AN4027 datasheet(21/41 Pages) STMICROELECTRONICS

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AN4027

Functional check

order to prevent loss of soft-switching during the initial switching cycles which typically is not

guaranteed by the usual soft-start procedure. At startup, the voltage across the resonant

capacitor is often quite different from Vin/2, as during normal steady-state operation, 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 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, in a typical controller working

with fixed 50% duty cycle, as the duration of the two half-cycles is the same, the current may

not reverse in a switching half-cycle. Therefore, 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 almost

eliminated. Its operation is such that current reversal in every switching half-cycle and, then,

soft-switching, is ensured. In Figure 21 it can be noted that at the beginning of operation the

duty cycle of the half bridge is initially considerably less than 50%, the tank current has

lower peak values and changes sign every half-cycle, while the DC voltage across the

resonant capacitor reaches the steady-state. The device goes to normal operation after

approximately 50 µs from the first switching cycle. This transition is almost seamless and

just a small perturbation of the tank current can be observed.

4.2

Burst mode operation at light load

In Figure 22 some burst mode pulses are captured during 250 mW load operation. The

burst pulses are very narrow and their period is quite long, therefore the resulting equivalent

switching frequency is very low, ensuring high efficiency. The resulting output voltage ripple

during burst mode operation is about 200 mV peak-to-peak.

In Figure 23 the detail of the burst is reported: the first initial pulse is shorter than the

following ones avoiding the typical high current peak at half bridge operation restarting, due

to the recharging or the resonant capacitor. The maximum operating frequency of the half

bridge, set by the resistor R34 in series to the optocoupler, is around 77 kHz.

Figure 20. Startup at 115 Vac - full load

Figure 21. Startup at full load - detail

CH1: HB voltage

CH2: LVG

CH3: CSS

CH4: ISEN

AM11413v1

CH1: HB voltage

CH2: LVG

CH3: CSS

CH4: ISEN

AM11414v1

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