L6699D datasheet(28/38 Pages) STMICROELECTRONICS

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Capacitive-mode detection function

L6699

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Doc ID 022835 Rev 2

Capacitive-mode detection function

Normally, the resonant half bridge converter operates with the resonant tank current lagging

behind the square-wave voltage applied by the half bridge leg, like a circuit having a

reactance of an inductive nature. In this way the applied voltage and the resonant current

have the same sign at every transition of the half bridge, which is a necessary condition in

order for soft-switching to occur (zero-voltage switching, ZVS at turn-on for both MOSFETs).

Therefore, should the phase relationship reverse, i.e. the resonant tank current anticipates

the applied voltage, like in circuits having a capacitive reactance, soft-switching would be

lost. This is termed capacitive-mode operation and must be avoided because of its

significant drawbacks:

Both MOSFETs feature hard-switching at turn-on, like in conventional PWM-controlled

converters (see

Figure 21). The associated capacitive losses may be considerably

higher than the total power normally dissipated under “soft-switching” conditions and

this may easily lead to their overheating, since heatsinking is not usually sized to

handle this abnormal condition.

The body diode of the MOSFET just switched off conducts current during deadtime and

its voltage is abruptly reversed by the other MOSFET turned on (see

Figure 21).

Therefore, once reverse-biased, the conducting body diode keeps its low impedance

until it recovers, therefore creating a condition equivalent to a shoot-through of the half

bridge leg. This is a potentially destructive condition (see next point) and causes

additional power dissipation due to the current and voltage of the conducting body

diode simultaneously high during part of its recovery.

There is an extremely high reverse dv/dt (many tens of V/ns!) experienced by the

conducting body diode at the end of its recovery with the other MOSFET turned on.

This dv/dt may exceed the rating of the MOSFET and lead to an immediate failure

because of the second 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 dv/dt-induced

failure is much more likely. The L6699 may be also damaged if its OUT pin is subject to

a dv/dt exceeding the AMR (50 V/ns).

Figure 21.

Details of hard-switching transition during capacitive-mode operation

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