Application information
Figure 30. Lossless current sensing technique, with capacitive shunt
L6599
10
fmin
VCrpk
≈
τ
1N4148
CA RA
1N4148
6
ISEN
L6599
ICr
pk
Cr
RB
CB
The device is equipped with a current sensing input (pin 6, ISEN) and a sophisticated
overcurrent management system. The ISEN pin is internally connected to the input of a first
comparator, referenced to 0.8V, and to that of a second comparator referenced to 1.5V. If the
voltage externally applied to the pin by either circuit in Figure 29 or Figure 30 exceeds 0.8V
the first comparator is tripped and this causes an internal switch to be turned on and
discharge the soft-start capacitor C (see Chapter 7.3: Soft-start). This will quickly
SS
increase the oscillator frequency and thereby limit energy transfer. The discharge will go on
until the voltage on the ISEN pin has dropped by 50mV; this, with an averaging time in the
range of 10/f , ensures an effective frequency rise. Under output short circuit, this
min
operation results in a nearly constant peak primary current.
It is normal that the voltage on the ISEN pin may overshoot above 0.8V; however, if the
voltage on the ISEN pin reaches 1.5V, the second comparator will be triggered, the L6599
will shutdown and latch off with both the gate-drive outputs and the PFC_STOP pin low,
hence turning off the entire unit. The supply voltage of the IC must be pulled below the
UVLO threshold and then again above the start-up level in order to restart. Such an event
may occur if the soft-start capacitor C is too large, so that its discharge is not fast enough
SS
or in case of transformer's magnetizing inductance saturation or a shorted secondary
rectifier.
In the circuit shown in Figure 29 where a sense resistor R in series to the source of the
S
low-side MOSFET is used, note the particular connection of the resonant capacitor. In this
way the voltage across R is related to the current flowing through the high-side MOSFET
S
and is positive most of the switching period, except for the time needed for the resonant
current to reverse after the low-side MOSFET has been switched OFF. Assuming that the
time constant of the RC filter is at least ten times the minimum switching frequency f , the
min
approximate value of R can be found using the empirical equation:
S
Vspkx
5 ⋅ 0.8
4
--------------
------------------
--------------
RS
=
≈
≈
ICrpkx
ICrpkx
ICrpkx
where I
is the maximum desired peak current flowing through the resonant capacitor
Crpkx
and the primary winding of the transformer, which is related to the maximum load and the
minimum input voltage.
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STMICROELECTRONICS [ ST ]
