L6564T datasheet(19/33 Pages) STMICROELECTRONICS

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L6564T

Application information

Doc ID 022671 Rev 1

19/33

6.2

Feedback failure protection (FFP)

The OVP function described above handles “normal” overvoltage conditions, i.e. those

resulting from an abrupt load/line change or occurring at startup. In case the overvoltage is

generated by a feedback disconnection, for instance when the upper resistor of the output

divider (R1) fails to open, the comparator detects the voltage at pin INV. If the voltage is

lower than 1.66 V 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. To restart the system it is necessary to recycle the input power, so that the

Vcc voltage of the L6564T goes below 6 V.

The pin PFC_OK doubles its function as a not-latched IC ‘Disable’: a voltage below 0.23 V

shuts down the IC, reducing its consumption below 2 mA. To restart the IC simply let the

voltage at the pin go above 0.27 V.

Note that these functions offer 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 a large trade-off 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

the minimizing of all the above-mentioned issues. It consists in 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 28).

L6564T Datasheet(PDF) 19 Page - STMicroelectronics