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# Address last gasp concerns in smart meter design

Posted: 24 Jan 2012     Print Version

Keywords:Smart meter  switch-mode power supply  flyback  capacitor

Smart meter designers have an unusual predicament: the meter is powered from the same bus that the meter is monitoring. When power is lost, the meter must record state information to flash memory or send out a wireless signal—the meter's last gasp. Some utilities also desire to disconnect subscribers from the grid during power outages so that the inrush demands are minimised when power is ultimately restored. Disconnecting the subscriber after power is lost also requires stored energy in either electrical or mechanical form.

The problem of efficiently and cost-effectively providing hold-up energy typically falls to power supply designers. Here we will review three options a power supply designer has to solve this problem and evaluate their benefits and costs in a flyback switch-mode power supply.

The circuits
Figure 1 shows a basic off-line flyback circuit. The supply accepts 85VAC-265VAC and generates a single 3.3VDC, 5W output.

 Figure 1: Basic flyback schematic with highlighted locations for energy storage.

Let us set the holdup requirement of the load to be 50% power (2.5W) for 0.5s, or 1.25J.

Three sections where energy can be stored (A, B, and C) are highlighted in the schematic. Option A stores the hold-up energy in the high voltage capacitor, Cbus. Option B stores the energy in a 20V intermediate voltage capacitor with a down-stream DC/DC buck regulator that steps down the voltage to the load working voltage at 3.3V. Option C is simpler and stores energy in a large capacitor at the output.

Electric potential energy
Since all the options involve storing energy as electric potential in a capacitor, we should review the relationship of voltage, capacitance, and potential energy shown in the following equation:

To calculate a change in the potential energy for a given change in the voltage across the capacitor, we have:

where V f is the final voltage, and V0 is the initial voltage. The change in voltage for a given change in potential energy is:

where U f and U0 are the final and initial potential energy, respectively.

Option A: Primary-side capacitance
The first option is to increase the capacitance of the high voltage bulk electrolytic capacitor on the primary side, Cbus.

This capacitor is typically sized to store just enough energy to continue power conversion during the AC cycle valleys—for a full-wave rectified input this is 1/120 s, or 8.3ms.

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