L6699 datasheet(29/38 Pages) STMICROELECTRONICS

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L6699

Capacitive-mode detection function

Doc ID 022835 Rev 2

29/38

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 enter 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

definitely 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 IC 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

OCP procedure (see

Section 8: Current sensing, OCP and OLP) 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 after a

time TSH causing intermittent operation and reducing thermal stress.

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

converter's 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.

L6699 датащи(PDF) 29 Page - STMicroelectronics