Jan Matthews Technical Director for Young Electronics Group discusses the need for high reliability electronic components in critical applications.
Electronic products are at the heart of almost all everyday activities that affect our lives. These products can be simple disposal aids that are only expected to last for the warranty period to critical safety devices that would be expected to last the life of the system and most certainly significantly longer than any manufacturers’ warranty period. Only the use of the highest possible quality controlled component manufacturing can ensure such critical and long life electronic products fulfil their needs. Young Electronics Group has over 35 years of dedicated experience stocking and distributing the highest reliability components from component manufacturers dedicated to zero defect manufacturing controls into the UK electronics manufacturing base.
One such electronic component that can cause major reliability problems in electronic circuits is the electrolytic capacitor. This device is not often seen as a potential weak point in circuit design but is has such a critical task to perform in almost all circuitry and will definitely be used within a system if a power supply or point of power transfer is required. Electrolytic capacitors are simple electro-mechanical devices that many off shore companies can manufacture but only a few have the experience and control of the critical constituent components that make zero defect high reliability capacitors. Virtually all such manufacturers are vertically integrated in their electrolytic capacitor manufacturing processes. They will have been focussed in electrolytic capacitor manufacturing, have decades of capacitor technology experience and be totally dedicated to producing the highest quality and longest operating life components to the world’s most discerning electronic equipment manufacturers.
On the application or design side of the equation is the question which capacitor to choose to provide a long operating life. It’s no good choosing the best capacitor in the world if it isn’t suited to the application or local environment. After selecting the correct capacitance, voltage and rms current ratings for the circuit the product’s intended use must be considered.
The level of shock and vibration must be considered or severe acceleration forces can stress the capacitor’s leads and over time cause the lead to foil stitching to fracture or break. This will lead to either increasing ESR and local heating or a direct open circuit. To over come this problem especially with SMD electrolytic capacitors manufacturers include dummy terminals that act as extra circuit board anchors to minimise the mechanical effects.
The next point to consider is the voltage that can be applied to the capacitor. During manufacture the foil is formed and an oxide layer is grown to a depth that suits the dc “working” voltage of the capacitor. Generally this voltage is set to be in the order of 20% higher than the rated value in order to provide a high level of margin especially for large scale manufacture. This is not a licence for applications to push the capacitor in order to use a lower rating or lower cost part as applying higher than specified voltages will in the long term cause local points on the foil to generate excess gas (vaporising the electrolyte). Gas generation causes extra heating and so the cycle will build up and accelerate the loss of electrolyte. Eventually the ESR will increase and the capacitance will decrease and this in turn leads to more stress on the capacitor and even more local heat is generated and so shortens the design life of the product considerably.
Lastly and most likely the least considered point from a design angle is temperature. The local ambient operating temperature is such an important factor to consider, especially if the product is intended to be installed within another system. Electrolytic capacitor manufacturers will publish their own methods of determining the best operating conditions depending upon the capacitor series chosen. Following these guidelines and using them to select the optimum part will save money and lower risk and certainly ensure the expected and intended operating life is achieved without suffering premature failures.
Below is a real example of testing the suitability of three electrolytic capacitors used in a power supply.
Two power resistors were mounted directly on to the PCB and in very close proximity to this capacitor. This had caused circuit failure during prototype life testing due to the seal drying out and the subsequent loss of electrolyte had caused erosion of the PCB tracking. Lifting these resistors away from the PCB would help solve the issue but what is the impact on the capacitor?
This electrolytic capacitor is a Nippon Chemi-Con KMEVB100/35-8 Taking suitable temperature measurements of this part the expected operating life can be determined.
Operating the power supply with a 90% and allowing the temperatures to stabilise over 5 hours.
Case temperature of the dummy (working capacitor is removed and connected via wires) capacitor: +69C (T)
Peak Ripple current measured flowing through capacitor is 140mA. By following the manufacturer’s guidelines for estimating the heating effect factor (? T), this gives an rms core heating effect (? T) of 0.45 and gives a positive heating effect of 0.41C to the surface of this capacitor. Use ? T = (Ix/Io)2 x ? To,
Where: Ix is the measured ripple current
Io is the maximum specified ripple current
? To is the series base factor
The expression for calculating the expected life NCC capacitors is given by: Life Hours = (Base Hours) X (Ambient factor) X (Ripple Current factor)
Where Lbase for a KMEVB is 1000 hours
Life Hours = 22800 hours or 2.6 years continuous operation. If this life time was not long enough then the designer could choose to lower the local ambient temperature, choose a capacitor with a longer base life hours or a capacitor with a larger can or higher ripple current rating to lower the internal heating effects. Of course normally a combination and compromise of these three factors would be considered.
This is the bulk smoothing capacitor for the power factor controller circuit’s dc output. This part is a KMHVN150/450-25 X 40 and is heated locally by the effects of two power resistors, the PFC choke the convertor’s transformer.
Case temperature of the dummy capacitor: +53.3C (T)
Ripple current measured flowing through C2 is 1.6A @ 100Khz, this gives an rms core heating effect (? T) of 3.39
This gives a positive heating effect of 2.42C to the surface of C9
Lbase for a KMHVN is 2000h
Using the life expression above gives in this instance an expected life of 10 years. Again the designer would consider if this is excessive or this actually provides a good margin level of safety.
This is the local decoupling capacitor for the power factor controller integrated circuit. This part is a KMEVN2.2/100 – 5 and is heated locally by the effects of the same power resistors and the PFC choke. There is no evidence of this part having caused any previous circuit failure.
Case temperature of the dummy capacitor: +63.2C (T)
Ripple current measured flowing through C5 is 53mA @ 100Khz
This gives an rms core heating effect (? T) of 1.27
This gives a positive heating effect of 1.15C to the surface of C5
Where Lbase for a KMEVB is 1000h
Using the life expression above gives an expected life of 3.4 years continuous operation.
After all the testing and measurements have been completed the level of risk of failure judged against the operating conditions, product duty cycle and the determined expected operating life times can be made with the associated compromises taken for cost against performance and most importantly reliability.
YEG Components is a specialist distributor of electronic components with over 35 years experience supporting global electronic product OEMs and CEMs. They are part of Young Electronics Group which has 5 divisions, each focusing on a specific discipline, YEG’s experienced support team offer design in and logistics support to their customers.
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