The heating technician was baffled and an additional cost of £200-300 was mentioned, along with a further two day delay while parts were procured. Our team member was reluctant to go more nights without heating and so, with nothing to lose, climbed up into the loft to inspect the boiler. The technician removed the cover and showed him the suspect board. Along with the microprocessors, relays and connectors, there was a power supply laid out on the PCB.
Knowing the boiler had been turned off before without any issues last summer, he suspected the cold weather was causing the problem. He was also mindful of a recent press release TDK-Lambda had written for its ZMS100 power supply and how the product design had addressed capacitor life. A heater was brought into the loft and used to warm up the myriad of small electrolytic capacitors clustered around the power supply control IC. Once the ambient temperature had risen, the boiler started within 20 seconds, suitably impressing the technician.
There is a tendency in power supply reliability to focus on the larger aluminium electrolytic capacitors used in the design. Ranging from 10mm to greater than 50mm in diameter, these capacitors are used either as storage or to reduce output voltage ripple. In the case of the 'bulk' or hold-up capacitor, usually rated at 400 to 450V dc, energy is stored to allow the power supply to continue to operate during a short AC interruption. The output capacitors are used to reduce high frequency ripple voltage and improve response times to sudden load changes. One of the key elements in a capacitor specification is manufacturer’s life time numbers.
Many customers recognise electrolytic capacitor life-time as a key reliability factor. The concern is that over time, with elevated temperatures, electrolyte is lost due to diffusion through the rubber seals causing a loss in capacitance and an increase in ESR. For the bulk capacitor, this reduction in capacitance can reduce power supply hold-up times. As the output capacitors age, the output ripple voltage can increase to a point where the power supply can become unstable.
On the other hand, the smaller electrolytic capacitors that are associated with the housekeeping circuit and start up circuitry are often overlooked and considered relatively unimportant.
Capacitor manufacturers have responded to market needs and now offer a variety of economically priced electrolytic capacitors that have 10,000 hours with temperature ranges of up to 105°C. One would assume that if sufficient care is taken to ensure the ripple current and surrounding temperatures are correctly managed, the problem would be solved.
Not quite. Looking at one of the major capacitor manufacturer’s datasheets for a widely used long-life capacitor, it can be seen that for a 12.5mm diameter capacitor the life is stated at 10,000 hours. The life for a 6.3mm diameter capacitor in the same series at 105°C though is only 4000 hours. If the power supply is in operation 24 hours a day, that equates to less than six months. At that point, the datasheet says the capacitance can be 75% of the original value.
The widely used rule of thumb for capacitor derating is for every 10°C reduction in temperature, the life of an electrolytic doubles. Unfortunately, it is not unknown to find those small diameter capacitors located in a hot environment, close to the main power transformer. In a convection cooled application, at 40°C ambient, the case temperature could be as high as 85°C. Our 4000 hour capacitor would now have 16,000 hours, or 1.8 years before the capacitance value was reduced by 25%.
To compound the problem, electrolytic capacitors are notoriously sensitive to cold temperatures. A change in ambient temperature from 40° to zero can reduce the capacitance value by up to 10% and double the impedance.
Typically, when a power supply is switched on, current flows from the rectified high voltage through a series resistor to charge a capacitor connected across the supply pins of the control IC. When that voltage reaches the minimum threshold, the power supply converter will start switching and generate its own power. For the first few cycles though, that capacitor will provide a peak current to energise the power components. If there is insufficient energy stored, the voltage falls to less than the minimum threshold and the power supply will not operate. Oversizing that capacitor to compensate for aging effects can result in a longer charge time and an unacceptable turn on delay.
With the boiler power supply, the small diameter start up capacitor would have decreased in capacitance over the years. The sub-zero weather was probably just enough to reduce it to the point where it could not provide sufficient energy to initiate the converter. Some external heat was just enough to help.
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. As long as AC power was available, the power supply functioned correctly. If the power was interrupted, then the power supply would not restart, forcing an expensive recall.
The engineering team designed a start-up circuit that uses a longer-life ceramic capacitor in place of the commonly used electrolytic. Ceramic capacitors do not contain liquid electrolyte and are not susceptible to that aging failure mode and hence overcome this particular failure mode.
Author profile:
Martin Southam, director of marketing, TDK-Lambda EMEA
