viper22a short circuit protection problem

[MehmetKaya]

Hello, I use viper22A for 220VAC to 14VDC-1A PSU. When I shorted output, around 3A current flows from Ampermeter. Viper22A datasheet explain that if output is short circuit no feedback will come from optocoupler, FB pin voltage will be 0V (my measurement voltage on FB pin 0V too), and viper22A switch-off the Mosfet.

Why mosfet switched continuously although I did output shortcircuit?

[Peter BENSCH]

Please upload the schematics of your current design to be able to help you in a meaningful way.

Regards
/Peter

[MehmetKaya]

Hello, I attached schematic, when I shorted output FB pin seems 0V, but viper22A seems not switched off and 3A shorted current flows from output.

 

[AScha.3]

Hi,

how you get the idea, it would "switch off" ?

from ds i see:

So at short : current limit at max. drain current.

[MehmetKaya]

Hi Ascha, 

I am not meaning that full switched off. As your picture it must be controlled on and off (mosfet is off long time than on). When I shorted FB pin3 with power GND pin-1-2  and connect to 220VAC, I see clearly this situation mosfet on very short time and than off long time. But, When I shorted output and FB pin is 0V, not seen same situation and mosfet seems switched always.

There seems overtempareture, overcurrent and overvoltage protection on Viper22 datasheet. If output is shorted, how can it be protection?

Best regards.

[Peter BENSCH]

@MehmetKaya

• Do you understand how a flyback converter works?
• Have you read section 6.5 of the data sheet, which states, among other things: "In case of overload or short-circuit on the output, the IC runs at ILIM."?

Limiting the current in this way is in fact a form of protection, otherwise the converter would attempt to bring the voltage that has collapsed at the output back to the setpoint value - for which purpose the converter pushes energy into the output until this target is achieved. Without this limitation, at least one link in the chain would go up in smoke: rectifier, converter or the transformer.

Regards
/Peter