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AN1686 APPLICATION NOTE

Adaptations and modifications of the original design

With reference to the schematic of figure 1, here follows a step-by-step discussion on the modifications needed

for fulfilling the spec of table 1.

load the input power will be around 90 W. Estimating about 5 W power loss before the transformer, the power

managed by the transformer will be 85 W. With the worst-case spread (-20 %) of C1, that is with C1=80 µF, the

valley voltage across C1 is expected to be around 54 V. Under these conditions the maximum peak primary

current will be around 3.24A, exceeding the saturation current of the transformer (2.84A).

Moreover, the maximum duty cycle (@ Vin = 54V) will be around 59%: since the system is working in CCM,

slope compensation will be needed to avoid unconditional instability of the current loop and the resulting sub-

harmonic oscillations.

To avoid remaking the transformer and adding the slope compensation circuit, an attempt can be done using a

larger C1. Assuming there is no change in power levels, with 150 µF capacitance (120 µF, worst case), the val-

ley voltage across C1 will be 78 V, the maximum peak primary current 2.88 A and the maximum duty cycle 50%.

Although necessary, this step is not enough to guarantee that the transformer will not saturate and then the sys-

tem is still very close to the instability limit. The transformer needs then to be modified anyway (see the following

points).

value should be 0.92 V / 2.88 A = 0.319

Ω. This will be achieved by paralleling a 1Ω resistor (1%, metallic film)

to the existing 0.47

Ω. The worst-case peak current, will be 1.08 V / 0.319Ω = 3.39 A.

3.39A primary current; 2) to reduce the maximum duty cycle below 50%, to avoid the use of a slope compen-

sation circuit. Design constraint is the core size, which must be unchanged. The increase of total losses should

be as low as possible.

Starting from point 2), to maintain the same maximum duty cycle as in the original design, the turn ratio should

be reduced proportionally to the reduction of the valley voltage across C1. Formerly, with 45 W load, the worst-

case valley voltage was 83 V and now is 78 V, then the new turn ratio is (50/12)·(78/83) = 3.916.

Table 2. Modified transformer specification

To make sure that the core will not saturate with 3.39A, keeping the same inductance value, the number of turns

of the primary winding should be raised from 50 to 66 and the air gap from 0.7 to 1.5 mm. To reduce the copper,

it is acceptable to reduce the primary inductance by 10%: in this way the primary winding turn number will be

60 and the air gap 1.4 mm.

The secondary turn number will be 60/3.916=15.321, rounded off to 16. Keeping the same wire diameters, the

primary resistance will be increased by a factor 60/50, that is 20%, and the secondary resistance by 33% and

so will be the respective conduction losses (the RMS currents change very little). However, the flux swing is

Core

Philips EFD30x15x9, 3C85 Material or equivalent

Bobbin

Horizontal mounting, 12 pins

Air gap

≈1.4 mm for an inductance 2-6 of 360 µH

Leakage inductance

< 10 µH

Windings

Spec & Build

Winding

Wire

S-F

Turns

Notes

Pri1

AWG27

2-4

30

Sec (a)

AWG25

11-7

16

Bifilar with Sec (b)

Sec (b)

AWG25

12-8

16

Bifilar with Sec (a)

Pri2

AWG27

4-6

30

Aux

AWG32

3-1

14

Evenly spaced

Note: sec (a) and sec (b) are paralleled on the PCB