Designers commonly look at the MTBF (mean time between failure) ratings of power supplies to make sure they will operate reliably in an intended application. Although it can be a useful indicator, MTBF doesn’t give you the entire story about a supply’s reliability.
For instance, MTBF does not predict the power supply’s lifetime. It is the total functional life divided by the number of failures. But that expected time between failures can be longer than the life expectancy of the power supply’s internal components.
Electrolytic capacitors are usually the first components to break down inside a power supply. If the capacitor cannot reliably store energy as needed, the power supply’s reliability suffers. Several conditions can cause a capacitor to fail, so consult with your power supply manufacturer to determine how the capacitor will handle the rigors of the application environment.
Why Capacitors Fail
Some of the common reasons why capacitors fail or wear out include:
- Voltage rating. Applying a higher voltage than the capacitor’s voltage rating can cause catastrophic failure.
- Ripple currents. Extreme ripple currents can heat up the capacitor and dry out the electrolyte.
- Heat. Hot operating conditions shorten the capacitor’s life. Or, the circuit board can transmit heat to cause the electrolyte to vaporize.
- Short or open circuits. Short circuits can occur between the electrodes, and mounting errors can cause open circuits.
- ESR. A capacitor with a higher equivalent series resistance (ESR) is less able to handle high ripple currents. High temperatures can raise a capacitor’s ESR.
- Reduced capacitance. A capacitor’s performance can decline over time. As ESR increases, the capacitor heats up and dries the dielectric.
- Storage life. If you expect your power supply to be inactive for long periods, remember that electrolytic capacitors have a limited storage life.
- Other. Chemical leakage (which leads to corrosion), high leakage current, cold temperatures, capacitor size and many other factors affect the life of an electrolytic capacitor.
Internal Capacitors May Decide Your Power Supply’s Reliability
MTBF is not the only reliability metric to look at when selecting your power supply. Internal components like electrolytic capacitors have limited lifetimes, so be sure to take them into account. Capacitor lifetimes can be determined by a host of both operating and environmental factors, especially when it comes to thermal conditions.
Although it is desirable to look for built-in capacitors that offer long lifetimes in a wide range of operating environments, not all capacitors are equal: Better-quality capacitors use better-quality electrolytes. Also, be sure to get as much information as possible about the built-in electrolytic capacitor from your power supply manufacturer in order to determine whether it is appropriate for the intended application. Polytron Devices can provide the technical information, expertise and testing pertaining to internal electrolytic capacitors to help you select a power supply that will meet your expectations in the field.
For more information about Polytron power supplies, visit www.polytrondevices.com.