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EVL6563H-100W

250 W transition-mode PFC pre-regulator with the new L6563H

STMicroelectronics 

Test results and significant waveforms

AN3118

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 resistor ratio of the feedback output

divider. In the L6563H the PFC_OK pin (#7) has been dedicated to monitor 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 preset value (VOVP), usually larger than the maximum Vout that can be

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

For the EVL6563H-250W we have:

Vo = 400 V, Vovp = 434 V. Select: R21+R25+R26=8.8 MΩ; then:

R33=8.8 MΩ·2.5/(434-2.5)=51 kΩ.

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

voltage on the PFC_OK pin drops below 2.4 V. An example is seen in Figure 31. Notice that

both feedback dividers connected to the L6563H pin (#1) and the 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 L6563H is above the UVLO

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

voltage of the L6563H goes below Vccrestart. Note that this function offers 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.

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 below 2 mA. To restart the IC simply let

the voltage at the pin go above 0.27 V.

20/31

Doc ID 16847 Rev 2

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