L6599
Application information
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
circuit from pin 4 (RF ) to ground (see Figure 27).
min
Initially, the capacitor C is totally discharged, so that the series resistor R is effectively
SS
SS
in parallel to RF
and the resulting initial frequency is determined by R and RF
only,
min
SS
min
since the optocoupler's phototransistor is cut off (as long as the output voltage is not too far
away from the regulated value):
1
f
start= ------------------------------------------------------------------------
| |
3 ⋅ CF ⋅ (R(Fmin RSS))
The C capacitor is progressively charged until its voltage reaches the reference voltage
SS
(2V) and, consequently, the current through R goes to zero. This conventionally takes 5
SS
time constants R ·C but, before that time, the output voltage will have got close to the
SS SS
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
exponential charge of C , that is, initially it will change relatively quickly but the rate of
SS
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
RFmin
4
Fmin
R
SS
SS
R
L6599
Css
1
C
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STMICROELECTRONICS [ ST ]
