AN3233

Main characteristics and circuit description

Doc ID 17595 Rev 1

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the minimum switching frequency. The R-C series R44 and C18 sets both soft-start

maximum frequency and duration.

L6599A overload and short-circuit protection

The current into the primary winding is sensed by the loss-less circuit R41, C27, D11, D10,

R39, and C25 and it is fed into the ISEN pin (pin 6). In the case of overcurrent, the voltage

on the pin overpasses an internal threshold (0.8 V) that triggers a protection sequence. The

capacitor (C45) connected to the DELAY pin (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 input

average power.

Open loop protection

Both circuit stages, PFC and resonant, are equipped with their own overvoltage protections.

The PFC controller L6563H monitors its output voltage via the resistor divider connected to

a dedicated pin (PFC_OK, pin 7) protecting the circuit in case of loop failures or

disconnection. If a fault condition is detected, the internal circuitry latches the L6563H

operations and, by means of the PWM_LATCH pin (pin 8), it also latches the L6599A via the

DIS pin (pin 8). The converter is kept latched by the L6563H internal HV start-up circuit that

supplies the IC by charging the Vcc capacitor periodically. To resume converter operation, a

mains restart is necessary.

The output voltage is monitored by sensing the Vcc voltage. If Vcc voltage overrides the D12

breakdown voltage, Q9 pulls down the L6563H INV pin latching the converter.