AN3329 datasheet(24/47 Pages) STMICROELECTRONICS

24 / 47 page

Functional check

AN3329

24/47

Doc ID 18376 Rev 1

Overcurrent and short-circuit protection

The L6599A is provided with a current sensing input (pin #6, ISEN) and a dedicated

overcurrent management system. Current flowing in the resonant tank is detected and the

signal is fed into the ISEN pin, which is internally connected to a first comparator, referenced

to 0.8 V, and to a second comparator referenced to 1.5 V. If the voltage externally applied to

the pin exceeds 0.8 V, the first comparator is tripped causing an internal switch to be turned

on and discharge the soft-start capacitor CSS, increasing the switching frequency and so

limiting the output power, while the second, referenced to 1.5 V, latches the L6599A,

protecting the circuit against dead short-circuit. These two comparators, together with the

DELAY pin (#2) are dedicated to offering the possibility of implementing the overload

protection in either auto-restart or latch mode, according to the whole equipment needs. In

this case, being a flat TV, the final application to which this demonstration board is

dedicated, the desired overload and short protection is latched.

If the 12 V output is failing short-circuit as in Figure 46, the ISEN pin (#6) voltage rapidly

rises over 1.5 V, triggering the second comparator of the overcurrent protection. In this case,

as shown, the protection intervention is very quick, protecting properly the circuit. Then, the

L6599A shuts down and operation can be resumed after an off-on signal or a mains voltage

recycling. In Figure 47 details of half bridge voltage and current during the higher current

peaks have been captured, showing the correct ZVS operation by the circuit.

If the 24 V output is failing shorts (Figure 48), the L6599A ISEN pin reaches 0.8 V, triggers

the first comparator, and the capacitor C10 connected to the DELAY and DIS pins, starts

being charged by an internal 150 µA current generator while the soft-start capacitor C21 is

discharged, increasing the switching frequency to limit the output power. When the voltage

on the DELAY and DIS pins reaches 1.85 V, an internal comparator is triggered, then the

L6599A is shut down and its consumption is reduced. The PFC_STOP pin is pulled down.

The capacitor C10 is discharged by the external resistor R12. The latch condition is kept

until the L6599A Vcc drops below the UVLO. Therefore, to resume the operation an off-on

signal or a mains voltage recycling is necessary. Figure 48 details HB switching transitions

as soon as the short is detected, where the borderline between inductive and capacitive

mode could be critical. As can be seen, the resonant current has the correct polarity during

both HB transitions and the converter is properly working in the inductive region even in this

case.

Figure 44.

12 V - 2 A; 24 V 0 ÷ 6 A - 300 Hz

transition at 115 Vac - 60 Hz

Figure 45.

24 V - 6 A; 12 V 1 ÷ 2 A - 300 Hz

transition at 115 Vac - 60 Hz

CH4: +24 V O/P current

CH2: +12 V O/P voltage CH3: +24 V O/P voltage

CH4: +12 V O/P current

CH2: +12 V O/P voltage CH3: +24 V O/P voltage

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