AN4026 datasheet(9/40 Pages) STMICROELECTRONICS

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AN4026

Main characteristics and circuit description

Doc ID 022603 Rev 1

9/40

stops switching and the internal generator is turned off, so that the voltage on the pin decays

because of the external resistor. The IC is soft-restarted as the voltage drops below 0.3 V. In

this way, under short-circuit conditions, the converter works intermittently with very low input

average power.

Please note that some preliminary demonstration boards use the PCB of the EVL6599A-

90WADP reworked for L6699. On these boards the silk screens report the original reference

designators for D10 and D11 but have mounted on those positions two resistors in place of

the diodes. The name of the boards reported on the PCB silkscreen top side is “90 W

adapter with L6563H and L6599A Rev. 1.0”.

More recent demonstration boards have updated the reference designators and the

silkscreen of D10 and D11 has been updated to R63 and R64 respectively. The name of

these more recent boards reported on the PCB top side is “90 W adapter with L6563H and

L6699 Rev. 1.0”. There is no circuit difference between the two boards.

1.7

Overvoltage and open loop protection

Both circuit stages, PFC and resonant, are equipped with their own overvoltage protection.

The L6563H controller monitors the PFC output voltage via the resistor divider connected to

a dedicated pin (PFC_OK, #7) protecting the circuit in the case of loop failures or

disconnection of the feedback loop divider connected to the INV pin (#1). If a fault condition

is detected, the PFC_OK circuitry latches the L6563H operations and, by means of the

PWM_LATCH pin (#8), it latches the L6699 too, via its DIS pin (#8). The converter is kept

latched by the L6563H HV circuit, which supplies the IC by charging the VCC capacitor

periodically. To resume converter operation, a mains restart is necessary.

The LLC open loop protection is guarantee by the Zener D8 sensing the voltage from the

transformer T1 auxiliary winding. In case of open loop the Zener stops the operation by

triggering the DIS pin (#8) of the L6699.

1.8

Light load operation

The board implements a burst mode function allowing a significant power saving during light

or no-load operation.

The L6699 STBY pin (#5) senses the optocoupler's collector voltage that is related to the

feedback control and is proportional to the output load. This signal is compared to an

internal reference (1.24 V); if the load decreases and the voltage on the STBY pin becomes

lower than the reference, the IC enters an idle state and its quiescent current is reduced.

Once the voltage exceeds the reference by 30 mV, the controller restarts switching. Burst

mode operation load threshold is programmed by properly choosing the resistor connecting

the optocoupler to pin RFmin (R34).

As already mentioned, the deadtime inserted between the two gate driver signals is

automatically adjusted to best fit the transition times of the half bridge midpoint, improving

the efficiency thanks to the transformer magnetizing inductance maximization. In detail,

increasing the transformer magnetizing inductance minimizes the magnetizing current,

providing a conduction loss decrease because of the lower total RMS current flowing into

the transformer primary side and half bridge MOSFETs. Because of the low current value at

MOSFET turn-off, a longer transition time of the half bridge is observed, for this reason the

deadtime takes longer in order to ensure the correct zero voltage switching operation by the

MOSFETs.

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