AN2450 datasheet(27/35 Pages) STMICROELECTRONICS

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DocID12784 Rev 6

27/35

AN2450

Design example

The chosen standard value of the resonant capacitor is 47 nF. The transformer has been

designed using a two slot coil former and integrating both the series inductance Lr and the

shunt inductance Lm, in order to obtain a magnetic component with the following

parameters:



primary inductance (with secondary windings open)



primary inductance with secondary windings shorted



transformer turn ratio

The number of primary turns has been found experimentally, by measuring the “specific

leakage inductance” (i.e. the leakage inductance per square turns) of a few suitable ferrite

cores, using a two slot winding configuration. The procedure consists of winding a few

layers of turns on both slots of the coil former (same copper area for primary and secondary)

and then measuring the inductance of one winding with the other one short-circuited.

Dividing this measured value by the squared number of turns gives the specific leakage

inductance of the core - coil former construction. The chosen ferrite core is a ER-49-27-17

type, material grade PC44, and the necessary number of primary turns to obtain the

required leakage inductance is 19. Therefore, the total number of secondary turns for 200 V

output is 18 (from the required turn ratio nt).

The secondary side of the transformer consists of two center tap windings, one for each

output, and the two output voltages (+75 V and +200 V) are obtained by series connecting

the two secondary windings on the DC side (refer to the electrical schematic in Figure 12 for

better understanding of circuit configuration). The bottom winding (for +75 V output) has

7 turns, while the top winding consists of 11 (18 - 7) turns.

LpSO



Lr Lm 240 H

LpSS



Lr 40 H





1.08

AN2450 Datasheet(PDF) 27 Page - STMicroelectronics