Test results and significant waveforms

AN3119

20/32

Doc ID 16849 Rev 2

4.5

PFC_OK pin and feedback failure (open-loop) protection

During normal operation, the voltage control loop provides for the output voltage (Vout) of

the PFC pre-regulator close to its nominal value, set by the resistors ratio of the feedback

output divider. In the L6563S the PFC_OK pin (#7) has been dedicated to monitoring the

output voltage with a separate resistor divider made up of R21, R25, R26 (high) and R33

(low), see Figure 2. This divider is selected so that the voltage at the pin reaches 2.5 V if the

output voltage exceeds a pre-set value (VOVP), usually larger than the maximum Vout that

can be expected, also including worst-case load/line transients.

For the EVL6563S-250W:

Vo = 400 V, Vovp = 434 V. Select: R21+R25+R26=8.8 M

Ω; then:

R33=8.8 M

Ω·2.5/(434-2.5)=51 kΩ.

Once this function is triggered, the gate drive activity is immediately stopped until the

both feedback dividers connected to the L6563S pin (#1) and PFC_OK pin (#7) can be

selected without any constraints. The unique criterion is that both dividers have to sink a

current from the output bus which needs to be significantly higher than the current biasing

the Error amplifier and PFC_OK comparator.

The OVP function described above is able to handle “normal” over voltage conditions, i.e.

those resulting from an abrupt load/line change or occurring at startup. In a case where the

overvoltage is generated by a feedback disconnection, for instance, when one of the upper

resistors of the output divider fails to open, an additional circuitry detects the voltage drop of

the INV pin. If the voltage on pin INV is lower than 1.66 V and at the same time the OVP is

active, a feedback failure is assumed. Therefore, 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 L6563S is above the UVLO

threshold. To restart the system it is necessary to recycle the input power, so that the Vcc

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.

Moreover, the PFC_OK pin doubles its function as a not-latched IC disable; a voltage below

0.23 V shuts down the IC, reducing its consumption to below 2 mA. To restart the IC simply

let the voltage at the pin go above 0.27 V.