SRC Forum - Message Replies
Forum: Reliability & Maintainability Questions and Answers
Topic: Reliability & Maintainability Questions and Answers
Topic Posted by: Reliability & Maintainability Forum
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
Posted by: Kevin Silke
Date posted: Mon Apr 7 17:43:30 US/Eastern 2003
Subject: MLCC Derating
In a power supply we are designing, there is a multilayer ceramic chip capacitor that can see a significant ripple current. Our normal derating criteria in this case is to calculate the temperature rise of the part, ensure that we still have 10C of margin against the rated max temperature in the worst case environment, and add some additional voltage derating if the part temperature gets within 40C of the max rated temperature. But this allows for a significant temperature rise in the part. Is there a limit on the allowable temperature rise that I should be using?
Subject: Chip Capacitor Temperature Rise
Reply Posted by: (firstname.lastname@example.org
Date Posted: Thu Apr 17 14:33:08 US/Eastern 2003
Thermal characteristics of ceramic chip capacitors are governed by the physical dimensions of the capacitor. Ceramic chip capacitors have a thick dielectric layers which result in low voltage gradients. As a result many capacitor manufacturers claim that these types of capacitors may be operated at specified voltages at the maximum rated temperatures, i.e. no temperature derating is required
To assist in thermal design, some ceramic capacitor manufactures supply a coefficient of thermal transfer, CTT, expressed as oC/W. Depending on the capacitance value and the construction of the chip, CTT is different for each different capacitance value.
While there is no rule of thumb for allowable temperature rise for a ceramic chip capacitor, the industry generally accepts a temperature rise of +20 degrees C when calculating the published maximum power ratings.
This information may be found at KEMET Electronics FAQs web site using the following links.
The RAC derating standard only has the 10 degrees C temperature safety margin along with voltage derating to 70% or 60% of maximum.