Design scheme of voltage monitor in electric meter application

As power grids around the world become increasingly smarter over time, sophisticated electronic design is necessary. At the same time, long-term system reliability cannot be compromised. Smart meters monitor various system voltages to prevent unexpected conditions that lead to inconsistent measurements or system failure.

The generic spreadsheet application shown in Figure 1 consists of several modules that work together to measure the power consumed.

Figure 1: Example of electricity meter system block diagram

This blog post explains the purpose of the DC/DC power, metering, and application modules (outlined in red) and delves into the importance of the supply voltage monitor/watchdog timer subsystem. From a general design perspective, systems that monitor power consumption need to be accurate, compact, and reliable.

DC/DC power subsystem requirements: wide V  IN and low I  Q

The meter typically receives 12 V to 24 V from the main AC power source and converts it to 3.3 V or 5 V DC to power the rest of the system. The voltage monitor ensures that the main supply and all power rails are within system requirements for correct system operation. If the voltage in that module fails or loses power, the voltage monitoring solution needs to flag the fault condition so that the rest of the system can shut down gracefully.

If the main power supply fails to provide sufficient power, the switch will connect backup batteries or supercapacitors to the system to temporarily provide the required power. Generally speaking, systems that rely on backup power reserves can benefit from any power savings during such critical operating conditions because lower power systems will last longer on power reserves. DC/DC power modules require a voltage monitor with a wide V IN to monitor voltages up to 24 V and achieve low power consumption when using power reserves.

Metrology and application subsystem requirements: high accuracy and programmable delays

The metering module is connected to the outside world via sensors and is responsible for taking the actual power measurements before sending the information to the microcontroller (MCU) for processing. The application block has another MCU that, in addition to displaying information, can also format and store data. This block is responsible for monitoring the entire system and serves as the main interface between the system and data output.

In these modules, accurate monitoring of the voltage rails powering the MCU and MCU activity is critical to ensuring reliable and consistent measurements. If the MCU is not in the correct operating state, a voltage monitor or watchdog timer can flag the fault before any other problems arise. When monitoring voltages, especially in systems that require precise, robust, and reliable measurements, voltage monitoring accuracy is important to quickly and accurately determine when a system is not operating at optimal performance.

Additionally, programmable startup delays may be required for sequencing whenever an application uses an MCU or multiple different peripherals. Programmable delays play an important role when an MCU or other peripheral requires a specific amount of time to start up or complete a task before the system begins functioning normally. Additionally, when a fault condition occurs, a specific reset delay may be required to allow the MCU and/or peripherals to complete their tasks before releasing reset. In this case, the programmable delay feature provides a programmable and simple solution for added flexibility.

Electric meter design requirements review

All three modules listed in Figure 1 require voltage supervisors with wide V IN, low power consumption, high accuracy, and programmable delays. A good option to consider is the TPS3840 as it offers a balance of these requirements in one device.

The TPS3840 features a wide input voltage range up to 24V or higher, with external resistors for monitoring high and low voltage rails, 1% typical voltage monitoring accuracy, and consumes only 350nA of power while offering programmable delay. In addition, the TPS3840 provides significant accuracy and flexibility that cannot be obtained by simply monitoring the internal analog-to-digital converter in the MCU. Compared to the internal ADC monitoring in the MCU, the TPS3840 offers greater flexibility in voltage monitoring threshold options, lower power-on reset voltage, and faster startup delays.

Power-on reset is defined as the minimum input voltage before the output is defined, which is critical to prevent glitches that can produce false faults or premature system start-up. The TPS3840's startup delay is only 220 µs, which means the TPS3840 can start monitoring the voltage before the rest of the system is even powered up. Overall, a voltage monitor ensures proper system functionality by continuously monitoring the voltage rails that power the meter internally.

An external watchdog timer in the metering and application module will ensure that the MCU does not latch or glitch periodically by detecting pulses sent from the MCU's general-purpose input/output pins. If the software fails and a pulse is missed, the external watchdog timer will reset the MCU.

The TPS3430 Programmable Watchdog Timer is a good choice because it provides programmable watchdog timeout and watchdog reset delays to meet the timing requirements of any MCU.