AN4027 datasheet(24/41 Pages) STMICROELECTRONICS

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AN4027

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DocID022604 Rev 3

4.4

Anti-capacitive mode protection

The EVL6699-150W-SR demonstration board has been designed in such a way that the

system does not work in capacitive mode during normal operation or failure conditions. As

seen in Figure 27, even in dead short condition the LLC operates correctly in the inductive

region, the same correct operation happens during load or input voltage transients.

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 leads the

applied voltage, such as 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 14). 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, as heatsinking is not usually sized to handle

this abnormal condition.

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

and its voltage is abruptly reversed by the other MOSFET turned on (see Figure 14).

Therefore, the conducting body diode (which does not generally have great reverse

recovery characteristics) keeps its low impedance until it recovers, and so originating

a condition equivalent to a shoot-through of the half bridge leg. This is a potentially

destructive condition (see point 3) 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

Figure 28. Short-circuit - hiccup mode

CH1: HB voltage

CH2: DELAY

CH3: CSS

CH4: ISEN

AM11630v1

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