AN4027 datasheet(10/41 Pages) STMICROELECTRONICS

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Main characteristics and circuit description

AN4027

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DocID022604 Rev 3

To overcome this issue, the L6563H device implements the fast voltage feed-forward

function. As soon as the voltage on the VFF pin decreases by a set threshold (40 mV

typically), a mains dip is assumed and an internal switch rapidly discharges the VFF

capacitor via a 10 k

 resistor. Thanks to this feature, it is possible to set an RC circuit with

a long time constant, assuring a low THD, keeping a fast response to mains dip.

1.5

L6699 overload and short-circuit protection

The current into the primary winding is sensed by the lossless circuit R41, C27, R78, R79,

and C25 and it is fed into the ISEN pin (pin 6). In the case of overload, the voltage on the pin

surpasses an internal threshold (0.8 V) that triggers a protection sequence. An internal

switch is turned on for 5 µs and discharges the soft-start capacitor C18. This quickly

increases the oscillator frequency and thereby limits energy transfer. Under output short-

circuit conditions, this operation results in a peak primary current that periodically oscillates

below the maximum value allowed by the sense resistor R78.

The converter runs under this condition for a time set by the capacitor (C45) on pin DELAY

(pin 2). During this condition, C45 is charged by an internal 150 µA current generator and is

slowly discharged by the external resistor (R24). If the voltage on the pin reaches 2 V, the

soft-start capacitor is completely discharged so that the switching frequency is pushed to its

maximum value. As the voltage on the pin exceeds 3.5 V, the IC 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.

This procedure allows the converter to handle an overload condition for a time lasting less

than a set value, avoiding IC shutdown in the case of short overload or peak power

transients. On the other hand, in the case of dead short, a second comparator referenced to

1.5 V immediately disables switching and activates a restart procedure.

1.6

L6699 anti-capacitive protection

The LLC resonant half bridge converter must operate with the resonant tank current lagging

behind the square-wave voltage applied by the half bridge leg. This is a necessary condition

in order to obtain correct soft switching by the half bridge MOSFETs. If the phase

relationship reverses, i.e. the resonant tank current leads the applied voltage, like in circuits

having a capacitive reactance, soft switching is lost. This condition is called capacitive mode

and must be avoided because of significant drawbacks coming from hard switching (refer to

the L6699 datasheet).

Resonant converters work in capacitive mode when their switching frequency falls below a

critical value that depends on the loading conditions and the input-to-output voltage ratio.

They are especially prone to run in capacitive mode when the input voltage is lower than the

minimum specified and/or the output is overloaded or short-circuited. Designing a converter

so that it never works in capacitive mode, even under abnormal operating conditions, is

certainly possible but this may pose unacceptable design constraints in some cases.

To prevent the severe drawbacks of capacitive mode operation, while enabling a design that

needs to ensure Inductive mode operation only in the specified operating range, neglecting

abnormal operating conditions, the L6699 provides the capacitive mode detection function.

The IC monitors the phase relationship between the tank current circuit sensed on the ISEN

pin and the voltage applied to the tank circuit by the half bridge, checking that the former

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