l6599atn STMicroelectronics, l6599atn Datasheet - Page 19

• Manufacturer Part Number: l6599atn
• Description: Improved High-voltage Resonant Controller
• Manufacturer: STMicroelectronics

L6599AT
6.4
Equation 5
where f
quite empirical and is a compromise between an effective soft-start action and an effective
OCP (see next section). Please refer to the timing diagram of
significant signals during the soft-start phase.
Current sense, OCP and OLP
The resonant half-bridge is essentially voltage-mode controlled; hence a current sense input
only serves as overcurrent protection (OCP).
Unlike PWM-controlled converters, where energy flow is controlled by the duty cycle of the
primary switch (or switches), in a resonant half-bridge the duty cycle is fixed and energy flow
is controlled by its switching frequency. This impacts the way current limitation can be
realized. While in PWM-controlled converters energy flow can be limited simply by
terminating switch conduction beforehand when the sensed current exceeds a preset
threshold (this is commonly known as cycle-by-cycle limitation), in a resonant half-bridge the
switching frequency (that is, its oscillator frequency) must be increased and this cannot be
done as quickly as turning off a switch: it takes at least the next oscillator cycle to see the
frequency change. This implies that to have an effective increase, capable of changing the
energy flow significantly, the rate of change of the frequency must be slower than the
frequency itself. This, in turn, implies that cycle-by-cycle limitation is not feasible and that,
therefore, the information on the primary current fed to the current sensing input must be
somehow averaged. Of course, the averaging time must not be too long, to prevent the
primary current from reaching excessively high values.
In
following paragraphs. The circuit in
sense resistor Rs may not be negligible, decreasing efficiency. The circuit in
more complex, but virtually lossless and recommended when the efficiency target is very
high.
Figure 11
start
is recommended to be at least 4 times f
, a few current sensing methods are illustrated that are described in the
R
Doc ID 15534 Rev 1
SS
=
Figure 11
f
f
RF
start
min
min
−
1
a is more simple, but the dissipation on the
;
C
SS
min
=
. The proposed criterion for C
3
R
⋅
10
SS
−
3
Figure 10
Application information
to see some
Figure 11
SS
b is
19/31
is