Application information

L6599AF

18/32

DocID028175 Rev 3

6.3

Soft-start

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

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

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

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

With the L6599AF converter, soft-startup is simply realized with the addition of an R-C

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

away from the regulated value):

Equation 4

voltage should be already close to the regulated value and the feedback loop already taken

over, so that it is the optocoupler phototransistor to determine the operating frequency from

that moment onwards.

During this frequency sweep phase the operating frequency decays following the

gets slower and slower. This counteracts the non-linear frequency dependence of the tank

circuit that makes the converter power capability change little as frequency is away from

resonance and change very quickly as frequency approaches resonance frequency (see

Figure 10, right).

Figure 10. Soft-start circuit (left) and power vs. frequency curve in a resonant half bridge (right)

As a result, the average input current smoothly increases, without the peaking that occurs

with linear frequency sweep, and the output voltage reaches the regulated value with almost

no overshoot.



SS

min

start

R

//

RF

CF

3

1

f

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