Power supply is the core of electronic equipment. But as crucial as it is, designing a power supply can be difficult due to an indirect feedback loop within design teams, especially when it comes to thermal solutions. It is often more difficult to know what the temperature should be as opposed to what the temperature will be.
Increasingly, the electronics industry is realizing that classic derating is no longer satisfactory; its broad assumptions can result in overly conservative and expensive designs or products with insufficient reliability. Either case can cause a loss in customers and a shrinkage of market share. A more effective approach is one that takes the results of thermal modeling or measurement and inserts these results into design rules or predictive tools that are based on reliability physics.
The component technology of greatest concern in regard to temperature and reliability in power supply design are:
- Magnetics (Transformers & Chokes)
- Optocouplers / Light Emitting Diodes (LEDs)
- Capacitors (Electrolytic, Ceramic, & Film)
- Integrated Circuits
- Solder Joints
Magnetics, such as transformers and chokes, are the components often least considered when concerns about temperature arise during design reviews. Since transformers are typically custom made, many do not come with a temperature rating. So, how do you determine what is too hot for magnetics? There are three key issues of concern.
The first is that the saturation current in ferrite material has a soft saturation curve which can tend to obscure when a material starts to saturate. Saturating the magnetic material will not damage a magnetic, but it will appear to be shorted to the electronic circuit and can cause the circuitry to fail.
Another issue is that designers are sometimes under the mistaken impression that maximum temperature rating is equal to the Curie temperature (this can be between 100C to 300C). However, core loss usually reaches a minimum at temperatures between 50C to 100C, which is below the Curie temperature. Depending on the ferrite design, structure and cooling, the magnetic can go into a thermal runaway if the core temperature is on the right side of this minimum.
Finally, thermal aging is primarily a concern for powder iron cores, which are lower cost and sometimes more appropriate than ferrite cores. Long-term exposure at elevated temperatures can induce ‘Thermal Aging’ of the binding agents. As Thermal Aging progresses, the eddy current loss becomes significantly higher. Increasing core loss results in higher core temperatures and failure of the magnetic component.
Light Emitting Diodes/Optocouplers
LEDs are incorporated into power supplies to be utilized as indicators. LEDs have a natural lifespan that ends in a wear-out mechanism. Defects within the active region can spur nucleation and dislocation growth and they are particularly affected by temperature.
Challenges with LEDs often arise due to their use in optocouplers. The issue is where to locate the optocoupler. Different locations have various advantages and disadvantages, but more importantly, different locations have distinct differences in temperature. Ultimately, optocouplers should be placed in areas that keep them as cool as possible to decrease the thermal stress on the LEDs.
Electrolytic Capacitors are the components designers need to be most concerned with in regard to temperature. Because electrolytic capacitors rely on liquid for functional operation, they have a limited lifetime due to the gradual evaporation of their liquid electrolyte. This loss of electrolyte leads a gradual decrease in capacitance and an increase in equivalent series resistance. As a result of this process, all electrolytic capacitor manufacturers provide a rated lifetime.
So, how do power designers take into consideration electrolytic capacitors and temperature? Most companies extrapolate manufacturer’s ratings to actual use environment using a classical Arrhenius equation to develop a conservative prediction of lifetime of electrolytic capacitors. But this strategy can be hit-or-miss.
Actual lifetime can vary depending upon the sensitivity of the circuit to change in component parameters. The manufacturers’ definition of lifetime is typically a 20% drop in capacitance. By that point, the equivalent series resistance could experience an increase of 2X to 5X. Depending on the sensitivity of the circuit, this could induce failure in the product before ‘failure’ of the capacitor.
When designers place electrolytic capacitors near hot components, the standard lifetime equation may not even apply as non-uniform temperature distributions across the capacitor can induce accelerated degradation and pressure increases that can induce rupture.
Ceramic capacitor manufacturers have aggressively increased the amount of capacitance available. This improvement in capacitor technology has required an increase in the number of dielectric layers and a decrease in the dielectric thickness. The rate of this decrease has exceeded the reduction in rated voltage, resulting in a significant rise in the electric field across the dielectic.
A combination of accelerated testing determined that 0603 / 10uF / 6.3V / X5R capacitors operated at 40C and 3.3VDC could see 2% failure after 10 years. It may not sound like much but once you consider all of the capacitors in your design, it starts to add up to a real problem.
Film capacitors can fail by one of two failure mechanisms, both of which are sensitive to case temperatures: partial discharge and embrittlement of the dielectric material. Unfortunately, there are no good formulas that separate the effect of two distinct failure mechanisms. Instead, the typical approach for predicting lifetime is to extrapolate from the standard IEC 60384-16 endurance test.
Lifetime of film capacitors is by far the most sensitive to variations in voltage. Because of this, expert designers are typically willing to allow film capacitors to get little hotter than electrolytics or ceramics because sufficient voltage derating can extend lifetime sufficiently for most applications.
Because complex integrated circuits within their designs may face wearout or even failure within the period of useful life, it is necessary to investigate the effects of use and environmental conditions on these components. The main concern is that submicron process technologies drive device wearout into the regions of useful life well before wearout was initially anticipated to occur.
The ability to analyze and understand the impact that thermal effects have on failure mechanisms and device reliability is necessary to mitigate risk of system degradation, which can cause early failure of a system. Reliability Physics Analysis knowledge and an accurate mathematical approach which utilizes semiconductor formulae, industry accepted failure mechanism models, and device functionality can access reliability of those integrated circuits vital to system reliability.
Solder joints provide electrical, thermal, and mechanical connections between electronic components and the substrate or board to which it is attached.
When experiencing changes in temperature, the component and printed board will expand or contract dissimilarly due to differences in the coefficient of thermal expansion. This difference in expansion or contraction will place the second-level solder joint under a shear load. This stress is typically far below the strength of the solder joint. Repeated exposure to temperature changes can introduce damage into the bulk solder. Each additional temperature cycle accumulates damage, leading to cracking and eventually the failure of the solder joint.
The failure of solder joints due to thermo-mechanical fatigue is one of the primary wearout mechanisms in electronic products, primarily because inappropriate design, material selection, and use environments can result in relatively short times to failure.
Finding a Solution
While the issues are clear, power supply designers have had a challenging time resolving these issues due to the lack of effective tools. A more viable approach is to utilize Reliability Physics Analysis, where degradation behavior is predicted, and tradeoff analyses are performed using validated algorithms that use environmental, material, and architectural information to provide accurate guidance and prediction on the performance of the power supply.
DfR Solutions’ Sherlock Automated Design Analysis™ software helps the power supply engineer clearly understand when hot is too hot. Standard design information is combined with comprehensive embedded databases to provide the inputs necessary to perform these complex calculations. Streamlined software architecture ensures that thousands of these calculations are completed and results displayed within a matter of minutes. Most important, this type of analysis can now be performed by the design team well before any prototyping activity.
By using Sherlock, power supply engineers can now be sure that their product will never be too hot.