AN4027 datasheet(11/41 Pages) STMICROELECTRONICS

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AN4027

Main characteristics and circuit description

lags behind the latter (Inductive mode operation). If the phase shift approaches zero, which

is indicative of impending capacitive mode operation, the monitoring circuit activates the

overload procedure described above so that the resulting frequency rise keeps the

converter away from that dangerous condition. Also in this case, the DELAY pin is activated,

so that the OLP function, if used, is eventually tripped after a time TSH causing intermittent

operation and reducing thermal stress.

If the phase relationship reverses abruptly (which may happen in the case of dead short at

the converter's output), the L6699 is stopped immediately, the soft-start capacitor C18 is

totally discharged and a new soft-start cycle is initiated after 50 µs idle time. During this idle

period the PFC_STOP pin is pulled low to stop the PFC stage as well.

1.7

Output voltage feedback loop

The feedback loop is implemented by means of a typical circuit using the dedicated

operational amplifier of the TSM1014A modulating the current in the optocoupler diode. The

second operational amplifier embedded in the TSM1014A, usually dedicated to constant

current regulation, is here utilized for burst mode as previously described.

On the primary side, R34 and D17 connect the RFMIN pin (pin 4) to the optocoupler's photo

transistor closing the feedback loop. R31, which connects the same pin to ground, sets the

minimum switching frequency. The RC series R44 and C18 sets both soft-start maximum

frequency and duration.

1.8

Open loop protection

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

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

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

disconnection. If a fault condition is detected, the internal circuitry latches the L6563H

operations and, by means of the PWM_LATCH pin (pin 8), it latches the L6699 as well via

the DIS pin (pin 8). The converter is kept latched by the L6563H internal HV startup circuit

that supplies the IC by charging the VCC capacitor periodically. To resume converter

operation, mains restart is necessary. The LLC open loop protection is realized by

monitoring the output voltage through sensing the VCC voltage. If VCC voltage overrides the

D12 breakdown voltage, Q9 pulls down the L6563H INV pin latching the converter. Even in

this case, to resume converter operation, mains restart is necessary.

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