AN4026 datasheet(24/40 Pages) STMICROELECTRONICS

24 / 40 page

Functional check

AN4026

24/40

Doc ID 022603 Rev 1

breakdown of the parasitic BJT intrinsic in its structure. If a MOSFET is hot, the turn-on

threshold of its parasitic BJT is lower, and this dv/dt-induced failure is much 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 into 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 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 the 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.

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