LNK304 input fusible resistor keeps blowing
I have designed a universal input 3.3VDC less than 1W power supply using the LNK304P. My input fusible resistor occasionally blows during operation (not due to inrush current).
The data sheet and design example I looked at suggested using an 8.2 ohm 1W fusible resistor. I couldn't find one so I used a 10 ohm 2W resistor.
My input curcuit is 1/2 wave rectified, has two 4.7uF caps with a 1mH inductor in between the caps.
Now that I have seen AN-37 I see that I could have used a 100 ohm resistor instead of the inductor. Since I don't think this is an inrush issue, I don't think this change will help, but don't know.
The micro that is powered by this circuit is connected to a laptop USB port via a programming header. The programmer is set up to NOT power the micro.
There is also a serial converter connected to the TX and RX pins of my micro and also connects back to the laptop USB port. Since this provides power to the TX line, I wonder if this could be back feeding the micro and causing my power supply to fight with it. If this is the case, could it be the cause of the fusible resistor blowing?
My power supply schematic is attached for reference.
Thanks,
-Bob
Comments
Doesn't the fact that the resistor opens during operation (not at power up) mean that it is not a problem with in rush current or are you talking about line surges?
If the resistor opens during a surge event, then I would say that it is surge related. If it opens during normal operation and the power supply has not been exposed to a surge event, then I would say it is inrush. You should verify with the resistor manufacturer that the part is suitable for this application.
Z. Cochrane
Excuse me for hammering this point, but there is no surge event, and the resistor does not open during power-up. Doesn't this eliminate in-rush current as the porbably cause of the failure?
I have attached the resistor data sheet, maybe ther is something there that would counterindicate it's use in this application.
I have experienced that film resistor degrade with inrush current. After a period of time, some will open. Usually what happens is the film erodes after inrush current. Eventually after many inrush events, the resistance increases to a point that the resistor will open (even during normal operation, not during the initial inrush). Wire wound resistors are more robust than film and will be more reliable.
If you would like to experiment, set a high voltage DC source to 325 Volts (the peak voltage of a 230 VAC input). Measure the resistance of a film resistor. Install it into a power supply and apply the DC voltage 100 times (make sure that the input caps are fully discharged before re-applying power). Measure the resistance after this test. It is likely that the resistance has increased.
Z. Cochrane
Your schematic looks OK. Make sure that your resistor is specified for surge at power up. Many film resistors will degrade and eventually open. You should not need to use a 100 Ohm resistor to prevent your fusible resistor from opening.
Z. Cochrane