I am having problems with the caps failing and shorting input power. I have found a lot of pages on Tantalum Capacitor failures, but very little satisfactory fixes.
Here is set up: I have an 18.5 volt (7.2 amp) linear regulated power supply. It is connected to a DC to DC power supply board with a number of linear and switching power supplies. This power supply board is connected to another electronic board. The overall electronics draw about .5 amps at 18.5 volts. The caps that fail are on the input of the DC to DC power supply board. I am using 47 uF 35 volt Tantalum Low ESR capacitors. Actually, I have three of these caps in parallel and two more in parallel individually shielded by a ferrite bead. All fail over time.
How is the problem occurring? I have many other boards in a similar set up that the caps are not failing, but in this setup the caps are failing. I believe it is because this device has can be disconnected between the regulated power supply and the DC-DC power supply. Tantalum capacitors are sensitive to instantaneous voltage changes (dv/dt). This means that they should never be connected to a mechanical switch or in this case hot plugging the device. (See
Possible solutions
- Shoot and kill all service techs and clients that hot plug device. Unfortunately, this is not an option my manager will let me pursue.
- Exchange the Tantalum caps with Aluminum electrolytic. I see this advice mentioned a lot, but the major problem is the life of electrolytic. Also, the manufactures also don't recommend this, of course it is not to their advantage... (See formatting linkThe solution recommended is to fix the problem and not switch to a different component with other problems. Also, my switching power supplies on the DC-DC power supply board recommend tantalum caps... I don't want to change to Aluminum electrolytic.
- Add a series resistor between regulated power supply and rest of input of DC-DC power supply board. The idea here is to limit inrush current. But that resistor would need to be very high wattage (18.5 volts at .5 amps)... I don't think that is a feasible option in this circuit.
- Add a MosFET in series with input to DC-DC power supply board. The idea here is to have a RC startup delay. (This is recommended formatting linkunder section "keep a handle on inrush current.") This seems to be the only viable option. I was wondering if there is anyone that has tried this. If so could you please discuss results? (This article also recommends using slow turn on for power supply. This is already incorporated as mush as possible on the board in question.)
- Other options? Please let me know.
Thanks, Matthew