Smart meters are next-generation meters that replace existing meters, which still use technology that was developed decades ago. Smart meters use a secure connection network to automatically and wirelessly send the energy usage to the utility companies. This means that customers will no longer receive estimated energy bills or have meter readers come into their homes to read the meter.
Smart meters use more advanced communication interfaces as compared to conventional IR (Infra-Red) and IrDA (Infra-red Data association) interfaces; they also require more memory and a more powerful microcontroller. These features increase the power consumption which necessitates the use of switch-mode power supply(SMPS) rather than a capacitive-drop power supply .The single-phase energy meter has to operate from minimum 100VAC up to 500VAC.The three-phase energy meter has to operate with minimum of single-phase present (100VAC) up to 300VAC in each of the phases . Compliance with efficiency standard - and more importantly compliance with lower power-consumption requirements – poses more challenges for SMPS designers as you cannot bill customers for the energy that the meter use. On the other hand the smart meter energy usage should not place unacceptable power demands on utilities either.
Utility companies have been experiencing revenue loss due to tampering of energy meters worldwide. Since the first electronic energy meter was deployed in the field, unethical people have attempted to alter meters to steal electricity without paying for it. They will devise some ways to fail the power-supply .So this tampering may be the most important problem that a smart meter power-supply designer has to confront. Most power supplies use ferrite core based transformers which are cheap and efficient but susceptible to strong magnetic fields generated by permanent rare-earth magnets placed a few centimeter away. As soon as the magnet comes near the vicinity of the transformer, it saturates the transformer and creates an over-load condition, damaging the switch (MOSFET/Bipolar junction transistor) and hence destroying the power supply. Most of the power supply controllers available today have an integrated function of overcurrent protection built in. If transformer becomes saturated, a fast current comparator turns off the switch thus protecting the power supply. But the drawback is that there will be no power available to the metrology block and thus no metering. However this is what energy meter’s foremost function is. To maintain continuous operation during any attempt at tampering, one option is to shield the transformer with a magnetic shield material. But this is an expensive alternative and adds to the assembly cost –as each ferrite core based transformer would need shielding.
Another option is to use a high reluctance powdered iron core instead of ferrite core in the transformer. A powdered iron core has a much higher flux density of 1.2 – 1.4 Tesla as compared to 0.4 – 0.5 Tesla for ferrite core. It costs less than putting a magnetic shield around the transformer. But suffers from higher core loss which can significantly reduce power-supply efficiency. This drawback may not be particularly significant in smart-meter power supplies, however.
Peak power consumption in smart meters jumps to 1W-10W levels depending upon the wireless communication protocol used.
For sub 1 GHz peak power consumption is around 0.5W; for ZigBee it is around 1W and for Global System for Mobile communication (GSM) it is around 10W.
But the meter typically consumes less than 1W for most of its operating life. The efficiency of the power-supply at low/light loads is critical in determining the overall power consumption of the meter. This imposes high light-load efficiency as a target for the power-supply designer. Several approaches can increase efficiency at lower/ light loads such as reducing the quiescent current of the switching controller, using a primary-side controller with a high voltage start-up integrated circuit, or using a switching controller that contains a mixture of frequency and amplitude modulation. The reduction in switching frequency and thus current with the reduction in load reduces core loss, thus justifying the use of powdered iron core based transformers.
A high operating voltage is essential in smart meters to protect against the accidental connection of a single-phase meter to two phases (500VAC) or the presence of a 300VAC in each of the three phases of a three-phase meter. But this high voltage increases power complexity, component count and cost.
A flyback topology is the simplest and the cheapest topology for a smart meter power-supply design. In a flyback topology, the switching component has to withstand up to 1000V: 730VDC (300VAC *√3 *√2) plus 220VDC clamp voltage plus 50VDC overshoot due to the snubber diode conduction delay. The switching component should have a voltage rating of 1200V assuming 15% derating. The switching converter (with a built-in 700V/800V MOSFET) uses another 500V MOSFET in cascode configuration to meet the 1200V requirement. Another low-cost technology uses a using a single 1200V BJT with switching controller to take care of high-voltage protection. The cost of BJT with the same voltage and current rating price is one-third the price of a MOSFET.
Thus the optimal SMPS design would feature a powdered iron core based transformer, control law in switching converter to take care of light-load efficiency and a cascode configuration to handle high input voltages.
Additional resources:
- Discover the UCC28722 and start designing with the Ultra-wide Input Range, Dual Output, Offline AC/DC Bias Supply with PSR Control and BJT Switch Reference Design and Dual-Output 5V at 300mA and 15V at 100mA Isolated Flyback From 85V to 440V AC Mains Reference Design.
- Read the white paper, “Smart power opens door to more efficient electrical use”