Smart electricity metering system design is looking to use completely novel power supply topologies,writes Tolga Aydemir.
Smart electricity meters are big news a with cost savings promised for suppliers and consumers a but they also demand increasingly sophisticated power supplies(PSUs)compared with their digital counterparts.
Energy suppliers expect more advanced communication interfaces,larger memories,and more powerful processing but also lower system costs.Added to this are wider/higher operation voltages(up to 500Vac);compliance with efficiency standards;and more importantly lower power consumptions as almost every installed meter is utilising energy that cannot be billed to the householder.
Several groups are considering completely novel single-mode power supply(SMPS)topologies.For example:metering company Itron has patented a low cost,single switch approach that uses a series of capacitors and diodes configured for series charge and parallel discharge to deliver a high efficiency PSU(see US 2009/0296431).
However,most other companies are looking to integrate a range of more established,widely available techniques to secure the required performance.
Iron cores versus ferrite
Most of todaya's SMPS designs use ferrite core transformers but they can also be easily saturated with strong magnets by consumers trying to stop their meter from registering consumption.Ferrites are low cost and efficient but also highly susceptible to strong magnetic fields.
Saturating the transformer core usually overloads and damages the switching component which destroys the power supply,unless it has an integrated protection function.
To overcome this,many smart meter PSU designers are now specifying iron-cores on account of their much better saturation characteristics.
For example:the saturation level of an E25 iron-powder core positioned 1cm away from a 2T magnet is 30%;whereas that figure jumps to over 90%for the same size ferrite core.
Iron offers a higher degree of magnetic immunity and lower costs compared with magnetic shielding around a ferrite core but the downside is that their greater core losses can significantly reduce the supplya??s overall efficiency.
Ultra capacitors deliver peak power
Smart meter PSUs are inevitably more powerful than their conventional digital meter counterparts because of the metera??s increased range of power-hungry features such as Zigbee,GSM or advanced processing capabilities.
The peak power consumption could jump to over 10W depending on the specific features,but the meter itself will typically consume less than 1W per hour(24Wh per day)for most of its operating life.
As a result,the efficiency of the PSU at low load can be critical in determining the metera??s overall power consumption,with regulators and energy suppliers focused on keeping this to an absolute minimum.
Energy suppliers want more features,which is driving more innovative ways to significantly improve light-load efficiency.For example:by reducing the quiescent current of the switching controller or techniques such as cycle-by-cycle current limiting,adaptive switching frequency control and frequency hopping.
One novel approach,which avoids the need for a higher-rated PSU that spends most time operating at low load,is to use a smaller,lower cost and more energy-efficient PSU,rated at 1 or 2W combined with ultra/super capacitors.
These components can deliver the required short bursts of peak power and their price is now low enough to enable them to be exploited in more cost-sensitive applications.
Helper MOSFETS for HV protection
High withstand voltage is virtually essential in meters:to protect against a single phase meter being accidentally connected to two phases,rather than phase to neutral,or for use in some dual-phase metering configurations.But the feature also adds design complexity,components and cost.
Rectifying 500Vac generates 720V high voltage DC and in a flyback topology that means the switching component has to withstand up to 1kV;namely 720Vdc plus the reverse voltage of the flyback transformer,which is typically a few hundred volts.
To address this,some designers are incorporating novel helper mosfet circuitry as a simple,low cost way to protect the power switch.
The schematic shows the basic topology:where Q1 works as a helper mosfet and when IC1 conducts,Q1 also conducts.When IC1 is open,the voltage on the source pin of Q1 does not exceed the zener voltage and in this case the high voltage on the drain pin of Q1 is shared between Q1 and IC1.
Using helper mosfets appear to offer the perfect protection but the downside is that it also reduces the total operating efficiency of the power supply.
However the same PCB layout and fundamental transformer design can be used for PSUs with or without the feature.If high withstand protection is not required,then the helper mosfet circuit components are not added to the board.
Piezoelectric transformers replace magnetics
Some design teams are actively investigating piezoelectric transformers(PT)as a very promising alternative to electromagnetic transformers for smart meter PSUs.PTs are already being used in some specialist AC/DC and DC/DC converters and offer major advantages including:very low profiles,higher power density,no need for windings(ideal for automated manufacture),high insulation and potentially low cost.
They also generate very little EMI enabling simpler designs and easier regulatory compliance but one of their biggest benefits in the battle against electricity theft is magnetic immunity.
Is there an optimal solution?In an ideal world power supply manufacturers would design one low cost,performance-optimised solution to fit any smart electricity meter application but the complete opposite is happening.
Manufacturers have to develop multiple designs in 120 and 230V versions to comply with a wide range of country-level requirements on areas such as energy-efficiency,safety and security,as well as different metering features and configurations.
The optimal PSU design would probably feature an iron-cored transformer for improved magnetic immunity;ultra capacitors to deliver peak power;and helper mosfets to provide high voltage withstand protection.
However,many manufacturers see PT-based approaches as offering the ultimate in lower cost,smaller footprint designs,although more reliable fabrication technologies and sophisticated drive electronics are still needed.And with complete magnetic immunity,one benefit is certainly guaranteed:consumers who are expert in using magnets in attempts to defraud their utility company will need to develop alternative strategies.
Tolga Aydemir is an electronics&telecommunication engineering consultant at 42 Technology,a product design and development consultancy,based near Cambridge.
