5.10

Overload and short-circuit protection

To manage an overload or a short-circuit condition, an internal up-down counter is incremented or decremented

with an internal clock, if the FB pin voltage is, respectively, above or below the upper saturation limit VFBH.

If the overload/short-circuit event persists for a time greater than TOVL (50 ms typical value), the switching is

inhibited and VDD is recycled between VDD_RESTART and VDD_ON by the periodical activation of the HV current

source. After TRESTART from protection tripping, the IC restarts switching with soft-start as soon as VDD is

recharged to VDD-ON. After soft-start (during which the overload counter is disabled), if overload is still present,

the switching activity is disabled again as previously described, otherwise it resumes normal operation. If the

overload/short-circuit event persists for a time smaller than TOVL, the counter is decremented cycle-by-cycle down

to zero and the IC is not stopped.

This fault management ensures low repetition rate restart attempts, so that the converter works safely with

extremely low power throughput and avoids the IC overheating in case of repeated overload events, and a prompt

resumption of normal operation at fault removal.

In case of transformer saturation, short-circuit of secondary side rectifier or deep CCM, a further protection feature

named OCP_LEB is embedded: if the drain current value is higher than IOCP_LEB at the end of LEB time for two

subsequent cycles the VIPERGAN50 shuts down for TSD-REST seconds and performs a new startup.

In case of VDD clamp activation with a current sunk from the pin higher than IDD-CL (26 mA typ.) and for a time

duration higher than TDD-CL (100 us typ.), the IC is shut down then restart after VDD recycling.

Figure 25. Timing diagram of OLP protection

5.11

Output overvoltage protection

The voltage on the ZCD pin is monitored at the end of the transformer’s demagnetization, where the auxiliary

winding accurately tracks the converter output voltage and compares with an internal reference.

If the sampled voltage exceeds the OVP threshold VOVP (2.5 V typ.), an overvoltage condition is assumed.

With reference to the external configuration of Figure 15, once ROPP is fixed by feed-forward considerations (see

relevant section), it is possible to calculate the value of the ROVP resistor to activate the OVP protection for a

certain output voltage level, VOUT-OVP.

Equation 9

(9)

RFB= VOVP

NAUXNSEC∙VOUT−OVP−VOVP∙ROPP

To reduce sensitivity to external noise and prevent the protection from being erroneously activated, the OVP

comparator must be triggered for four consecutive oscillator cycles before the device is stopped.

A counter, which is reset every time the OVP comparator is not triggered in one oscillator cycle, is provided for

this purpose.

Best accuracy of OVP protection is achieved when demagnetization time is longer than 2 μs.

VIPERGAN50

Overload and short-circuit protection

DS13888 - Rev 2

page 22/37