Datasheet

Main characteristics and circuit description AN3172
8/31 Doc ID 17230 Rev 1
1.5 Output voltage feedback loop
The feedback loop is implemented by means of a typical circuit using a TL431 modulating
the current in the optocoupler diode.
On the primary side, R34 - connecting pin RFMIN (#4) to the optocoupler’s phototransistor -
closes the feedback loop and its value sets the maximum switching frequency at about 130
kHz. This value has been chosen to limit the switching losses at light-load operation. R31,
connecting the same pin to ground, sets the minimum switching frequency. The R-C series
R44 and C18 sets both soft-start maximum frequency and duration.
1.6 L6599A overload and short-circuit protection
The current into the primary winding is sensed by the lossless circuit R41, C27, D11, D10,
R39, and C25 and it is fed into the ISEN pin (#6). In the case of overcurrent, the voltage on
the pin overpasses an internal comparator threshold (0.8 V), triggering a protection
sequence. The capacitor (C45) connected to the DELAY pin (#2) is charged by an internal
150 µA current generator and is slowly discharged by the external resistor (R24). If the
voltage on the pin reaches 2 V, the soft-start capacitor is completely discharged so that the
switching frequency is pushed to its maximum value. As the voltage on the pin exceeds 3.5
V the IC stops switching and the internal generator is turned off, so that the voltage on the
pin decays because of the external resistor. The IC is soft-restarted as the voltage drops
below 0.3 V. In this way, under short-circuit conditions, the converter works intermittently
with very low average input power.
1.7 PFC overvoltage and open-loop protection
Both circuit stages, PFC and resonant, are equipped with their own overvoltage protections.
The L6563H PFC controller monitors its output voltage via the resistor divider connected to
a dedicated pin (PFC_OK, #7) protecting the circuit in the case of loop failures or
disconnection of the feedback loop divider. In the case where a fault condition is detected,
the PFC_OK circuitry latches the L6563H operations and, by means of the PWM_LATCH
pin (#8), it also latches the L6599A, via the DIS pin (#8). The converter is kept latched by the
L6563H HV circuit that supplies the IC charging the Vcc capacitor periodically. To resume
converter operation mains restart is necessary.