By Capacitors Age, and Sometimes Fail
An aluminum electrolytic capacitor is a wear-out component: its capacitance falls and its ESR rises as the electrolyte dries over time, at a rate that depends on the core temperature and the ripple. A film capacitor is far more stable but is defeated by a poor layout. Most capacitor problems are therefore either normal aging that is faster than expected or a specific stress, and this article presents a method for diagnosing the common issues in TDK EPCOS capacitors.
Capacitance Loss
Capacitance loss is expected with age, and the datasheet specifies how much change is allowed after the endurance test. If the loss is faster than expected, check the applied voltage against the rating with margin, the polarity, the ripple margin and the case temperature. Over-voltage and reverse voltage accelerate the aging, and excessive ripple dries the electrolyte faster. Measure the capacitance and the ESR over time to establish the aging rate and compare it with the estimated life.
Overheating
A hot capacitor is usually carrying too much ripple, sitting near a heat source, or starved of airflow. Check the ripple current against the rating at the measured case temperature, check the local ambient, and check the airflow. Recheck the derating for the temperature and the frequency, and improve the layout if the capacitor is in the shadow of a heat sink or next to hot magnetics. Because ESR rises as the capacitor ages, the heating can creep up over the life.
Venting and Failure
Venting, where the safety vent opens and electrolyte escapes, usually follows an over-voltage, a reverse voltage, an over-temperature or excessive ripple. Check the applied voltage against the rating with margin, confirm the polarity of every capacitor in the bank, and verify the ripple and the temperature are within limits. In a bank, a single failure often points to an imbalance: if one capacitor carries more than its share of the ripple or sees a higher voltage, it ages faster and fails first. Verify the sharing across the bank.
High Overshoot in a Film DC Link or Snubber
When a film DC-link or snubber capacitor fails to limit the voltage overshoot, the cause is almost always the layout rather than the capacitor. The benefit of a low-ESL film capacitor depends on a low-inductance loop, so a long busbar, a wide loop or a distant snubber defeats it. Keep the commutation loop small, mount the snubber directly across the module, and measure the overshoot at the switch terminals.
A Systematic Method
Work from the simple to the complex: check the applied voltage and polarity first, then the ripple and the case temperature, then the bank sharing, then the connections and the layout. Keep a reference capacitor that is known good, and record the capacitance and the case temperature of each new design on the bench, so a later change is immediately visible. BeiLuo supplies genuine TDK EPCOS capacitors with import declaration, certificate of origin and RoHS documents, and our FAE team can help you diagnose a problem and choose a design change that resolves the root cause.