AN4026 datasheet(21/40 Pages) STMICROELECTRONICS

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AN4026

Functional check

Doc ID 022603 Rev 1

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

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

guaranteed by the usual soft-start procedure. At startup, the voltage across Cr is often quite

different from Vin/2, as during the 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, 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, in a typical controller working with fixed 50% duty

cycle, with the duration of the two half-cycles being 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 nearly eliminated. Its operation

is such that current reversal in every switching half-cycle and, then, soft-switching is

ensured. In Figure 22 it is possible to note 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 nearly seamless and just

a small perturbation of the tank current can be observed.

4.3

Overcurrent and short-circuit protection

The L6699 is equipped with a current sensing input (pin #6, ISEN) and a dedicated

overcurrent management system. The current flowing in the resonant tank is detected and

the signal is fed into the ISEN pin. It is internally connected to a first comparator, referenced

to 0.8 V, and to a second comparator referenced to 1.5 V. If the voltage externally applied to

the pin exceeds 0.8 V, the first comparator is tripped causing an internal switch to be turned

on and to discharge the soft-start capacitor CSS.

Under output short-circuit, this operation results in a nearly constant peak primary current.

With the L6699, the user can program externally the maximum time that the converter is

allowed to run overloaded or under short-circuit conditions. Overloads or short-circuits

Figure 21.

Startup at full load

Figure 22.

Startup at full load - detail

CH1: HB voltage

CH3: CSS

CH2: LVG

CH4: ISEN

CH1: HB voltage

CH3: CSS

CH2: LVG

CH4: ISEN

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