DocID025611 Rev 1

7/20

AN4414

Description of SMPS

20

1.2

Anti-tampering requirement

The E-meter SMPS requires a 0.5 T external magnetic immunity test and generally ferrites

saturated with a 0.5 T external magnetic field. In order to comply this norm, a lot of attention

is required in making proper shield transformers, which is very expensive and sometimes

still difficult to comply the norms easily. The alternative to this is to adapt a ferrite less or air

core transformer approach for such low power output requirement.

Considering the classical equations for a DCM (discontinuous conduction mode) flyback

converter as described in Section 1.1: Basic flyback equations will result in a higher

magnetizing inductance of primary of transformer. So using no ferrite material (we call this

as “Air core”) to build the high frequency transformer would result in higher number of turns

to achieve the required magnetizing inductance and with air core, the primary inductance in

the range of 1 mH is itself difficult to achieve with reasonable number of primary turns.

So the first most important criterion to this approach is to select a low primary inductance

value, Lp, but the value must be greater than a threshold in order not to cause an excess

peak drain current. The high voltage flyback switcher is specified with a finite peak current of

devices, we have this current as: Idlim = 0.4 A.

So during the wide mains operation, the peak primary current should be well below this

value to avoid any protection to activate in a nominal operation. The VIPER17 has a hiccup

mode activation above this current level.

As we know, the important parameters for a flyback design are:

Dmax = max duty cycle,

VR = reflected voltage,

Lp = primary inductance,

n = turn ratio, np/ns

Ipkp = peak primary current

fsw = switching frequency

Equation 15

Primary turns

where:

Ae = cross-sectional area of core chosen

As we know, selecting higher duty cycle will have below impact:

Equation 16

Np

Lp Ipkp



Bmax Ae



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