February 2014

DocID025611 Rev 1

1/20

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.

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