AN4027 datasheet(23/41 Pages) STMICROELECTRONICS

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AN4027

Functional check

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 keeps

going, a protection procedure is activated that shuts down the L6699 and, in the 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. When the

voltage on C45 is 3.5 V, the L6699 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 becomes lower 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 L6699 shuts down and C45 is charged

to 3.5 V. Even in this case, the operation is resumed once the voltage on C45 drops below

0.3 V.

In Figure 26 a dead short-circuit event has been captured. In this case the overcurrent

protection is triggered by the second comparator referenced at 1.5 V which immediately

stops switching by the L6699 and discharging of the soft-start capacitor; at the same time

the capacitor connected to the DELAY pin (#2) begins charging up to 3.5 V (typ.). Once the

voltage on the DELAY pin reaches 3.5 V, the L6699 stops charging the delay capacitor

(C45) and the L6699 operation is resumed once the DELAY pin (#2) voltage decays to 0.3 V

(typ.) by the parallel resistor (R24), via a soft-start cycle. If the short-circuit condition is

removed, the converter again starts operation, otherwise if the short is still there, the

converter operation results in an intermittent operation (Hiccup mode) with a narrow

operating duty cycle of the converter, in order to prevent overheating of power components,

as can be noted in Figure 28.

In Figure 27 details of peak current with short-circuit occurring is shown. It is possible to see

the ZVS correct operation by the half bridge MOSFETs.

Figure 26. Short-circuit at full load

Figure 27. Short-circuit at full load – detail

CH1: HB voltage

CH2: DELAY

CH3: CSS

CH4: ISEN

AM11628v1

CH1: HB voltage

CH2: DELAY

CH3: CSS

CH4: ISEN

AM11629v1

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