STCMB1 datasheet(44/59 Pages) STMICROELECTRONICS

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Application information

STCMB1

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DocID028958 Rev 4

Figure 26. Start-up with traditional controllers

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, thus invoking its reverse recovery. More hard switching

cycles may follow. These events are few but potentially hazardous: they could cause both

MOSFETs destruction, should the resulting dV/dt across the low-side MOSFET exceed its

maximum rating. To understand the origin of transformer flux imbalance it is worth reminding

that normally the half-bridge is driven with the 50% duty cycle. This implies that, under

steady state conditions, the voltage across the resonant capacitor Cr has a dc component

equal to Vin/2.

Figure 27. STCMB1 start-up waveforms

Consequently, the transformer's primary winding is symmetrically driven by a ± Vin/2 square

wave.

At the start-up, however, the voltage across the Cr is often quite different from Vin/2 and it

takes some time for its dc component to reach this value. During this transient, the

transformer is not driven symmetrically in voltage and, then, with the 50% duty cycle there is

a significant V • s imbalance in two consecutive half-cycles. This causes a significant

difference in the up and down slopes of the tank current and, being the duration of the two

half-cycles the same, the current may not reverse in a switching half-cycle, as shown in the

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