AN1673 APPLICATION NOTE

20/24

# Because the Lm/Ls ratio, a not negligible portion of current managed in the transformer's primary should

not be transferred to the load. For this reason, using the formulas for calculate the primary side current

(21& 22), the results will be significantly lower than the tested one see fig 22.

Bulk capacitor C-Bulk.

The relations for design:

[23]

Where

[24]

The key points for selecting the bulk capacitor are in sequence: the rated voltage, the capacitance related

to the 100Hz ripple and the Holdup time (on request) see also PFC documentation.

Choose C-bulk (6) = 33

µF-450V that involve ±9V voltage ripple without fixing any hold-up time.

Control circuits.

The control of the output voltage and current has been realized with twice loop using the TSM103 (Dual

amplifier and voltage reference). This component, in addition to an Optic Coupler, allows profiling a full

control of the output both of the voltage and the current profile (see fig.23). The control's parameter for the

Since the control of the output could not be longer ensured in case of a heavy load (because of very low

output voltage e.g.< 5V), at the primary side the current is sensed (by the C17, R26, D12+D13) and as the

signal, filtered and feed to the EN2 of the L6598, exceeds the threshold limit the soft start is reactivated.

-1.5

t=5

µs/div

-1.0

-0.5

0

0.5

1.0

(A)

I(QI)

D03IN1457

I(Cres)

CBulk

P

o

2

π 2ƒ

V

o

∆

V

o

⋅⋅

⋅

⋅

-------------------------------------------------

=

CBulk

2P

o

t_Hold

⋅⋅

V

o

2

V

omin

()

2

–

--------------------------------------

≥