L6699D datasheet(18/38 Pages) STMICROELECTRONICS

18 / 38 page

Application information

L6699

18/38

Doc ID 022835 Rev 2

Figure 10.

Comparison startup behavior: traditional controller (left), with L6699

(right)

A non-zero initial voltage on the resonant capacitor may cause the very first turn-on of the

high-side MOSFET to occur with non-zero drain-to-source voltage while the body diode of

the low-side MOSFET is conducting, therefore invoking its reverse recovery. More hard-

switching cycles may follow (see the left-hand image in

Figure 10). These events are few but

potentially hazardous: they could cause the destruction of both MOSFETs, should the

resulting dv/dt across the low-side MOSFET exceed its maximum rating (see

Section 9:

Capacitive-mode detection function for more details).

To prevent this hard-switching cycle(s) with body diode reverse recovery, the L6699 waits

about 50 µs after the pre-charge time before starting switching (see the right-hand image in

Figure 10). This idle time is normally long enough to let the tank current decay to essentially

zero in case of an initially charged resonant capacitor. On the other hand, it is too short for

the bootstrap capacitor to be significantly discharged.

To understand the origin of transformer flux imbalance it is worth remembering that the half

bridge is driven with 50% duty cycle, so that under steady-state conditions the voltage

across the resonant capacitor Cr has a DC component equal to Vin/2. Consequently, the

transformer's primary winding is symmetrically driven by a ± Vin/2 square wave.

At startup, however, the voltage across Cr is often quite different from Vin/2, so it takes

some time for its DC component to reach the steady-state value Vin/2. During this transient,

the transformer is not driven symmetrically and, then, there is a significant V·s imbalance in

two consecutive half-cycles. If this imbalance is large, there is a significant difference in the

up and down slopes of the tank current and, the duration of the two half-cycles being the

same, the current may not reverse in a switching half-cycle, as shown in the left-hand image

Figure 11. Once again, one MOSFET can be turned on while the body diode of the other

is conducting and this may happen for a few cycles.

To prevent this, the L6699 is provided with a proprietary circuit that modifies the normal

operation of the oscillator during the initial switching cycles, so that the initial V·s unbalance

is nearly eliminated. Its operation is such that current reversal in every switching half-cycle

and, then, soft-switching is ensured.

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