L6599A
Application information
7.3
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 L6599A converter, soft-startup is simply realized with the addition of an R-C series
circuit from pin 4 (RFmin) to ground (see Figure 27, left).
Initially, the capacitor CSS is totally discharged, so that the series resistor RSS is effectively
in parallel to RFmin and the resulting initial frequency is determined by RSS and RFmin
only, since the optocoupler phototransistor is cut off (as long as the output voltage is not too
far away from the regulated value):
Equation 4
1
fstart
=
3 ⋅CF ⋅
(
RFmin //RSS
)
The CSS capacitor is progressively charged until its voltage reaches the reference voltage
(2 V) and, consequently, the current through RSS goes to zero. This conventionally is
imposed 5 times by selecting the constants RSS·CSS. Before reaching 2 V on Css, the
output 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
exponential charge of CSS, that is, initially it changes relatively quickly but the rate of change
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 27, right).
Figure 27. Soft-start circuit (left) and power vs. frequency curve in a resonant
half bridge (right)
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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.
Doc ID 15308 Rev 7
21/35
STMICROELECTRONICS [ ST ]
