LNK306 based power supply blowing fuses at startup

Posted by: thinkautomatic on 04/30/2013

We're building a smart wall switch dimmer that utilizes a power supply design based on the LNK306GN (see attached schematic). Notice the two 0.125A pico fuses which are required by UL to keep us under 15 watts. Most of the time we are well below 15W in the 0.020A range. However, at startup the fuses are blowing which we believe is due to the sudden current inflow. We are considering replacing the pico fuses with these fuseable power resistors: http://www.digikey.com/product-detail/en/4-1676966-7/A103603CT-ND/2730241

My question is whether that is likely to alleviate our fuse blowing issue by handling the startup surge better while still providing the circuit protection required by UL?

Thanks!!

Comments

The fusible power resisters will probably help as they will slow down the inrush current and minimize the peak value of the current waveform. For the fuses that are currently blowing, you might want to check and see if slow-blow fuses will work in your application. Very fast fuses can respond to the inrush current while slow-blow fuses have a longer time constant and react to longer duration fault conditions. Another item you can consider is a NTC thermistor. These devices increase their resistance when hot (during inrush conditions) and return to a low resistance state once they have cooled. If you're borderline passing on an efficiency spec, the thermistors could be helpful as they will have a lower steady state resistance than the fusible resisters.

-The Traveler

Thanks Traveler! Can you recommend which thermistor we should try using?

I would try something in the 5-20 ohm territory that is rated for the continuous input curren that it will see during normal use.

-The Traveler

i am using 10Ohm NTC. while operating conditions NTC temperature is 50 Degree celsius. is it fine to use. in my application it will switch on off every min once.

Yes, it is okay to use. This can prevent high inrush current especially if the input voltage is at high line and if your input bulk capacitor is fully discharge prior to switch ON of your application.