120W TOPSwitch-HX TOP259EN Power supply

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Dear PI experts!


I need to build 19,5V 6.3A power supply. I've used PI Expert software, calculated and built power supply with it's recommendations. 


But the trouble with it - it won't start when the 3Ohm load is connected at start-up. I've tried installing a soft-finifsh network (diode, capacitor, resistor) , but no luck! It starts when I connect load after 5-10 seconds after power-on.


See attached design.


Any ideas how can I acheive smooth startup? 

TOPSwitch-HX_PIDesign4.uds284.5 KB



Will you please provide a schematic so that I can review how exactly the soft-finish is connected?


Does the unit appear to hiccup? If yes, what is the time duration?




Dear PI-Sarek


See attached schematics of the power supply and the soft finish circuit.  Sorry, I don't have them at one drawing now. 


The power supply apperars to hiccup with an interval of approximately 1 second. 


When I shunt the TOPswitch D and S pins with a 15pF capacitor in parallel with 600kOhm resistror, the power supply starts ok at full load. But is this right to do so? What if this disables some kind of protection?


Please help, do you have any advice?


With best wishes,

Artem Moroz

I need to know few things


The supply does not start up at RLOAD = 3 Ohms.   Is this happening at Vin=90VAC?



If it the case,  what happens when you increase your imput voltage to 115VAC?


 Can you test the supply changing the feedback of the TL431 to a single  0.1uF capacitor instead of the complex feedback you are showing int he soft finish circuit?


 Let me know the what kind of results you are obtaining