STCMB1 datasheet(43/59 Pages) STMICROELECTRONICS

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STCMB1

Application information

Figure 25. ACP intervention

7.23

HB: Smooth start-up function

The timeshift control allows the prevention of any hard switching at the start-up: it's inherent

in the control methodology to ensure that the half-bridge toggles when the tank current has

the right sign in order for the MOSFETs to achieve soft-switching, which is not ensured by

the usual soft-start procedure.

Sweeping the operating frequency from an initial high value 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, thus avoiding the excessive inrush

current. On the other hand, it makes the converter to initially work at frequencies higher than

the upper resonance frequency of the LLC tank circuit, which ensures the inductive mode

operation (e.g.: 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, e.g.: when the

operating point of the resonant tank is slowly varying around 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 switching activity by turning on

the low-side MOSFET for a preset time to pre-charge the bootstrap capacitor and ensure

proper driving of the high-side MOSFET since the first cycle. In traditional controllers,

normal switching starts right at the end of the pre-charge time, as shown in Figure 26.

STCMB1 Datasheet(PDF) 43 Page - STMicroelectronics