AN4414 Datasheet, PDF (1/20 Pages) STMicroelectronics – Anti-tampering ferrite less power supply for 3-phase energy meter

AN4414
Application note
Anti-tampering ferrite less power supply for 3-phase energy meter
Harjeet Singh
Introduction
This power supply design is dedicated to energy meter applications and an other low power
auxiliary supply for systems with galvanic isolation for a single phase or 3-phase input mains
supply. The SMPS can cater the super wide input voltage ranging 90 VAC to 440 VAC single
phase or 3-phase / 4-wire configurations with low component counts. For energy meter
applications, anti-tampering feature is a must nowadays which is governed by electricity
board regulatory agencies.
Normally the SMPS are designed with ferrite cores, either a flyback transformer or a buck
inductor, which saturates in an environment where strong magnetic fields are available. To
avoid the same, some type of magnetic shielding are required, which adds a cost and
complexity in manufacturing process of the magnetics. In case of tampering where SMPS
are available in energy meters, commonly a strong magnetic field of about 0.5 Tesla
strength is applied, which interfaces with the ferrite core and saturates it. Due to saturation,
sometimes the failure of power devices may also take place, or the SMPS with integrated
switches gets into the protection mode and limiting the output voltage, consequently the
meter stops which is a tempered condition.
To overcome this situation, in the present case, an example to design the flyback converter
without using any ferrites or a high permeability core for a required magnetizing inductance
and so called an air core magnetic approach is described in this application note. If no
ferrites are involved, no saturation phenomenon can take place under the influence of an
external strong magnetic field and this will comply the energy meter anti-tampering
standard, which requires 0.5 Tesla magnetic immunity. The only drawback could be the
large number of turns and associated copper losses. Since the required power is very low,
the copper losses are acceptable up to a certain extent. The specification of overall input
power to be within the limits as per energy meter power supply specifications needs to be
complied.
February 2014
DocID025611 Rev 1
1/20
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