Turn up the heat on stacked capacitors

Electronics that require elevated operating temperatures are going to be a key technology to support the next generation of products. We are all aware of the growing requirements for electronics in automotive applications such as hybrid vehicles and the demand for more electronics that control and monitor every aspect of our vehicles. This demand can also be seen in the control of motors and engines.

Consumers are driving an ever increasing pressure on the petroleum industry to provide the petrochemicals that make our lives comfortable.  The petroleum industry has responded by searching out new pockets of reserves and this means placing these controls in harsh environments.

For these types of applications, temperatures can easily surpass 150°C and sometimes even 200°C.  These devices are not only subjected to elevated temperatures, but also extreme shock and vibration.  Furthermore, these parts need to be able to supply high currents and also operate over a wide voltage range.

Prior to the adoption of 200 degree ceramics, these electronics were placed in a relatively safe environment like the inside of an airplane fuselage or perhaps even in a remote temperature controlled environment.  This “distance” from the source resulted in higher power losses in the cables, inefficiency and the need for larger power supplies to overcome these issues.

Today it is not uncommon to see a jet engine with ceramic capacitors on board the engine mount itself.  Obviously at 35,000ft the external temperature is well below zero and the power supply is not exposed to great amounts of heat.

However, within 5-10 minutes the aeroplane can be parked on the ground where ground temperatures can exceed 100°F (ambient) and the heat from the rotating engine will seep into the controls.  A similar situation also exists in the down hole logging environment.

SMX capacitors are rated for -55 to 200°C.  These capacitors were designed to endure the severe shock and vibration associated with harsh environments and mission critical applications such as down hole logging, the rigors of avionics and even space applications. Another feature is an ability to retain capacitance over temperature and frequency.

The capacitors were designed to meet varied conditions such as voltage ratings from 25Vdc to 500Vdc with a capacitance of 340µF at 25Vdc.

Many design constraints had to be overcome to create this technology.  A stacked capacitor not only has to be able to withstand the shock and vibration of everyday applications, but also meet the 200°C operating temperatures.  To do this a special high temperature solder was used to adhere the leadframe of the structure to the body of the individual capacitors in the stack.

This solder has a melting point well in excess of 250°C and approaching that of 300°C.

One of the other drawbacks to using a traditional capacitor composed of X7R dielectric material is that of capacitance loss over temperature.  Some traditional capacitors exhibit a capacitance loss of over 80%.  Using a specially developed dielectric, the 200°C capacitor has a minimal capacitance loss over temperature. This provides a typical loss of 55% of rated capacitance value at 200 degrees.

For these applications in the past, several competing capacitor technologies have been used, but the main one has been wet tantalum.  The advantages of a stacked ceramic or the “supracapacitor” stem from the core materials of these capacitors.

The lowest ESR capacitors have traditionally been ceramic capacitors and by placing these into parallel stacks, one gets the benefit of lower ESR and the added value of higher current and surge handling capability. The ceramic can handle in excess of x3.5 the ripple rating of a similar tantalum, while maintaining a minimal loss of capacitance.

One item that many engineers overlook is the capacitance loss under operating conditions.  These conditions can be a combination of frequency, voltage and temperature.  The table above clearly indicates this phenomenon.  If a design calls for 10µF of capacitance, the engineer will often either specify much higher capacitors in anticipation of this capacitance loss or place multiple parts in parallel, both of which can significantly add to the cost of the design as well as the physical size of the board layout.

X7R capacitors are designed to operate from -55 to 125°C.  X8R capacitors are designed to operate up to 150°C.  This new formulation allows users to operate the capacitor up to 200°C and still retain the X8R characteristics of cap loss over temperature.  In several data points the high temperature capacitors hold better performance over the extended temperature range than the typical equivalent X8R ceramics

Perhaps one of the most “telltale” characteristics of a capacitor is a prediction of how the capacitor is going to perform over an extended period.  When dealing with temperatures and potential thermally induced failures, an Arrenhius plot is often used to predict the outcome.

The Arrhenius plot is very useful when you are trying to find the “Activation Energy” of a certain process. The relationship could involve many variables as function of temperature, i.e. in high temperature capacitors you could trace an Arrhenius plot with leakage current as a function of 1000/T[K]. It gives you the information about the exact moment when the free carriers “jump” under certain conditions. In other words, when the device could potentially become unstable.

Or simply stated, what is the life expectancy of the capacitor?  As with any component, the higher the voltage, that is applied and/or the temperature that the device is exposed to, the shorter the life expectancy of the device.  This was also a key design parameter of the capacitor and dielectric.

We have performed extended testing of these high temperature ceramic capacitors. The initial testing was to put the device under an accelerated test plan which equates to over 1,500,000 hours of operating life.  Initial laboratory test results indicated a failure rate of 0.43% per 1000 hours at 200°C and full rated voltage.  Actual environmental and operating results have proven to be significantly better with zero failures over 20,000 hours of actual operation.

High temperature capacitors can be used in a variety of applications ranging from power controls in hybrid vehicles, power supplies for satellites in orbit, jet airplane engine controls on commercial, private and military related aircraft where operating conditions can be well in excess of 150 degrees.

Some of today’s oil logging tools and space exploration vehicles are experiencing  200°C operating environments and manufacturers are developing future equipment to operate in near 300°C environments of tomorrow.  These high temperature stacked capacitors are suitable to meet the demanding needs of these environments.