P₁ ports in smart meters.

References

1. Texas Instruments, eFuses in Smart Electricity Meters, application brief.
2. Texas Instruments, TPS2662x 60-V, 800-mA Industrial eFuse with Integrated Input and Output Reverse Polarity Protection, data sheet.
3. Dutch Smart Meter Requirements, P1 Companion Standard, 2016.