L6599

Application information

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7.3

Soft-start

Generally speaking, purpose of soft-start is to progressively increase converter's power

capability when it is started up, so as to avoid excessive inrush current. In resonant

converters the deliverable power depends inversely on frequency, then soft- start is done by

sweeping the operating frequency from an initial high value until the control loop takes over.

With the L6599 converter's soft start-up is simply realized with the addition of an R-C series

since the optocoupler's phototransistor is cut off (as long as the output voltage is not too far

away from the regulated value):

regulated value and the feedback loop taken over, so that it will be the optocoupler's

phototransistor to determine the operating frequency from that moment onwards.

During this frequency sweep phase the operating frequency will decay following the

change will get slower and slower. This counteracts the non-linear frequency dependence

of the tank circuit that makes converter's power capability change little as frequency is away

from resonance and change very quickly as frequency approaches resonance frequency

(see Figure 28).

Figure 27. Soft-start circuit

f

start

1

3CF

R F

min

R

SS

||

()

()

⋅⋅

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

=

L6599

Css

4

1

RFmin

| Z ( f ) |