Isolated Flyback Converters: Performance Evaluation and Check Points
Critical checkpoint: Aluminum electrolytic capacitors
We have explained that in addition to the specifications, the following "Critical checkpoints" should be examined when evaluating the performance of an isolated flyback converter. This time, we will consider the last of these checkpoints, "Electrolytic capacitors".
- MOSFET VDS and IDS, and rated voltage of output rectifier diode
- Transformer saturation
- Output transient response and rising output voltage waveform
- Measuring temperature and loss
- Electrolytic capacitor
Lately, electrolytic capacitors have come to include tantalum and functional polymer electrolytic capacitors, in addition to aluminum electrolytic capacitors. In this discussion, however, we shall essentially focus on aluminum electrolytic capacitors, which remain the most standard type. Aluminum electrolytic capacitors are comparatively cheap and can attain high capacitances, and so should be thought of as the default choice for the input/output bulk capacitors used in AC/DC converters in particular. For this reason, often Aluminum electrolytic capacitors are used without too much detailed study, but here we mention some matters that require attention.
Aluminum Electrolytic Capacitors Require an Awareness of Service Lifetime
All components have a lifetime, but in the case of ICs and other semiconductor components, for example, excluding ramdom failures, their anticipated lifetimes are sufficiently long that they will outlast the useful life of the equipment without requiring any special consideration. However, in general aluminum electrolytic capacitors have relatively short lifetimes, and there is the possibility of performance degradation as a consequence of ageing in an operating power supply.
The lifetime of aluminum electrolytic capacitors is shortened considerably at higher temperatures. In general, they conform to the Arrhenius rule, also known as the "10-degree rule". That is, for every 10°C increase in temperature, the coefficient of ageing acceleration is doubled, which means the lifetime is halved. Of course this also means that, conversely, if the temperature can be lowered by 10°C, the lifetime is doubled (actual lifetime calculations for separate components should be performed according to equations or the like provided by the capacitor manufacturer).
If a ripple current flows in a capacitor, heat generation occurs due to losses caused by the internal impedance. Aluminum electrolytic capacitors have a comparatively high ESR, and it is important to recognize that should a large ripple current flow, the resulting heat generation would be substantial.
For example, if an aluminum electrolytic capacitor with a predicted lifetime of "2000 hours at 105°C" could be used at 75°C, then the predicted lifetime would be extended to 16,000 hours, but if the temperature is 95°C, a lifetime of 4000 hours would need to be assumed. And from such predicted lifetimes it should be clear that they have dramatically shorter lifetimes than do ICs or other components.
When an Aluminum Electrolytic Capacitor Deteriorates
So, what happens when an aluminum electrolytic capacitor degrades as it reaches its lifetime? In essence, its electrostatic capacitance decreases. This phenomenon may be called liquid leakage or capacitance loss. When the capacitance decreases, the power supply circuit cannot obtain the capacitance it would need to function normally, and so the following issues occur, and problems appear in the operation of devices being fed by the power supply.
- In the case of an input capacitor ⇒ Rise in ripple voltage, reduced retention time (because only a small amount of charge can be stored)
- In the case of an output capacitor ⇒ Rise in ripple voltage, reduced stability of output control loop (response is affected)
Points to be Noted
The essential point is to keep the capacitor temperature as low as possible. The following should be memorized as basic principles.
- Check the rated ripple current for capacitors that are to be used, and select only capacitors with ratings that are sufficiently higher than any ripple currents in the circuit.
- During evaluations, check the actual ripple current, as in the graph on the right.
- Similarly, thoroughly confirm the capacitor temperature and predict its lifetime.
- Depending on the conditions, perform derating, distance the capacitor from heat-generating sources, or take other measures to lower the temperature even a little bit.
- Perform a lifetime prediction, and based on the result, display the predicted lifetime and perform maintenance, including preventive maintenance.
We have described various points to be noted pertaining to aluminum electrolytic capacitors. This information may already be well known to designers of power supplies, but one hears of cases in which these matters are taken into consideration when selecting components in the design stage, but upon mass production a changeover is made to a general-purpose aluminum electrolytic capacitor with the same capacitance value, so that problems arise in actual use. Even components one is familiar with often have aspects that demand attention, as we have seen in the above.
・Aluminum electrolytic capacitors should be selected with particular attention paid to the lifetime.
・The lifetime of an aluminum electrolytic capacitor is shortened dramatically at higher temperatures; in general, the "10-degree rule" for halving of the lifetime applies.
・Capacitance loss due to degradation is highly likely to cause malfunctions in circuit operation.