AN4248 datasheet(13/46 Pages) STMICROELECTRONICS

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AN4248

Output overvoltage protection (ZCD setting)

There are two functions of the ZCD pin:

–

De-magnetizing input in QR operation by taking feedback from auxiliary winding

–

Sensing output voltage reflection through auxiliary winding and initiate output

overvoltage shutdown at desired output OVP level

The zero current detection (ZCD) and triggering blocks switch on the external MOSFET if a

negative-going edge falling below 50 mV is applied to the input (pin 11, ZCD). To do so the

triggering block must be previously armed by a positive-going edge exceeding 100 mV. This

feature is usually used for sensing transformer demagnetization in QR operation to start

another switching cycle. The signal for the ZCD input is obtained from the transformer's

auxiliary winding used also to supply the L6566B. The triggering block is blanked for

TBLANK = 2.5

μs after MOSFET's turnoff to prevent any negative-going edge that follows

leakage inductance demagnetization from triggering the ZCD circuit erroneously.

The same pin also performs OVP protection. The resistor divider network at the ZCD pin

sets the OVP level where protection is required. If the voltage on the pin exceeds an internal

5 V reference, a comparator is triggered, an overvoltage condition is assumed and the

device is shut down. An internal current generator is activated that sources 1 mA out of the

VFF pin (15). If the VFF voltage is allowed to reach 2 Vbe over 5 V, the L6566B will be

latched off. If the impedance externally connected to pin 15 is so low that the 5+2VBE

threshold cannot be reached or if some means is provided to prevent that, the device will be

able to restart after the VCC has dropped below 5 V. So in that case this OVP protection by

ZCD pin will be auto restart type. On the other hand if VFF pin is grounded with a 10 K

resistor, in that case as soon as ZCD senses 5 V due to overvoltage at output, the internal

current generator starts providing current to the VFF pin and voltage of VFF pin reaches 6.4

V. In that case the device gets latched due to output OVP. So both auto restart as well as

latched shutdown protections are possible.

Figure 8. Output overvoltage setting using Z the CD pin

The ZCD pin will be connected to the auxiliary winding through a resistor divider RZ1, RZ2

as shown above. The divider ratio kOVP = RZ2 / (RZ1 + RZ2) will be chosen equal to:

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