L6699

Application information

Doc ID 022835 Rev 2

17/38

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characteristics of the specific MOSFET and the speed its gate is driven

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the gate-drive signal of the other MOSFET going high; this includes the detection time

as well as the propagation delay along the downstream logic circuitry up to the driver

output.

other hand, is counted starting from the negative-going edge of the gate-drive signal, so it

Finally, it is worth stating that the adaptive deadtime function does not significantly increase

efficiency by itself. It is a degree of freedom that must be exploited for this purpose when

designing the resonant tank. Essentially, it allows the use of a higher magnetizing

inductance in the transformer, which minimizes the magnetizing current and, then, the

the minimum required to achieve soft-switching, therefore reducing turn-off switching losses

in MOSFETs. Efficiency at medium and light load greatly benefits from this optimization.

6.3

Safe-start procedure

In the L6699 a new startup procedure, termed “safe-start”, has been implemented to

prevent loss of soft-switching during the initial switching cycles, which is not 100%

guaranteed by the usual soft-start procedure.

Sweeping the operating frequency from an initial high value, that should not exceed

300 kHz, down to the point where the control loop takes over, which is commonly referred to

as soft-start, has a twofold benefit. On the one hand, since the deliverable power depends

inversely on frequency, it progressively increases the converter's power capability, therefore

avoiding excessive inrush current. On the other hand, it makes the converter initially work at

frequencies higher than the upper resonance frequency of the LLC tank circuit, which

ensures inductive-mode operation (i.e. with the tank current lagging the square wave

voltage generated by the half bridge) and, therefore, soft-switching.

However, the last statement is true under a quasi-static approximation, i.e. when the

operating point of the resonant tank is slowly varying around a steady-state condition. This

approximation is not correct during the very first switching cycles of the half bridge, where

the initial conditions of the tank circuit can be away from those under steady-state.

Therefore, hard-switching is possible during the transient period needed to reach the slowly

varying steady-state condition dictated by the soft-start action. A non-zero initial voltage on

the resonant capacitor Cr and transformer flux imbalance during the previously mentioned

transient period are the possible causes of hard-switching in the initial cycles.

In high voltage half bridge controllers it is customary to start the switching activity by turning

on the low-side MOSFET for a preset time to pre-charge the bootstrap capacitor (see

Section 12: Bootstrap section) and ensure proper driving of the high-side MOSFET from the

first cycle. In traditional controllers, normal switching starts right at the end of the pre-charge

time, as shown in the left-hand image in

Figure 10.