User`s guide

SMPS AC/DC Reference Design User’s Guide

From Equation 2-36, the on-time for the switch at 400V is 1.35 μs and the duty cycle is

0.19. Therefore, the rms current in the primary is 0.11 Arms. A suitable winding wire

diameter is 0.16 mm with 5.5 Amm

-2

. The 100ºC resistance of this wire is 1.1 Ωm

-1

, and

from the mean turn length of the bobbin (41.2 mm), the predicted primary resistance is

5Ω. Therefore, the primary copper loss is 60 mW.

The secondary turns must be selected to ensure that the referred voltage across the

TNY277 does not exceed its maximum blocking voltage rating and for discontinuous

current operation given the operating duty cycle. The peak voltage across the TNY277

when energy is transferred to the secondary is given by Equation 2-41.

EQUATION 2-41:

A sensible limit on the maximum switch voltage is 540V, so for the main 17V secondary

output, including a 0.8V diode forward voltage drop, the required turns ratio is 7.86.

Therefore, the secondary turns on the 17V winding is 14. A separate tapping at six

turns on this winding can be used for the 7V secondary. The secondary winding is

constructed using TEX-E wire to give the required 2500 Vrms galvanic voltage

isolation.

The time taken for the flyback transformer to be de-fluxed is given by Equation 2-42.

EQUATION 2-42:

Therefore, the off period is 3.9 μs, which is lower than the maximum available period

to ensure discontinuous operation under normal operating conditions.

The secondary peak and rms currents in the winding are given by the following formula

in Equation 2-43 and Equation 2-44.

EQUATION 2-43:

EQUATION 2-44:

Therefore, the secondary current in the main 17V output is 0.7 Arms. The secondary

winding resistance is 0.1Ω and the total copper loss in the secondary winding is

estimated to be 50 mW. Keeping a similar winding current density as the primary

means that a 0.4 mm wire gauge can be chosen for the secondary windings. The two

auxiliary supply windings on the primary side follow directly from the chosen

transformer turns ratio. The total transformer power dissipation is dominated by the iron

loss and is roughly 0.8W. This will lead to a temperature rise of 40ºC, based on the

published thermal characteristics of an EF20 transformer. Figure 2-15 illustrates the

flyback transformer construction.

DC o

VV V

off