STCMB1 datasheet(39/59 Pages) STMICROELECTRONICS

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STCMB1

Application information

the transition is completed, the output goes low. A monostable circuit, sensitive to negative-

going edges, releases a pulse that marks the end of the deadtime.

The derivative of HB voltage needed for the ADT function is implemented sensing the signal

current flowing through both the intrinsic Cgd capacitor of the external power MOS and the

LVG pin.

The signal current (isource/sink) is proportional to the derivative of HB voltage and Cgd value:

Equation 24

In order to ensure the right behavior of the ADT function, this current signal has to fulfill the

following:

Equation 25

isource/sink > Sdiff (SdiffMAX 4 mA)

where Sdiff is the minimum current signal which can be detected by the edge detector.

Cgd is a function of drain-source voltage of the LVG power MOS and the HB voltage slope is

not constant but it depends on the value of the current flowing in the resonant tank.

Therefore, in view of obtain reliable derivative detection, the following relationship has to be

fulfilled at least at the end of HB transition:

Equation 26

The adaptive dead time generator gives a minimum dead time of 220 ns (max.) and a

maximum dead time of 700 ns (min.), depending on the slope of the half bridge node. The

actual minimum observable dead time is about 120 ns, defined and limited by the internal

propagation delay. If, for some reason, the end of the half bridge transition were not

detected, then the dead time would last indefinitely.

To avoid this stall condition, a timeout of about 1

s on the dead time is applied: unless the

anti-capacitive protection mechanism is active (dead time made longer to let the current to

take the right sign), the turn-on of the MOSFET is forced at about 1

s from the turn-off of

the other.

According to Equation 26, a MOSFET with the small Crss and small switched tank current

could induce the unwanted generation of a minimum dead time to which a hard switch event

could be associated (MOSFET turns on before the end of the half bridge transition, but no

cross conduction). In these cases, a ceramic capacitor, up to 10 pF, between the drain of the

low side MOSFET and the LVG pin can solve this adaptive dead time generation issue. A

resistor, up to some k

, in series with the added capacitor and a Schottky diode, for

instance the BAV21, between the LVG and GND, will made the solution more robust. The

ceramic capacitor has to be high voltage rating. Figure 22 shows the proposed solution.

d t

d V

H B

g d

C g d

s o u r c e / s i n

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