Small diameter electrolytic capacitors in power supplies
There has been a great deal of emphasis recently on power supply electrolytic capacitor life. No more apparent than with the launch of several new products, like TDK-Lambda’s ZMS100 series to the market, where life-time is stated as a major competitive differentiator. Many customers recognise e-cap life-time as a key reliability factor and will ask for capacitor temperatures and calculated lifetimes before they even test a sample.
Many engineers, both customers and power supply designers alike, focus on the larger sized capacitors. In particularly, the “bulk” capacitor that provides the hold-up energy and the output capacitors that reduce the output ripple and improve transient load response. Unfortunately, the smaller capacitors that are associated with the housekeeping circuit and start up circuitry are often overlooked and considered relatively unimportant.
Looking at one of the major capacitor manufacturer’s datasheets for a long-life capacitor, it can be seen that for a 10mm diameter capacitor, the life is stated at 10,000 hours. The life for a 6.3mm diameter capacitor at 105oC though is only 6,000 hours. If run at 24 hours a day, 365 days a year, that equates to only 250 days. At that point the datasheet says the capacitance can be 25% less. After a couple of years that will be significantly reduced further, but unless the power supply suffers a loss of input power, it might go un-noticed.
Electrolytic capacitors are also notoriously sensitive to temperature. A change in ambient temperature from 40 to 0oC can reduce the capacitance value by up to 10% and double the impedance.
A recent incident in the home of one of our technical marketing team members revealed the effect of this combination. His central heating system was being upgraded and the 7-year old gas boiler had been turned off for 2 days. When the boiler was switched on, it failed to start. The heating technician was baffled and an additional cost of £200-300 was mentioned, along with a two day delay. As it had been snowing, our team member was reluctant to wait for spare parts and asked to see the boiler. Examining the power supply, he applied heat to the small diameter electrolytic capacitors and 20 seconds later the boiler started up.
When questioned how he knew what the fault was by the suitably impressed technician, he answered “poor power supply design; aged electrolytic capacitors and freezing temperatures!”
During the initial development stage of the ZMS100 series of 100W open frame power supplies, TDK-Lambda received similar customer feedback of large scale field failures with a competitor product. The Engineering team designed the start-up circuitry using a longer-life ceramic capacitor in place of commonly used electrolytic. Ceramic capacitors do not contain liquid electrolyte which over time will dry out and cause a loss of capacitance and an increase in ESR. This known failure mechanism can lead to failures and power supply start-up problems.
