TOP260YN design - Short circuit current limit

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I have inherited a design using your TOP260 device which is a 60W dual output unit. All the design seems to follow you application notes and overall the unit works very well. However when we tested the short circuit current limit with a low impedance the output did not go into trip and restart. When I increase the load steadily on the outputs and go into an overload the unit goes into trip & restart without any issue. However when I put a low impedance across the output (1 milliohm shunt) the output does not appear to go into trip and restart. The output trips once and then when restarting it does not appear to trip again and just stays providing a very high current. I have observed the voltage at the C pin stays around 5.5V and does not drop to the 4.8V needed to make it trip again. This is with an 18k on the External current limit pin (X).
If I increase the value of resistor this problem goes away and the device trips OK, but I cannot get the output power I need.
I tested the circuit by putting a switch between the feedback opto and the C pin so that when the device locked up in this way I could disconnect the opto therefore proving that it is not current coming from there. I am not sure if this is something due to the device getting locked in a start up mode where the internal supply from the drain is holding pin C up but I cannot understand why it would do this and not eventually trip off again.

Hi Palmond,
Thanks for posting your question into the forum and we appreciate your time and patience.
We just need clarification from you, you said"I tested the circuit by putting a switch between the feedback opto and the C pin so that when the device locked up in this way I could disconnect the opto therefore proving that it is not current coming from there." From this comment, is that part going into autorestart by opening the switch from feedback opto to control? Could you please clarify on this? Also Could you please send us the Schematic ,Layout and the documentation whatever you have for your design.

Thanks,
PI-KSAN

I need to check the possibility of sending schematics etc as I would not be able to generally attach them the a forum such as this. that said the circuit is taken almost exactly from your TOP252-262 data sheet, fig 43 for the 20W dual output unit. What appears to happen is that under certain conditions if you apply a short circuit to the output the device trips due to the over current and then on restart as there is no output the 'C' pin rises from the charge from the drain and then when it switches over to the feedback the 'C' pin drops and the device trips again. What I am seeing however under certain conditions is that the voltage on the 'C' pin when the device restarts after the initial over current trip rises to approx. 5.2V and stays there. I initially thought that the feedback opto must be sourcing some current so I fitted a switch between it and the resistor capacitor network on the C pin. What I found was that if I opened the switch after the initial overcurrent trip so that the opto could not be feeding current into the C pin, the C pin still rose to about 5.2V and stayed there. The effect of this was that the output current would be running continuously at a very high level (4 or 5 times the trip level). Part of this I think is due to the blanking of the initial current sensing but part also in that the over current circuit did not seem to trip the output. I seem to be in a situation where we are using a larger device than needed (to get the extra efficiency etc) and having to fit a size of resistor on X that gives this effect. If I increase the value of the resistor on X this effect does not appear, however the current limit is then too low for the power we are trying to provide.
I am looking at other ways of introducing a proper current limit as I cannot see a way of ensuring the trip & restart on the device operates correctly under all conditions.

Thanks for your reply.
The behaviour that you are seeing in your design is not common.When the Output Current running continously at very high level, the Auto restart feature should not be lost and it should be working. We are suspecting that Control Pin Circuit GND is not connected to Source Pin of the TopSwitch, instead it might be connected to Input return.
Could you please confirm us that " All the Circuitry related to Control is Connected to Source Pin of TopSwitch in the lay out".Please look at the AN-43 and figure No:19 for recommended lay out. If the lay out is not as per the recommended, you might see that issues. Please let us know if the layout is as per the recommendation and you still have the problem
Regarding sharing the layout and schematics information, if you having any concern regarding confidentiality ,please contact our local sales representative near to you and express your concerns. They can help you about the confidentiality and sharing information.

Regards,
PI-KSAN