AN4027 datasheet(25/41 Pages) STMICROELECTRONICS

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AN4027

Functional check

a MOSFET is hot, the turn-on threshold of its parasitic BJT is lower, this dv/dt-induced

failure is then far more likely.

When either MOSFET is turned on, the other one can be parasitically turned on too, if

the current injected through its Cgd and flowing through the gate driver's pull-down is

large enough to raise the gate voltage close to the turn-on threshold. This would be

a lethal shoot-through condition for the half bridge leg.

The recovery of the body diodes generates large and energetic negative voltage spikes

because of the unavoidable parasitic inductance of the PCB subject to its di/dt. These

are coupled to the OUT pin and may damage the L6699.

There is a large common-mode EMI generation that adversely affects EMC.

Resonant converters work in capacitive mode when their switching frequency falls below

a critical value that depends on the loading conditions and the input-to-output voltage ratio.

They are especially prone to run in capacitive mode when the input voltage is lower than the

minimum specified and/or the output is overloaded or short-circuited. Designing a converter

so that it never works in capacitive mode, even under abnormal operating conditions, is

certainly possible but this may pose unacceptable design constraints in some cases.

To prevent the severe drawbacks of capacitive mode operation, while enabling a design that

needs to ensure Inductive mode operation only in the specified operating range, neglecting

abnormal operating conditions, the L6699 provides the capacitive mode detection function.

The L6699 monitors the phase relationship between the tank current circuit sensed on the

ISEN pin and the voltage applied to the tank circuit by the half bridge, checking that the

former lags behind the latter (inductive mode operation). If the phase-shift approaches zero,

which is indicative of impending capacitive mode operation, the monitoring circuit activates

the anti-capacitive mode protection procedure so that the resulting frequency rise keeps the

converter away from that dangerous condition. Also in this case, the DELAY pin is activated,

so that the OLP function, if used, is eventually tripped, causing intermittent operation and

reducing thermal stress.

If the phase relationship reverses abruptly (which may happen in the case of dead short at

the converter output), the L6699 is stopped immediately, the soft-start capacitor CSS is

totally discharged and a new soft-start cycle is initiated after 50 µs idle time. During this idle

period the PFC_STOP pin is pulled low to stop the PFC stage as well.

AN4027 Datasheet(PDF) 25 Page - STMicroelectronics