L6563H
Application information
6.2
Feedback failure protection (FFP)
The OVP function above described handles “normal” over voltage conditions, i.e. those
resulting from an abrupt load/line change or occurring at start-up. In case the overvoltage is
generated by a feedback disconnection, for instance when the upper resistor of the output
divider (R1) fails open, an additional circuitry behind the pin PFC_OK detects the voltage
gap with respect to pin INV. If the voltage gap is greater than 40 mV and the OVP is active,
the FFP is triggered, the gate drive activity is immediately stopped, the device is shut down,
its quiescent consumption is reduced below 180 µA and the condition is latched as long as
the supply voltage of the IC is above the UVLO threshold. At the same time the pin
PWM_LATCH is asserted high. PWM_LATCH is an open source output able to deliver 2.8 V
minimum with 0.25 mA load, intended for tripping a latched shutdown function of the PWM
controller IC in the cascaded dc-dc converter, so that the entire unit is latched off. To restart
the system it is necessary to recycle the input power, so that the Vcc voltage of both the
L6563H goes below 6V and that one of the PWM controller goes below its UVLO threshold.
The pin PFC_OK doubles its function as a not-latched IC disable: a voltage below 0.23V
shutdown the IC, reducing its consumption below 2 mA. In this case both PWM_STOP and
PWM_LATCH keep their high impedance status. To restart the IC simply let the voltage at
the pin go above 0.27 V.
Note that these functions offer a complete protection against not only feedback loop failures
or erroneous settings, but also against a failure of the protection itself. Either resistor of the
PFC_OK divider failing short or open or a PFC_OK pin floating results in shutting down the
IC and stopping the pre-regulator.
6.3
Voltage feedforward
The power stage gain of PFC pre-regulators varies with the square of the RMS input
voltage. So does the crossover frequency fc of the overall open-loop gain because the gain
has a single pole characteristic. This leads to large trade-offs in the design.
For example, setting the gain of the error amplifier to get fc = 20 Hz @ 264 Vac means
having fc 4 Hz @ 88 Vac, resulting in a sluggish control dynamics. Additionally, the slow
control loop causes large transient current flow during rapid line or load changes that are
limited by the dynamics of the multiplier output. This limit is considered when selecting the
sense resistor to let the full load power pass under minimum line voltage conditions, with
some margin. But a fixed current limit allows excessive power input at high line, whereas a
fixed power limit requires the current limit to vary inversely with the line voltage.
Voltage Feedforward can compensate for the gain variation with the line voltage and allow
minimizing all of the above-mentioned issues. It consists of deriving a voltage proportional to
the input RMS voltage, feeding this voltage into a squarer/divider circuit (1/V2 corrector) and
providing the resulting signal to the multiplier that generates the current reference for the
inner current control loop (see Figure 39).
Doc ID 16047 Rev 2
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STMICROELECTRONICS [ ST ]
