STMicroelectronics 19V - 90W Adapter with PFC for Laptop computers using the L6563H and L6699 EVL6699-90WADP EVL6699-90WADP Ficha De Dados
Códigos do produto
EVL6699-90WADP
Application information
L6699
18/38
Doc ID 022835 Rev 2
Figure 10.
Comparison startup behavior: traditional controller (left), with L6699
(right)
(right)
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, therefore invoking its reverse recovery. More hard-
switching cycles may follow (see the left-hand image in
high-side MOSFET to occur with non-zero drain-to-source voltage while the body diode of
the low-side MOSFET is conducting, therefore invoking its reverse recovery. More hard-
switching cycles may follow (see the left-hand image in
). These events are few but
potentially hazardous: they could cause the destruction of both MOSFETs, should the
resulting dv/dt across the low-side MOSFET exceed its maximum rating (see
resulting dv/dt across the low-side MOSFET exceed its maximum rating (see
for more details).
To prevent this hard-switching cycle(s) with body diode reverse recovery, the L6699 waits
about 50
about 50
µ
s after the pre-charge time before starting switching (see the right-hand image in
). This idle time is normally long enough to let the tank current decay to essentially
zero in case of an initially charged resonant capacitor. On the other hand, it is too short for
the bootstrap capacitor to be significantly discharged.
the bootstrap capacitor to be significantly discharged.
To understand the origin of transformer flux imbalance it is worth remembering that the half
bridge is driven with 50% duty cycle, so that under steady-state conditions the voltage
across the resonant capacitor Cr has a DC component equal to Vin/2. Consequently, the
transformer's primary winding is symmetrically driven by a ± Vin/2 square wave.
bridge is driven with 50% duty cycle, so that under steady-state conditions the voltage
across the resonant capacitor Cr has a DC component equal to Vin/2. Consequently, the
transformer's primary winding is symmetrically driven by a ± Vin/2 square wave.
At startup, however, the voltage across Cr is often quite different from Vin/2, 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, the duration of the two half-cycles being the
same, the current may not reverse in a switching half-cycle, as shown in the left-hand image
in
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, the duration of the two half-cycles being the
same, the current may not reverse in a switching half-cycle, as shown in the left-hand image
in
. Once again, 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.
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.