AN4026 datasheet(23/40 Pages) STMICROELECTRONICS

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AN4026

Functional check

Doc ID 022603 Rev 1

23/40

4.4

Anti-capacitive mode protection

The EVL6699-90WADP 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 24, even in dead short condition the LLC operates correctly in the inductive

region, the same correct operation occurs during load and 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 leading 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 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, since 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 (Figure 14). Therefore, the

conducting body diode (which does not generally have great reverse recovery

characteristics) keeps its low impedance until it recovers, therefore originating a condition

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

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

Figure 25.

Short-circuit - hiccup mode

CH1: HB voltage

CH3: CSS

CH2: LVG

CH4: DELAY

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