Publication date: 20 October 2009
The new ‘smart grid’ concept is rapidly gaining favor with electricity suppliers worldwide. Faced with a growing need to utilize infrastructure more efficiently while reducing greenhouse gas emissions, utilities are using the smart grid to intelligently control and monitor the distribution and consumption of electricity. Suppliers expect this implementation to result in a dramatic savings in energy consumption and more efficient use of generating capacity.
The aim of the smart grid is to even out electricity consumption throughout the day and reduce peak loading by changing user behavior through variable charging schemes or by directly controlling the consumer’s equipment. By leveling out the network load, utilities can more efficiently manage their generating and transmission capacity, reducing emissions and avoiding power outages.A core element of the smart grid is the smart power meter. Much more than a standard (dumb) power meter, the smart power meter communicates with both the energy supplier and the consumer to provide real-time information on energy consumption and cost. Smart meters allow the utility to implement a variety of control and incentive programs (e.g., real-time variable pricing based on peak network loading). With features such as in-home energy displays or “eco-panels,” the consumer can be warned when a peak loading rate is being charged. The consumer can then make an informed choice to postpone the use of heavy consumption appliances such as washers or dryers until a lower rate for electricity is available.
The smart meter is envisioned to be at the center of a home information network, gathering data from smart appliances and communicating with the energy supplier (see Figure 1). The communication handling and security features in smart meters must be both flexible and upgradeable. Power management within the meter and achieving maximum power efficiency are of prime importance. The power supply must work over a wide dynamic range with fast response to changing demands when transmitting data or fulfilling other functions. This requires careful power circuit design.
The technical requirements for a smart meter power supply can vary, but there are some requirements that are highly desirable for all applications. These include current limiting, thermal shutdown, automatic fault recovery, reduced electromagnetic interference (EMI), high efficiency, and very low standby power consumption. These requirements predicate the use of a switch-mode power supply, even for very low consumption applications. Power Integrations (PI) is one manufacturer offering a range of SMPS controllers suitable for smart metering. The controllers all include a 700-volt power MOSFET combined with comprehensive control and protection circuitry on a single chip.
For a given application, several different products could be used, but one would generally lend itself better due to its specific characteristics. PI has developed several utility meter applications using both its LinkSwitchâ and TinySwitchâ product families. For the designer, there are several choices to consider. The majority of applications will call for an isolated supply, in which case a flyback converter would be employed, but for a lower cost, non-isolated design, a buck converter using LinkSwitch-TN could be implemented. For tight voltage regulation (±2%) an opto-isolator would be required in the flyback control loop. A clampless design could be implemented with the LinkSwitch-XT which provides both a ±2% CV and ±5% CC characteristic. If ±5% voltage regulation is acceptable, then a substantial saving in component count can be achieved in a design employing primary-side regulation with no optocoupler required. For this, the LinkSwitch-CV would be suitable for power outputs from below 1 W up to 8.5 W. For higher power levels up to 28 W, the TinySwitch family could be employed.
A typical requirement for a smart meter power supply would be to deliver 5 V and 12 V multiple outputs from a universal 85 to 265 VAC with efficiency meeting California Energy Commission (CEC) / ENERGY STARâ EPS v2 and conducted EMI meeting CISPR22B / EN55022B. Figure 2 shows an example of a power supply meeting these requirements1.
This circuit employs a LinkSwitch-CV switch-mode controller (U1) in a primary side control flyback circuit 2. Protection features provided by this circuit include auto restart for control loop open/short faults and output open circuit conditions, and hysteretic thermal shutdown. An extended creepage distance between high- and low-voltage pins on the package prevents arcing in humid environments. This is important for metering applications where the equipment is often situated in uncontrolled conditions.
In the primary side of the flyback circuit, the 700 V MOSFET in U1 switches the primary current through winding 1 3 of T1. An RCD-R clamp (D5, R3, R4, and C3) limits the leakage inductance drain voltage spike. U1 employs a unique ON/OFF control scheme to provide constant voltage (CV) regulation of the output characteristic. Primary-side control is achieved with the output being sensed by a feedback winding on the transformer and applied to the FB pin of U1. The switching of U1 is controlled in response to the current into the FB pin. In this way, an effective closed-loop control of output voltage is achieved without the need for any secondary-side control or optocoupler. The pi (π) filter (L1, L2, C1 and C2) minimizes conducted differential mode EMI; the circuit is rugged to surges and designed to meet IEC950 and UL1950 Class II safety requirements.
Despite the very low component count, a regulation of ±5% on the 5 V output and ±10% on the 12 V output is achieved across the full input voltage and load range.
The installation of smart meters and their employment in smart grids is accelerating. Their use can make a significant contribution to the more efficient distribution and utilization of energy in the future. With the large number of smart meters that are anticipated to be installed worldwide over the coming decades, every economy in materials use and power consumption will help to maximize their effectiveness and accelerate their adoption. The utility meter power supply designs developed by PI provide an efficient foundation for all metering applications.
For more information on smart grid metering, please visit the Power Integrations microsite www.powerint.com/smart-grid-metering.
1. DER-213 Design Example Report: 3.8 W 2-Output Supply Using LNK623PG, Power Integrations Inc.2. LNK623-626 LinkSwitch-CV Family: Energy-Efficient, Offline Switcher with Accurate Primary-side Constant-Voltage (CV) Control, Power Integrations Inc.
Silvestro Fimiani is product marketing manager of appliance and industrial applications at Power Integrations (PI). Prior to joining PI in 2005, Silvestro served as director of engineering of high-power products at International Rectifier. He holds a Bachelor of Arts in Physics from the University of Naples, Italy.
