AN3233

Functional check

Doc ID 17595 Rev 1

17/35

In Figure 17 and 18 the transitions from full load to no load and vice versa have been

checked. As seen in the images, both transitions are clean and there isn’t any output voltage

dip

.

Overcurrent and short-circuit protection

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

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

the signal is fed into the ISEN pin. It 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 the soft-start capacitor CSS to be discharged.

Under output short-circuit, this operation results in an almost constant peak primary current.

With the L6599A, the board designer can externally program the maximum time that the

converter is allowed to run overloaded or under short-circuit conditions. Overloads or short-

circuits lasting less than the set time do not cause any other action, therefore providing the

system with immunity to short duration phenomena. If, instead, the overload condition

continues, a protection procedure is activated that shuts down the L6599A and, in case of

continuous overload/short-circuit, results in continuous intermittent operation with a user

defined duty cycle. This function is realized with the DELAY pin (pin 2), by means of a

capacitor C45 and the parallel resistor R24 connected to ground. As the voltage on the

ISEN pin exceeds 0.8 V, the first OCP comparator, in addition to discharging CSS, turns on

an internal 150 µA current generator that, via the DELAY pin, charges C45. As the voltage

on C45 is 3.5 V, the L6599A stops switching and the PFC_STOP pin is pulled low. Also the

internal generator is turned off, so that C45 is now slowly discharged by R24. The IC

restarts when the voltage on C45 is less than 0.3 V. Additionally, if the voltage on the ISEN

pin reaches 1.5 V for any reason (e.g. transformer saturation), the second comparator is

triggered, the L6599A shuts down and the operation is resumed after an off-on cycle.

Figure 19

shows intermittent operations caused by an output short-circuit: average output

current is limited, preventing the converter from overheating and consequent failure.

Figure 17.

Transition full load to no load

at 115 Vac – 60 Hz

Figure 18.

Transition no load to full load

at 115 Vac – 60 Hz

CH1: LVG pin

CH2: PFC gate

CH1: LVG pin

CH2: PFC gate

CH3: Output voltage

CH4: Output current

CH3: Output voltage

CH4: Output current